GEOLOGIJA 40, 247-281 (1997), Ljubljana 1998
Zeolites in the Smrekovec volcaniclastic rocks, northern
Slovenia
Zeoliti V vulkanoklasticnih kamninah smrekovškega podgorja
(severna Slovenija)
Polona Kralj
Institute of Geology, Geotechnics and Geophysics,
Dimičeva 14, 1000, Ljubljana, Slovenia
Key words: hydrothermal alteration, laumontite, analcime, zeolites, volcanicla-
stics
Ključne besede: hidrotermalne spremembe, laumontit, analcim, zeoliti, vulkano-
klastiti
Abstract
Volcaniclastics from the Upper Oligocene Smrekovec volcanic complex comprise
autoclastic deposits, locally resedimented hyaloclastite deposits, pyroclastic depo-
sits, volcaniclastic debris flow and turbidite ash flow deposits and reworked turbi-
dite ash flow deposits. Particularly coarser-grained rocks underwent changes in mi-
neralogy characterised by the development of zeolites and related new-formed sili-
cate minerals: albite, quartz, chlorite, interlayered chlorite/smectite, prehnite, pum-
pellyite and sphene. Among zeolites, laumontite is the most widespread mineral; it
primarily occurs in veins and as interstitial cement but may also replace volcanic
glass, pj^ogenetic plagioclases and fine-grained matrix. Other zeolites - heulandite,
heulandite-clinoptilolite, analcime, stilbite, yugawaralite and thomsonite are less
abundant, and are more localised in occurrence. The formation of zeolites and other
new-formed silicate minerals is related to hydrothermal conditions generated by
emplacement of high-level intrusive bodies into soft, water-saturated sediments.
Kratka vsebina
Med vulkanoklastiti smrekovškega podgorja najdemo avtoklastične kamnine, lo-
kalno presedimentirane hialoklastite, piroklastite, vulkanoklastične debrite in tur-
bidite ter lokalno presedimentirane vulkanoklastične turbidite. Zeoliti in drugi av-
tigeni minerali: albit, kremen, klorit, glineni minerali z zmesno strukturo vrste klo-
rit/montmorillonit, prehnit, pumpellyit in sfen so nastali predvsem v bolj debelozr-
natih kamninah. Med zeoliti je najbolj razprostranjen laumontit, ki se najpogosteje
pojavlja kot žilni mineral ali pomi cement, ponekod pa lahko nadomešča tudi prvo-
tne sestavine kamnine - vulkansko steklo, pirogene plagioklaze ali drobnozmato
tufsko osnovo. Drugi zeoliti, heulandit, trdne raztopine heulandita in klinoptilolita,
analcim, stilbit, yugawaralit in thomsonit so v vulkanoklastičnih kamninah smre-
kovškega podgorja zastopani mnogo redkeje. Nastanek zeolitov je vezan na delova-
nje hidrotermalnih raztopin, ki so nastale s pregrevanjem pomih vod v vulkanokla-
stičnih sedimentih tedaj, ko je vanje intrudirala andezitna magma.
248_Polona Kralj
Introduction
Volcaniclastic material is particularly susceptible to alteration processes. Being
formed at much higher temperatures than that of the depositional medium, it is gene-
rally not in equilibrium with its low-temperature sedimentary environment. In res-
ponse to essentially diffrent chemical and physical conditions on the Earth's surface,
volcaniclastic constituents undergo the changes in mineralogy, characterised by reac-
tions of hydration. These changes are particulary pronounced in aqueous environ-
ments.
Volcaniclastic sediments may be deposited in environments with high chemical
gradient of reacting solutions, in areas of high-temperature gradients and/or hydro-
thermal activity, and in subsiding sedimentary basins. Herein, volcaniclastic material
is subjected to physical and chemical conditions departing further from those that
prevailed during deposition and early stage diagenesis. As a result, many of the initi-
ally stable minerals become unstable or metastable, whereas the stability conditions
of many of other secondary minerals are still far from being attained. The mineral re-
actions taking place are a response to this instability and tend to establish or re-esta-
blish equilibrium between various phases and between the phases and the environ-
ment.
The most pronounced new-formed minerals in volcaniclastic rocks are zeolites
(Steiner, 1953; Coombs et al., 1959; Kossovskaya & Shutov, 1961;
O t al or a, 1964; lijima & Utada, 1966; 1972; I i j i m a, 1978; 1980; 1984;
1988; Rostov, 1969; S eki et al., 1969; Hay & lijima, 1968a, 1968b; Utada,
1973; 1988; Hay, 1980). Most common zeolite species are clinoptilolite, heulandite,
analcime, phillipsite, chabasite, mordenite, erionite, laumontite and wairakite, so-
mewhat rarer in occurrence are yugawaralite, stilbite, natrolite, gonnardite, thomso-
nite, harmotome and levynite (Gottardi & Galli, 1985). They form in volcanic
rocks in varying geologic environments and due to diverse processes: weathering,
percolating of meteoric water, in saline-alkaline lake deposits, by deep sea hal-
myrolysis, upon burial diagenesis, and by contact metamorphism and hydrothermal
alteration (lijima, 1984; Utada, 1987).
Zeolites form by weathering upon surface conditions in alkali soils of semiarid
areas by interaction of volcaniclastic constituents with alkaline soil water. Hay
(1970, 1978) has described the alteration of trachytic glass being replaced by philli-
psite, chabasite and analcime.
Another environment favourable for the zeolite formation is the so-called open
hydrologie system. Meteoric water percolating through a tuff reacts with volcanic
glass to increase in pH and alkalinity until zeolites precipitate in interstitial pores
and voids of dissolved glass shards. Zeolites and other new-formed minerals are di-
stributed in vertical zones consisting of surface soil, fresh tuff or slightly altered
opal- and montmorillonite-cemented tuff, and zeolitic tuff. This type of zeolitization
was described for the first time in volcaniclastics of alkali-basaltic composition from
Hawaii by H a y and lijima (1968a, b) and lijima and H a r a d a (1968). The
open system alteration was also recognised in the Pliocene alkali basaltic volcanicla-
stic rocks at Grad, NE Slovenia (Kralj, 1995). Common zeolites encountered in
this type of environment are phillipsite, chabasite and analcime; gonnardite and na-
trolite may also occur In tuffs of rhyolitic composition clinoptilolite and abundant
smectites form by interaction of silicic glass with percolating ground water (lijima,
1984).
Zeolites in the Smrekovec volcaniclastic rocks_249
Alkaline saline lakes are closed hydrologie system where zeolites form by interac-
tion of volcanic glass and alkaline brines (Hay, 1966; 1978; Sheppard, 1973;
Surdam & Sheppard, 1978; Sheppard & Gude, 1968; 1969). Typical
zeolites produced in lake deposits of mafic composition are phillipsite, chabasite and
erionite. In volcaniclastic sediments of silicic composition, clinoptilolite and morde-
nite occur. Zonation of new-formed minerals is horizontal, extending from the lake-
margins to the lake-centre.
Halmyrolysis includes the reactions of volcanic glass from ash layers of youn-
ger geologic age, deposited on the bottom of the World's Oceans (I i j i m a,
1978; Kastner & Stonecipher, 1978; Honnorez, 1978). In mafic tuffs
and pelagic brown clay, encountered in the Pacific and Indian Oceans, phillipsite is
the dominant authigenic mineral. Clinoptilolite is more abundant in altered silicic
tuff, pelagic clay and siliceous oozes in the region of Atlantic Ocean and marginal
seas.
Diverse species of zeolites form upon burial diagenesis of volcaniclastic rocks in
widespread géosynclinal systems (Coombs et al., 1959; Kossovskaya &
S hut o V, 1961; lijima & Utada, 1966; Boles & Coombs, 1975; 1977;
Boles, 1989). During early diagenesis clinoptilolite and mordenite form in silicic
glass; upon progressive burial earlier developed zeolites alter further to analcime
and/or heulandite and finally to albite and laumontite.
Zeolitization in hydrothermally active environments is rather complex. According
to Utada (1987) it can be subdivided into four main types - Kuroko, Iceland, Oki-
nobe and Yellowstone, characterised by different zeolite zoning, the zone morphology
and extension, the temperature of zeolite formation and the chemistry of reacting so-
lutions. Some authors, i. e. lijima (1984), also cathegorise contact metamorphism
among hydrothermal occurrences.
The development of zeolites and other new-formed hydrous silicates in a hydro-
thermally active environment is strongly controlled by the temperature and chemi-
stry of reacting solutions. Chemical composition, porosity and permeability of the
host rock may also be important in zeolitization processes. In volcaniclastic sediments
and rocks of mafic composition, heulandite, stilbite, mordenite, laumontite, wairakite
and yugawaralite are the most significant zeolites (Kristmannsdottir &
Tomasson, 1978; Steiner, 1953; Utada, 1987). In tuffs of silicic composi-
tion, clinoptilolite, mordenite and analcime develop (Utada, 1988; Honda &
Muffler, 1970).
Geological setting of the Smrekovec volcaniclastics
The Smrekovec mountains, located in northern Slovenia (fig. 1) are characterised
by a widespread occurrence of coherent volcanic rocks and volcaniclastic deposits.
The complex encompasses an area of approx. 15 sq. km and includes three major mo-
untain peaks, Komen, Krnes and Smrekovec, reaching 1684 m, 1613 m and 1577 m
respectively. The basement consists mainly of Mesozoic carbonates encountered as
tectonically uplifted blocks on the NW and SE margins of the Smrekovec volcanic
complex. A NW-SE trending fault of the peri-Adriatic lineament zone separates this
complex from the Karavanke tonalité (M i o č, 1983). The Smrekovec volcanic com-
plex represents a part of a wider volcanic belt, named the "Smrekovec series", exten-
ding along a distance of about 100 km towards the southeast (M i o č, 1978; M i o č
250
Polona Kralj
Fig. 1. The Smrekovec volcanic complex: geological sketch map (after M i o č, 1983)
SI. 1. Poenostavljena geološka karta smrekovškega podgorja (po M i o č u, 1983)
et al., 1986). The Smrekovec volcanics are of Upper Oligocene stratigraphie age, as
determined on the basis of foraminifera fauna, found in the locally underlying mari-
ne marls and siltstones (R i j a v e c, 1966).
The present rather complicated situation in northern and north-eastem Slovenia is
associated with global tectonic processes of Late Cretaceous to Tertiary subduction and
collision of the continental African and oceanic European plates and their segmented
parts, Apulia and the Pannonian fragment (Oberhauser, 1980; Roy den, 1988;
Dercourt et al., 1986). In early Miocene, the Pannonian fragment separated from
Apulia and began to escape eastward from the collision zone in Eastern Alps. Due to
the mentioned eastward escapement, an extension of the Pannonian fragment began,
being followed by subsidence, and consequently, the formation of a back-arc basin -
the Pannonian basin.
Zeolites in the Smrekovec volcaniclastic rocks_5
It remains undefined whether the Smrekovec volcanism is related to an active con-
tinental margin or to one of the collision combinations: island arc - active continental
margin - passive continental margin (Gill, 1981). However, chemical composition of
the Smrekovec intermediate volcanics is not very characteristic of orogene andesites
(Kralj, 1997). It indicates that tholeiitic magma very possibly underwent a differen-
tiation due to crystal fractionation. Consequently, basalts, basaltic andesites, acid an-
desites, dacites and finally rhyodacites evolved in time, forming a volcanic suite. The
Smrekovec volcanism may be related to local extension and leakage at the plate boun-
dary, as it is the case in central California (Dickinson & Snyder, 1979a, b).
Smrekovec volcanic activity built a complex of submarine stratovolcano(es) with a
significantly elevated relief composed of lavas, high-level intrusive bodies, autocla-
stic deposits, pyroclastic deposits and syn-eruptive resedimented volcaniclastic de-
posits (Kralj, 1997). The early stage of volcanic activity was dominantly non-ex-
plosive. Basalts and basaltic andesites were emplaced as submarine lavas or high-le-
vel intrusive bodies. The style of fragmentation was mainly autoclastic, related to
chill and quench processes. The late-stage volcanic activity is characterised not only
by non-explosive volcanism of acidic andesitic to rhyodacitic composition, but also
by explosions, either combined hydrovolcanic and magmatic, or solely hydrovolca-
nic. Juvenile material, chiefly pumice and glass shards, became relatively abundant.
Explosive volcanic activity was probably instrumental in generation of volcaniclastic
debris flows and turbidite ash flows. Their deposits are recently the most widespread
throughout the Smrekovec volcanic complex.
Lithofacieses of volcaniclastic deposits were subdivided into four main groups
(Kralj, 1997):
1.	lithofacieses of autoclastic deposits and resedimented hyaloclastite deposits com-
prising sublithofacieses of hyaloclastite breccia, hyaloclastites, peperitic breccia, and
peperites;
2.	lithofacieses of assumed pyroclastic deposits;
3.	lithofacieses of volcanic debris flow and turbidity ash flow deposits comprising
sublithofacieses of polymict volcaniclastic breccia,volcaniclastic tuff-breccia, hori-
zontally stratified coarse-grained tuffs, horizontally laminated and vaguely lamina-
ted fine-grained tuffs, and massive fine-grained tuffs;
4.	lithofacieses of reworked turbidite ash deposits which comprise sublithofacieses
of massive tuffaceous sandstone, through-cross stratified tuffaceous sandstone and
massive tuffaceous sandstone.
Zeolites and accompanying secondary minerals in the Smrekovec volcanics
Some of the volcaniclastic, autoclastic and coherent volcanic rocks have undergo-
ne the changes in mineralogy, characterised by the development of zeolites and other
new-formed minerals: interlayered chlorite/smectite, albite, quartz, prehnite, pum-
pellyite, epidote, sphene, apophyllite, alkali feldspars and amphiboles (K o v i č &
Krošl-Kuščer, 1986; K o v i č, 1988). They are abundantly developed on the
contacts of high-level intrusive bodies with the enclosing sediments or in their vici-
nity. Particularly zeolitization was strongly controlled by porosity and permeability
of sediments and is more pronounced in the coarser-grained volcaniclastics. This can
easily be recognised in interbedded coarser- and finer-grained volcaniclastic rocks
from the same profile: in coarser-grained varieties, laumontite, prehnite and pumpel-
252_Polona Kralj
lyite developed, whereas interbedded, well-sealed fine-grained volcaniclastics do not
contain either laumontite or prehnite and pumpellyite.
Rock composition controlled the kind of zeolite developed, although to some ex-
tent only. Laumontite occurs in the rocks of various composition, from basaltic to
rhyodacitic. It replaces the primary constituents (volcanic glass, fine-grained matrix
or plagioclases) or infills interstitial space, voids or fissure systems. On the other
hand, clinoptilolite and heulandite occur mainly in hyaloclastites of acid andesitic to
dacitic composition replacing volcanic glass and infilling vesicles and the rock pore
space. Analcime and thomsonite developed in some complexly altered rocks of basal-
tic to basaltic andesitic composition. Herein, analcime replaces formerly developed
laumontite, and albitised plagioclases. Yugawaralite and stilbite are characteristic
vein minerals, and do not seem to be influenced by the host rock composition.
Studies of zeolites and accompanying new-formed minerals in the Smrekovec vol-
canics are based on X-ray diffraction (determination of mineral composition of 94
powdered samples, and determination of cell parameters for 3 analcimes), pétrogra-
phie investigation under the microscope (86 thin sections), elemental analysis by
scanning electron microscope and energy dispersive X-ray spectrometry (30 zeolites
and accompanying new-formed minerals), and combined chemical analysis - wet,
atomic absorption spectrometry and emission spectrometry with inductively coupled
plasma source (4 zeolite bearing rocks and 3 separated analcimes).
Laumontite - Ca^iAl^Si^ßO^g).I6H2O
Laumontite is the most widespread new-formed zeolite in the Smrekovec volca-
nics. Very commonly, it can be encountered in veinlet systems (plate 1, fig. 1). It also
infills vesicles of volcanic lithic fragments (plate 2, fig. 1, 2) and interstitial pore spa-
ce (plate 2, fig. 3). Replacements of the primary constituents - pyrogenetic plagiocla-
ses (plate 2, fig. 4) or volcanic glass (plate 3, fig. 1, 2) are somewhat less abundant. In
general, the amount of laumontite rarely exceeds 20 wt.% of the whole rock, even in
the most extensively altered volcanics. The average laumontite content, determined
by X-ray diffraction method in 48 of the laumontite-bearing rock samples, ranged
between 5 and 15 wt.%. The accompanying new-formed minerals determined are qu-
artz, albite, chlorite and interlayered chlorite/smectite, written in the order of de-
scending abundance. The amounts of prehnite, sphene, pumpellyite, epidote or
apophyllite were beyond the X-ray detection limits; these minerals can only be reco-
gnised under the microscope.
Laumontite crystals are very seldom transparent in a hand specimen (plate 1, fig. 1);
most commonly they are earthy whitish. Crystal size ranges from some 10 pm up to 2
mm; the largest crystals formed in veins. Elemental analysis of ten laumontite sam-
ples by scanning electron microscope and energy dispersive X-ray spectrometry
(SEM-EDX) revealed that laumontite may also contain small amounts (approx. up to
2 wt.%) of K2O (fig. 2) besides calcium; sodium has not been detected in any of the
examined samples.
Laumontite commonly replaces volcanic glass along with albite and quartz as ac-
companying new-formed minerals (plate 3, fig. 1, 2). Intergrowths of laumontite, al-
bite and quartz are sometimes very fine-grained, detectable by X-ray diffraction
only, although sometimes they may be recognised under the microscope, too. Replace-
ments of volcanic glass by laumontite are often observed in hyaloclasts of autobrecci-
Zeolites in the Smrekovec volcaniclastic rocks
253
Fig. 2. Energy dispersive X-ray spectrum (EDX) yielding the major
elements of laumontite (sample Sm 29, northern slopes of Smrekovec)
SI. 2. Energijsko disperzijski spekter rentgenskih žarkov (EDX), na
katerem so prikazane glavne prvine laumontita (vzorec Sm 29, severno
pobočje Smrekovca)
ated lavas, high-level intrusive bodies and peperitic breccias. Rapid heat transfer
from the emplaced high-level intrusive body into soft volcaniclastic sediments cau-
sed sudden increase in temperature of the enclosing sediments and their pore waters.
Consequently, local hydrothermal conditions arised affecting predominantly margi-
nal parts of the high-level intrusive body and the hyaloclasts of peperitic breccias.
The laumontite-albite-quartz mineral assemblage commonly replaces spherical areas
of hydrated volcanic glass produced by perlitic cracks (plate 3, fig. 1, 2).
Laumontite from the Smrekovec volcanics can also replace pyrogenetic plagiocla-
ses and alkali feldspars, although the majority of plagioclases is albitised. According
to Coombs et al. (1959), laumontite replaces the anorthite component in plagio-
clases whereas the albite component alters to fine-grained aggregates of albite. Some
of alkali feldspars are altered to laumontite and secondary alkali feldspars. The two
new-formed minerals are not intimately intergrown but replace crystal grains in the
form of irregular patches, attaining a few tenths of mm in length.
Microfissures developed in the Smrekovec volcanics are often infilled solely by la-
umontite (table 1) although veinlets containing besides laumontite also one or two
other zeolites - i.e. analcime (plate 3, fig. 3, 4), stilbite, yugawaralite, or yugawaralite
and analcime, can also be encountered. On the other hand, the prehnite association
with laumontite is rather common, not only in veins, but also in interstitial infillings
of volcaniclastic and autoclastic rocks (plate 1, fig. 3, 4; plate 2, fig. 3). Laumontite
postdates and also replaces prehnite. This is a very exceptional relationship between
the two minerals, since in burial environments where prehnite replaces laumontite,
the situation is opposite (Boles & Coombs, 1975; 1977, Thompson, 1971).
According to the activity diagram of phase relations for laumontite, heulandite and
prehnite (Boles & Coombs, 1977), heulandite alters either to laumontite or
254_Polona Kralj
Table 1. X-ray powder pattern of laumontite
Tabela 1. Zapis rentgenske difrakcije vprašenega vzorca iz laumontitne žilice
Sample (GN 38v) from a laumontite veinlet. Northern slopes of Komen;
Philips diffractometer, Ni filtered CuK^^ radiation (A,= 1.54051), slits 1°,
0.1 mm, 1°, scanning speed 17niin
prehnite when the activity of hydrous silica ajj4SiQ4(aq) decreases. Both reactions are
strongly controlled by the activity ratio аса2+/(ан+)^- The reaction from laumontite to
prehnite occurs unlikely in the presence of waters saturated with quartz, since silica
is, along with water and the H"^ ions the reaction byproduct (Boles & Coombs,
1977. Instability of prehnite and its conversion to laumontite evidenced in the Smre-
kovec volcaniclastic rocks could therefore be related to the decreased ratio
аса2+/(ан+)^ in reacting solutions; additional favourable conditions might be the in-
creased activity of hydrous silica aH4Si04(aq) ^^^^ the decreased temperature of reac-
ting solutions.
Heulandite (Na,K) Ca^(AlgSÌ2jOj2} • 24H2O and clinoptilolite
(Na,K)e(AleSÌ3o072) • 2OH2O
Heulandite and clinoptilolite form a continuous solid solution series along the join
between the stoichiometric fomulae given above (Mumpton, 1960; Gottardi
& Galli, 1985). Heulandite, clinoptilolite and numerous members of the heulandi-
te-clinoptilolite solid solution series altogether belong to the heulandite group; for
this reason, the name heulandite may sometimes refer to the whole genus. For proper
distinction of heulandite and clinoptilolite at least the thermal test of Mumpton
(1960) must be applied.
Heulandite is very common in hydrothermally altered basic volcanics as vesicle
and fissure filling. Sedimentary occurrences of heulandite with proper evidence are
Zeolites in the Smrekovec volcaniclastic rocks_9
rare. On the contrary, clinoptilolite is a veiy rare hydrothermal mineral and has been
shown to be the main constituent of many sediments, and is hence much more abun-
dant in the Earth's crust than heulandite (Gottardi & Galli, 1985).
In the Smrekovec volcanics heulandite replaces volcanic glass of acid andesitic
composition, being accompanied by cristobalite/quartz and montmorillonite; it also
infills pore space in the same rock. Heulandite has not been encountered as vesicle
and fissure filling in the rocks of more basic composition; therein, laumontite is the
predominant zeolite.
Common hostrocks of heulandite are resedimented hyaloclastites (plate 4, figs. 1,
3, 4) which also contain pumice lapilli and glassy, fine-grained matrix of similar
composition. Plagioclases are fresh. The new-formed minerals are very fine-grained
and can not be recognised under the microscope. Heulandite crystals sometimes atta-
in up to some hundred pim (plate 3, fig. 1); smectite montmorillonite occurs in globu-
lar aggregates having a few hundred pm in diameter (plate 3, fig. 1). Heulandite and
cristobalite or microcrystalline quartz are intimately intergrown when replacing vol-
canic glass. Besides heulandite, very small amounts of analcime may locally occur
According to Mumpton (1960) clinoptilolite remains stable after being heated for
twelve hours at 600°C, whereas the heulandite lattice collapses. X-ray diffraction
patterns of four thermally treated samples have confirmed the presence of heulandi-
te; in one of the samples solid solution heulandite-clinoptilolite with predominating
heulandite component has been determined (figs. 3a, 3b).
Resedimented hyaloclastites with pumice lapilli occur in the form of scarce,
small and isolated erosional remnants on the top of the mountain range from Ko-
men to Smrekovec. They are also to be found along the southern and northern
slopes of Komen, Krnes and Smrekovec, dipping outward from the top of Komen
towards the southeast and northwest, respectively. In general, the hyaloclastites
contain heulandite, heulandite-clinoptilolite, smectite and quartz. Locally, they
can be found altered to laumontite, albite, quartz, interlayered chlorite/smectite
and traces of analcime. The laumontite-albite-quartz-chlorite/smectite-(analci-
me) mineral assemblage occurring in the same rock layer as the heulandite-cri-
stobalite/quartz-smectite could indicate the presence of the progressive zeolite
reaction pattern: silicic andesitic/dacitic glass -> clinoptilolite-cristobalite/qu-
artz-smectite (mordenite)-heulandite-analcime-cristobalite/quartz-smectite
laumontite-albite-quartz-interlayered chlorite/smectite. However, the relation-
ship between laumontite and heulandite seems to be more complicated. In heu-
landite-bearing hyaloclastites, laumontite locally occurs in very small amounts
being developed as the replacement of volcanic glass in larger hyaloclasts or as
interstitial cement. Herein, laumontite was found to be partially replaced by cli-
noptilolite-heulandite (plate 4, fig. 2). Microscopic observation and X-ray analy-
sis indicate the transformation can be either direct or related to prior alteration
of laumontite to kaolinite or montmorillonite. The occurrence indicates that a
post-hydrothermal process, diagenesis or halmyrolysis, must be superimposed on
the earlier alteration.
Analcime Naiß(AlißSi320gß).16H20
Besides some subordinate occurrences of analcime developed during the progressi-
ve alteration of silicic andesitic or dacitic glass to heulandite, smectite and cristoba-
256
Polona Kralj
Zeolites in the Smrekovec volcaniclastic rocks_257
lite, analcime may also show very exceptional style of formation. In the northern
slopes of Smrekovec, extensively altered autoclastic and volcaniclastic rocks occur
containing up to 60% of analcime (plate 1, fig. 2). Herein, analcime replaces formerly
developed laumontite and albitised plagioclases, and is accompanied by interlayered
smectite/chlorite.
A complex alteration history leading to analcime development can be observed in
a 50 metres thick profile in the northern slopes of Smrekovec. The early stage of alte-
ration is characterised by an intrusion of basaltic andesite into volcaniclastic sedi-
ments. Andesite marginal parts were autobrecciated and autoclasts partially admix-
ted to the enclosing sediments. A plagioclase-rich dyke of similar composition cuts
the andesite. By contact metamorphism and hydrothermal activity related to the an-
desite emplacement laumontite extensively developed in the layer of autoclastic an-
desite along with albite, quarz, interlayered chlorite/smectite and traces of sphene.
Small amounts of prehnite and pumpellyite also occur in this autoclastic layer that
was situated immediately above the source of heat. Herein, pumpellyite may replace
plagioclases along with albite and prehnite (plate 5, fig. 1, 2) or infills vesicles in au-
toclasts (plate 5, fig. 4). The laumontite-albite-quartz-chlorite/smectite mineral as-
semblage is developed above the andesite intrusive body for over 120 metres, up to
the top of Smrekovec. However, the laumontite content in the section is fairly varia-
ble, but is generally much lower in volcaniclastic rocks (5-20 wt.%) than in the auto-
clastic layer where it may attain up to 50 wt.%. Interstratified fine-grained volcani-
clastic rocks, even if situated in close vicinity of the intrusive andesite, contain only
traces of zeolites, whereas plagioclases are completely albitised and the matrix re-
placed by interlayered chlorite/smectite.
This high-level intrusive body of basaltic andesitic composition is interrupted by
another andesite body - probably a feeder dyke - which is of acidic andesitic compo-
sition. Analcime is closely related to this late-stage intrusion and predominantly fol-
lows previous alteration replacing laumontite. It is very localised in occurrence; at a
distance of some 10 metres laterally from the intrusion, analcime becomes very scarce
- often below the X-ray detection limit. Herein, incomplete replacements of lau-
montite by analcime are commonly encountered (plate 3, fig. 3, 4). Closer to the in-
trusion, analcime becomes more pronounced, replacing not only laumontite but also
albitised plagioclases (plate 1, fig. 2). Analcime is particularly abundand in autocla-
stic rocks that previously underwent extensive laumontite alteration. The replace-
ment of laumontite by analcime is accompanied by crystallisation of alkali feldspars
(plate 5, fig. 3). The presence of alkali feldspars was confirmed by elemental analysis
of seven analcime-rich samples by scanning electron microscope and energy dispersi-
ve X-ray spectrometry (fig. 4). As already mentioned, laumontite may also contain,
besides calcium, small amounts (approx. up to 2 wt.%) of K2O (fig. 2); during the re-
action from laumontite to analcime, potassium might have been fixed by crystalisati-
on of alkali feldspars. Together with alkali feldspars, up to 200 mp sized exsolutions
of thomsonite sometimes occur
Chemical analyses of four analcime- or laumontite-bearing rocks important for in-
terpretation of analcime occurrence in the Smrekovec volcaniclastics is shown in ta-
ble 2. The rock samples no. 3 (Sm 34/51) and no. 4 (Sm 34/11) are texturally alike and
also, similarly extensively altered. The only conspicuous difference is in the type of
zeolite developed: the rock sample no. 3 contains analcime, and the rock sample no.
4, laumontite. It is very interesting that no obvious distinction between the abundan-
ces of major elements can be observed, although at least the difference in the sodium
258
Polona Kralj
Fig. 4. Ener^ dispersive x-ray spectrum (EDX) yielding the major ele-
ments of alkali feldspar exsolutions in analcime (sample Sm 34 c)
SI. 4. Energijsko disperzijski spekter rentgenskih žarkov (EDX), na kate-
rem so prikazane glavne prvine alkalnega glinenca, ki je kristaliziral med
nadomeščanjem laumontita po analcimu (vzorec Sm 34 c)
and calcium contents would be expected. The rocks could have undergone some ion
exchange processes in interlayered smectite/chlorite clay minerals after crystallisati-
on of zeolites.
Analcime has been separated almost completely from the bulk samples of extensively
altered rocks by the use of heavy liquids. Three relatively pure analcime samples contai-
ning no other minerals detectable by X-ray diffraction were obtained. The analcime
samples were investigated by the means of X-ray diffraction method (tables 3, 4, 5) and
combined wet chemical analysis, atomic absorption spectrometry and optical emission
spectrometry with inductively coupled plasma source (table 6). The results have
shown that analcimes are cubic, low-silica and calcian varieties. No solid solution with
wairakite (A oki & Minato, 1980; Harada & Sudo, 1976) can be assumed.
The analcime occurrence bears evidence of a very complex alteration history of the
Smrekovec volcaniclastic rocks. Analcime is superimposed on the earlier, laumontite
yielding alteration, and is related to the late-stage emplacement of an acid andesite
body - probably a feeder dyke. Experimental work on hydrothermal alteration of the
Smrekovec volcanics (table 1, sample no. 1) performed by Barth-Wirsching
(pers. comm.) indicates laumontite alters to analcime in closed or open system at the
temperatures of above 150°C by action of sodium-bearing reacting solutions. Hydro-
thermal fluids responsible for the laumontite to analcime transformation could have
been magmatic in origin but it is also possible that marine water from the sea-bottom
became superheated when penetrating along the fissures opening the pathway of the
ascending magma.
Zeolites in the Smrekovec volcaniclastic rocks_13
Table 2. Chemical composition of four analcime- or laumontite- bearing rocks
Tabela 2. Kemična sestava štirih vzorcev kamenin, ki vsebujejo analcim ali laumontit
1.	Ko-3, altered basaltic rock from northern slopes of Komen. Mineral composition, determined
by X-ray diffraction method: laumontite (20-25%), albite (15-20%), interlayered chlori-
te/smectite (50-65%^ quartz (<5%), K-feldspars in traces. Optically observer traces of pre-
hnite and sphene
2.	Sm 31a, altered coarse-grained volcaniclastic rock, northeren slopes of Smrekovec. Mineral
composition, determined by X-ray diffraction method: albite (35%), interlayered chlori-
te/smectite (20-30%), laumontite (20-30), quartz (15-16%)
3.	Sm 34/51, altered volcaniclastic rock, northern slopes of Smrekovec. Mineral composition de-
termined by X-ray diffraction method: analcime (40-45%), interlayered chlorite/smectite (45-
50%), quartz (<5%), albite in traces
4.	Sm 34/11, altered autoclastic rock, northern slopes of Smrekovec. Mineral composition, deter-
mined by X-ray diffraction method: albite (35%), laumontite (20-25%), interlayered chlori-
te/smectite (35-45%), quartz (<5%)
Sample (Ko-3) was analysed in X-RAL Activation Services Inc., Ann Arbor, Michigan. Samples
Sm 31a, Sm 34/51 and Sm 34Д1 were analysed in National Chemical Institute (KIBK), Ljubljana
Stilbite NaCa4(AlgSÌ270j2)-30H20 and yugawaralite Ca2Al2Sij2032-8H20
Stilbite and jaigawaralite are typical hydrothermal zeolites (Gottardi & Galli,
1985). In the Smrekovec volcanics both stilbite (fig. 5a) and yugawaralite (fig. 5b)
occur only as vein minerals, being always accompanied by laumontite. Stilbite com-
monly crystallises at lower temperatures than laumontite (lijima, 1984; Boles
& Coombs, 1975; L i o u, 1971a). Yugawaralite develops at higher temperatures
than laumontite and in comparison with wairakite at lower pressures (L i o u, 1971b).
In veins, yugawaralite and laumontite may also be accompanied by analcime. One
of the veinlets containing yugawaralite, laumontite and analcime occurs in a fine-
grained tuff which does not contain zeolites but is located in the vicinity of analcime-
rich rocks. Immediately above the contact with tuff, a few mm thick layer of fine-
grained laumontite and yugawaralite occurs. Above this layer, cubic crystals of anal-
260_Polona Kralj
Table 3. X-ray diffraction pattern of analcime (sample N 34 1/4 L)
Tabela 3. Zapis rentgenske difrakcije vprašenega vzorca analcima, izdvojenega iz kamenine v
težki tekočini (vzorec N 34 1/4 L)
Sample N 34 1/4 L, separated from altered volcaniclastic rock from nor-
thern slopes of Smrekovec; Philips diffractometer, Ni filtered CuK„ radi-
ation (X = 1.54051), slits 1°, 0.1 mm, 1°, scanning speed l°/min; cuMc cell
parameter a = 13.1950
cime developed; the analcime crystals are of approx. equal size of 2-3 mm. On the
analcime crystals, fine-grained laumontite occurs. The described succession of vein
zeolites indicates that laumontite crystallised before and after the analcime. Analci-
me, occurring between the layers of calcic zeolites seems to crystallise during short
episode of sodium-yielding hydrothermal activity.
Zeolite formation in the Smrekovec volcaniclastic rocks
The occurrence of zeolites and other new-formed minerals in the Smrekovec vol-
caniclastics is rather complex. The most common zeolite is laumontite; heulandite,
heulandite-clinoptilolite, analcime, yugawaralite, stilbite and thomsonite are subor-
dinate and more localised in occurrence. The accompanying new-formed minerals are
quartz, albite, chlorite, interlayered chlorite/montmorillonite, prehnite, pumpellyite,
sphene, epidote, zoisite and apophyllite.
Laumontite is a common zeolite in different environments. Upon burial and con-
tact metamorphism, it forms from a zeolite precursor - most frequently heulandite,
but also mordenite or clinoptilolite (Coombs et al., 1959; Boles & Coombs,
1975; 1977; lijima & Utada, 1966; Utada, 1973). On the other hand, hydro-
thermal genesis of laumontite, attributed to those crystals filling veins and fractures
with no obvious reaction of the mineralising fluid with the wallrock, is also rather
Zeolites in the Smrekovec volcaniclastic rocks_261
Table 4. X-ray diffraction pattern of analcime (sample Sm 34/31)
Tabela 4. Zapis rentgenske difrakcije vprašenega vzorca analcima, izdvojenega iz kamenine v
težki tekočini (vzorec Sm 34/31)
Sample Sm 34/31, separated from altered volcaniclastic rock from nor-
thern slopes of Smrekovec; Philips diffractometer, Ni filtered CuK„ radi-
ation (X = 1.54051), slits 1°, 0.1 mm, 1°, scanning speed l°/niin; cumc cell
parameter 13.7143
common (Gottardi & Galli, 1985). For comparison of the laumontite occur-
rence in the Smrekovec volcaniclastics, the alteration upon contact metamorphism,
encountered in Neogene sediments of Japan is particularly interesting. The following
text is a very breef summary of the comprehensive work of Utada (1973).
Neogene sediments surrounding volcano-plutonic masses underwent complex
changes in mineralogy related to contact metamorphic, diagenetic and hydrothermal
alteration. According to the assemblages of new-formed minerals eight alteration zo-
nes were recognised. Higher-grade zones are completely metamorphic and comprise:
the homblende-plagioclase zone, the actinolite-plagioclase-chlorite zone, the prehni-
te-epidote-plagioclase-chlorite zone, and the chlorite-epidote-plagioclase-quartz zo-
ne. Lower-grade alteration zones comprising abundant zeolites are the following: the
laumontite-chlorite-plagioclase-quartz zone, the analcime-heulandite-chlorite-mont-
morillonite-quartz zone, the mordenite-montmorillonite-opal/quartz or the clinopti-
lolite-mordenite-montmorillonite-opal zone, and the zone of altered volcanic glass,
montmorillonite and opal. The laumontite-bearing zone commonly spreads in the ou-
ter areas apart from the intrusive mass but sometimes it also immediately surrounds
intrusive bodies of small sizes. Laumontite replaces plagioclase phenocrysts, fine-
grained matrix and groundmass of various rocks, and is interspersed with other new-
formed minerals. It also occurs in druses and as a vein mineral. The original rock tex-
ture is relatively well preserved.
262_Polona Kralj
Table 5. X-ray diffraction pattern of analcime (sample Sm 34/60 L)
Tabela 5. Zapis rentgenske difrakcije vprašenega vzorca analcima, izdvojenega iz kamenine v
težki tekočini (vzorec Sm 34/60 L)
Sample Sm 34/60 L, separated from altered volcaniclastic rock from nor-
thern slopes of Smrekovec; Philips diffractometer. Ni filtered CuK„ radi-
ation (k = 1.54051), slits 1°, 0.1 mm, 1°, scanning speed IVniin; сишс cell
parameter 13.7231
In the Smrekovec volcaniclastics laumontite is the most widespread zeolite, deve-
loped as interstitial filling, a vein mineral or replacement of volcanic glass and plagio-
clases. The average laumontite content in altered volcaniclastic rocks rarely exceeds
20 wt.% of the bulk composition. The replacements of volcanic glass and pyrogenetic
plagioclases are more localised in occurrence and related to the proximity of high-le-
vel intrusive bodies. The degree of zeolitisation is also strongly dependent on porosity
and permeability of the host-rock; this relationship is the most obvious in the secti-
ons, composed of interbedded coarse-grained rocks containing abundant zeolites,
and fine-grained tuffs which lack of zeolites, except for fissure fillings.
Laumontite and other zeolites show no obvious zonal arrangement. Away from ex-
tensively altered rocks encountered in close vicinity of high-level intrusive bodies, la-
umontite-cemented volcaniclastics grade into the rocks in which zeolites do not oc-
cur any more, not even as vein minerals. The only occurrence which could indicate
the presence of two possible zones with defined progressive reaction pattern, is rela-
ted to resedimented hyaloclastites spreading from the top of Komen towards the so-
uth-east and north-west. The hyaloclastites are generally altered to heulandite, heu-
landite-clinoptilolite, quartz and montmorillonite. Locally, laumontite, albite, quartz
and interlayered chlorite/montmorillonite are encountered in the same type of rocks;
due to extensive erosion of hyaloclastites it is uncertain whether the two alteration
patterns occur in exactly the same layer. In heulandite-bearing rocks, scarce remains
Zeolites in the Smrekovec volcaniclastic rocks
263
264_Polona Kralj
Table 6. Analcimes: chemical composition, formulae on the basis of 96 oxygens and lattice con-
stants in Â
Tabela 6. Analcimi: kemična sestava, formule na osnovi 96 kisikov ter mrežne konstante v Â
1.	Sample N34 1/4L, separated analcime from volcaniclastic rock form northern slopes of Smre-
kovec. Chemical formula: (Na+K+2Ca)i4 28 Al^e.oe 86 ^96 •
2.	Sample 34/31 2L, separated analcime from volcaniclastic rock form northern slopes of Smre-
kovec. Chemical formula: (Na+K+2Ca)i4 93 Alj^5 4g SÌ32 28 O96 •
3.	Sample 34/60 L, separated analcime from volcaniclastic rock form northern slopes of Smre-
kovec. Chemical formula: (Na+K+2Ca)i5 Alj^5 45 SÌ32 33 Ogg . 17.3 H2O
Chemical analyses were performed in National Chemical Institute (KIBK) in Ljubljana. Cell di-
mensions were determined in University of Belgrade, Faculty for Mining and Geology, Yugoslavia
of laumontite occur being extensively replaced by clinoptilolite-heulandite. This re-
lationship between the two minerals would hardly justify the progressive reaction
pattern and the existence of zonal arrangement of zeolites. It strongly suggests that
Zeolites in the Smrekovec volcaniclastic rocks_265
other mechanisms - diagenesis or halmyrolysis - must have operated after the hydro-
thermal stage of alteration.
Yugawaralite is a vein mineral genetically related to crystallisation from hydro-
thermal fluids. Stilbite is very common hydrothermal zeolite although it can also be
encountered in burial environments as is the case in Taringatura Hills, New Zealand
(Boles & Coombs, 1975; 1977). Analcime in the Smrekovec volcaniclastics is of
hydrothermal origin formed during late-stage emplacement of a high-level intrusive
body - most probably a feeder dyke.
If the zeolite occurrence in the Smrekovec volcaniclastics is compared with the
previously described contact metamorphic alteration, it can be concluded that hig-
her-grade metamorphic zones are missing. Zeolites do not show any obvious zonal
arrangement although an enhanced rock alteration in close vicinity of the outcrops of
high-level intrusive bodies suggests their emplacement must have been instrumental
in the development of laumontite and other zeolites but was probably too small to
produce zonation recognisable on larger scale. On the other hand, laumontite and he-
ulandite locally replace volcanic glass indicating the precipitation from hydrother-
mal fluids could not have been the only mechanism responsible for the zeolite develo-
pment.
Conclusions
The Smrekovec volcaniclastic rocks underwent alteration characterised by the
development of zeolites and related silicate minerals: albite, quartz, chlorite and
interlayered chlorite/smectite. Laumontite is the most widespread in occurrence;
heulandite, heulandite-clinoptilolite and analcime may locally be abundant where-
as stilbite and yugawaralite can be encountered only as vein minerals. Laumontite
developed as replacement of the primary constituents - volcanic glass, pyrogenetic
plagioclases and a fine-grained matrix, and as abundant interstitial filling and a
vein mineral. Heulandite and heulandite-clinoptilolite occur abundantly in resedi-
mented hyaloclastites of acid andesitic to dacitic composition. Herein, they replace
volcanic glass and infill vesicles in glassy hyaloclasts or pumice lapilli. Analcime-
rich rocks are very localised in occurrence. Herein, analcime replaces previously
developed laumontite, and rarely also albitised plagioclases. It formed during the
late-stage emplacement of a dyke into already lithified and alterd volcaniclastic
rocks.
Zeolites developed in the Smrekovec volcaniclastics, their occurrence and associa-
tion with prehnite and pumpellyite indicate their formation to be closely related to
local hydrothermal conditions generated in water-saturated sediments by emplace-
ment of high-level intrusive bodies. This intrusives were obviously too small sources
of heat to produce zonation on kilometre scale as encountered in contact metamor-
phic settings.
Quartz, interlayered chlorite/smectite and albite are widely developed throughout
the Smrekovec volcanic complex, irrespective to finer- or coarser-grained texture of
volcaniclastic rocks, their position or zeolite content. For this reason, they could have
also developed upon shallow burial diagenesis.
266_Polona Kralj
Acknowledgements
I am much obliged to the Institute of Geology, Geotechnics and Geophisics for ha-
ving enabled me to work on this problem. The study was supported by the Ministry of
Science and Technology of the Republic of Slovenia. Many thanks to my mentor Prof.
Dr. Vera Gregorič.
I express my cordial thanks to Prof. Dr. Josip Tišljar from The University of Za-
greb, Croatia, for revision of the manuscript and many helpful comments.
Zeoliti V vulkanoklastičnih kamninah smrekovškega podgorja
Uvod
Piroklastičen material se je potem, ko je bil odložen na zemljini površini, hitro
spreminjal, saj so temperature in tlaki v novem okolju mnogo nižji kot tisti, ki je v
njem nastajala magma. Predvsem vulkansko steklo se zelo hitro hidratizira, še pose-
bno, če je bil piroklastični material odložen na jezerskem ali morskem dnu. Reakci-
jam hidratad j e slede spremembe v mineralni sestavi tako, da začno kristalizirati gli-
neni minerali, opal, odvisno od danih okoliščin pa tudi zeoliti.
Ker sedimentacijsko okolje ni statično, temveč se nenehno spreminja, se tudi vul-
kanski material prilagaja novim kemičnim in fizikalnim razmeram. Pri tem je kineti-
ka reakcij navadno počasnejša od hitrosti sprememb v okolju. Zato lahko - sicer v ne-
katerih danostih tlaka, temperature in kemične sestave delujočih fluidov - nestabilni
minerali zelo dolgo obstanejo kot metastabilne faze. Značilni minerali te vrste so
prav zeoliti, ki lahko obstajajo kot metastabilni celo več milijonov let.
Če se piroklastični sediment, v katerem so se že pričele spremembe, začne pogreza-
ti ali pa je izpostavljen hidrotermalnemu delovanju, visokemu termičnemu ali kemi-
čnemu gradientu, postanejo prej obstojni avtigeni minerali neobstojni. Namesto njih
prično kristalizirati drugi, ki so bolj prilagojeni novim razmeram v okolju. Pri tem
lahko nekatere faze dosežejo v danih okoljih termodinamično ravnotežje, druge pa
ne, vendar kljub temu lahko obstajajo še naprej kot metastabilne faze. V okolju dia-
geneze tonjenja se spremembe tlaka, temperature in sestave pomih raztopin spremi-
njajo sorazmerno počasi. Zato nastajajo in obstajajo določene združbe zeolitov in
drugih avtigenih mineralov v bolj ali manj debelih slojih, imenovanih zone zeolitov
(Coombs et al., 1959; Kossovskaya & Shutov, 1961; lijima & Utada,
1966; Boles & Coombs, 1975; 1977). V okoljih, kjer se pojavlja hidrotermalno
delovanje, pa so nihanja v temperaturi in predvsem v sestavi delujočih fluidov znatna
in so zato zone zeolitov le malokje razvite (lijima, 1984), razen na kontaktih glo-
bočnin in sedimentov v geosinklinalnih območjih (Utada, 1973).
Zeoliti so hidratizirani alumosilikati alkalnih in zemljoalkalnih kovin in so najbolj
razširjeni avtigeni minerali v piroklastičnih sedimentih (Utada, 1987). Nastajajo v
številnih geoloških okoljih in zaradi različnih procesov: s površinskim preperevanjem
vulkanskega pepela alkalne sestave, v slanih-alkalnih jezerih (zaprti hidrološki siste-
mi), v odprtih hidroloških sistemih zaradi pronicanja meteornih vod, s halmirolizo
vulkanskega stekla na dnu oceanov, pri diagenezi tonjenja in v hidrotermalno aktiv-
nih sistemih (lijima, 1984). Združbe zeolitov, ki pri tem nastajajo, so odvisne od
Zeoliti v vulkanoklastičnih kamninah smrekovškega podgorja_267
temperature, tlaka, sestave delujočih raztopin, sestave prikamnine in pogosto tudi
njene poroznosti in permebilnosti.
Geološko okolje smrekovških vulkanoklastitov
Smrekovško podgorje (si. 1) grade zgornjeoligocenske vulkanske kamnine, v kate-
rih se pojavljajo zeoliti in drugi avtigeni minerali. Raztezajo se na površini približno
petnajst kvadratnih kilometrov in predstavljajo osrednji del obsežnejšega vulkanske-
ga pasu, imenovanega tudi smrekovška serija (M i o č, 1978; 1983; M i o č et al.,
1986). Najvišje se vzpno vrhovi Komen (1684), Kmes (1613) in Smrekovec (1577).
Zgomjeoligocenska starost predomin je določena na osnovi foraminifeme favne, vse-
bovane v meljastih sedimentih podlage (Rijavec, 1966).
Vulkanizem je pričel delovati v morskem okolju, kjer je nastal vulkanski masiv z
enim ali več stratovulkanov in izrazitim pozitivnim reliefom. Sestava magme se je za-
radi frakcijske kristalizacije bazaltne taline s časom spreminjala od bazaltne prek
bazaltne andezitne in kisle andezitne do dacitne in tako ustvarila vulkanski diferen-
ciacij ski niz.
Smrekovške vulkanoklastične kamnine obsegajo tri glavne zvrsti - avtoklastične
kamnine s presedimentiranimi hialoklastiti, vulkanoklastične debrite in turbidite ter
lokalno presedimentirane sedimente vulkanoklastičnih tubiditov. Za pojav zeolitiza-
cije so najpomembnejše plitve intruzije magme v vlažne, še nekonsolidirane sedimen-
te, saj so le-te predstavljale glavni izvor toplote, zaradi katere so se pome vode v vla-
žnih sedimentih segrele in povzročile kristalizacijo zeolitov.
Zeoliti v vulkanoklastitih smrekovškega podgorja
Zeolite in druge avtigene minerale smo preiskovali z različnimi metodami: z mikro-
skopijo v presevni polarizirani svetlobi, kjer sem pregledala 86 zbruskov; z rentgensko
difrakcijo 94 vprašenih vzorcev, z elektronsko mikroskopijo, kombinirano z ener-
gij sko-disperzijskim spektrometrom rentgenskih žarkov (SEM-EDX), kjer smo pregle-
dali 30 vzorcev in s kemično analizo (kombinirana mokra analitska metoda, atomska
absorpcijska spektroskopija in emisijska spektroskopija z induktivno sklopljeno pla-
zmo), kjer smo analizirali 4 vzorce kamnine in 3 vzorce separiranega analcima.
Laumontit - Ca^AlgSiiß04^.1ßH20
Med zeoliti v vulkanoklastičnih kamninah smrekovškega podgorja je najbolj raz-
širjen laumontit. Pojavlja se kot žilni mineral (tabla 1, si. 1), zapolnitev votlinic plin-
skih mehurčkov (tabla 2, si. 1, 2) ali kot pomi cement (tabla 2, si. 3). Lahko pa nado-
mešča tudi plagioklaze (tabla 2, si. 4), vulkansko steklo (tabla 3, si. 1, 2) in drobnozr-
nato tufsko osnovo. Kot žilni mineral ali pomi cement se pogosto pojavlja sam, vča-
sih pa ga spremljajo tudi stilbit, yugawaralit, analcim (tabla 3, si. 3, 4) ali prehnit (ta-
bla 1, si. 3, 4). V kamninah je njegova zastopanost običajno skromna, saj le redkokje
presega 20 mas.% celotne kamnine. Najpogostejši minerali, ki v vulkanoklastitih
smrekovškega podgorja spremljajo laumontit, so albit, kremen, klorit in glineni mi-
nerali vrste klorit/montmorillonit. V slednih količinah je pogosto prisoten sfen, pone-
268_Polona Kralj
kod pa se pojavljata tudi prehnit in pumpellyit. Prav pojav prehnita z laumontitom je
zelo neobičajen, saj v vulkanoklastitih smrekovškega podgorja laumontit ne le da je
kristaliziral kasneje kakor prehnit, temveč ga tudi nadomešča (tabla 1, si. 3, 4). V
okolju diageneze tonjenja prehnit nadomešča laumontit. Iz diagrama stabilnosti heu-
landita, laumontita in prehnita (Boles & Coombs, 1977) je mogoče nakazati,
da bi morda iz prehnita retrogradno lahko nastajal laumontit ob povišani aktivnosti
kremenice aH4Si04(aq) zmanjšanem razmerju aktivnosti kalcijevih in vodikovih io-
nov аса2+/(ан+) •
Raziskave z vrstičnim elektronskim mikroskopom, kombiniranim z energijsko-dis-
perzijskim spektrometrom rentgenskih žarkov (SEM-EDX) so pokazale, da vsebujejo
nekatera kristalna zma laumontita poleg kalcija tudi manjše količine kalija (si. 2).
Laumontit lahko nadomešča tudi vulkansko steklo; tedaj nastajata poleg laumon-
tita še albit in kremen. Nadomeščanja vulkanskega stekla po laumontitu, albitu in
kremenu so manj pogosta in so vezana na bližino plitvo ležečih intruzivov, kjer je bila
temperatura dovolj visoka. Spremembe je mogoče opazovati v robnih, avtoklastičnih
delih intruzivov, kjer najdemo avtoklaste andezita z različno stopnjo spremen j enosti
steklaste osnovne mase.
Med heulanditom in klinoptilolitom obstaja cel niz trdnih raztopin (Gottardi
& Galli, 1985). Ime heulandit se najpogosteje nanaša na celotno skupino, sicer pa
oba končna člena niza trdnih raztopin ni mogoče ločiti samo z rentgensko difrak-
cijsko metodo, temveč je treba vpeljati vsaj termični test - po Mumptonu (1960).
Po segrevanju vzorca na 600°C se struktura heulandita poruši, klinoptilolit pa ostane
kot nespremenjena kristalna faza tudi po toplotni obdelavi.
V vzorcih vulkanoklastitov smrekovškega podgorja smo našli heulandit in trdno
raztopino heulandita in klinoptilolita (si. 3a, 3b). Toplotno obdelane vzorce je anali-
ziral M. Mišič iz Inštituta za geologijo, geotehniko in geofiziko v Ljubljani. Heulandit
in trdna raztopina heulandita in klinoptilolita se pojavljata v presedimentiranih hia-
loklastitih, kjer nadomeščata vulkansko steklo in zapolnjujeta prazne prostore v ka-
mnini. Spremljajoča avtigena minerala sta montmorillonit in kristobalit (ponekod
tudi mikrokristalni kremen). Plagioklazi so sorazmerno sveži. Zanimivo je, da se v
istem tipu kamnine pod vrhom Komna, kjer izdanja andezit, pojavljajo kot avtigeni
minerali laumontit, albit, kremen in glineni minerali z zmesno strukturo vrste klo-
rit/montmorillonit. Ti dve združbi avtigenih mineralov bi torej lahko predstavljali
progradni reakcijski niz oziroma dve zoni zeolitov.
Analcim Naiß(AlißSi320g6)-16H20
Poleg manjših količin analcima, ki spremlja zeolitizirane presedimentirane hialo-
klastite, izdanjajo na severnem pobočju Smrekovca tudi kamnine, ki vsebujejo do 60
mas.% analcima (tabla 1, si. 2). Pojav analcima v teh kamninah je zelo nenavaden,
kajti analcim nadomešča predhodno nastali laumontit, tu in tam pa tudi albitizirane
plagioklaze (si. 4; tabla 5, si. 1, 2). Obseg z analcimom bogatih kamnin je prostorsko
zelo omejen. Pojav analcima je vezan na kasnejšo intruzijo kislega andezita - verje-
Zeoliti v vulkanoklastičnih kamninah smrekovškega podgorja_269
tno dovodnega dyka v že spremenjeno kamnino. Glede na nadomeščanja kalcijskega
zeolita laumontita z natrijskim zeolitom analcimom so morale biti delujoče raztopine
bogate z natrijem. Eksperimentalno delo na vzorcih s smrekovškega podgorja, ki smo
ga opravili s sodelavci Tehniške visoke šole v Gradcu (Barth-Wirsching, osebna
komunikacija), je te domneve potrdilo. Analcim je nastajal iz laumontita v odprtem
in zaprtem sistemu pri temperaturah, višjih od 150°C.
Kemična sestava preiskanih vzorcev analcima (tabela 6) je pokazala, da pripadajo
nizkosilicijskemu kalcijskemu tipu kubične strukture. Tako je izključena možnost, da
bi analcim predstavljal trdno raztopino analcima in wairakita.
Stilbit NaCa4(AlgSÌ270-j2)-30H20 in yugawaralit Ca2Al2Sii2022-8H20
Yugawaralit in stilbit sta značina hidrotermalna zeolita, četudi se stilbit lahko po-
javlja tudi v okolju diageneze tonjenja (Boles & Coombs, 1975). Nastanek stil-
bita je navadno vezan na temperature, ki so nižje kakor za laumontit (lijima,
1984; L i o u, 1971 a). Yugawaralite pa nastaja pri temperaturah, ki so višje kakor za
laumontit; glede na wairakit kristalizira pri nižjih tlakih (L i o u, 1971 b).
V vulkanoklastitih smrekovškega podgorja dobimo stilbit in yugawaralit^navadno
v žilicah skupaj z laumontitom (si. 5a, 5b). Blizu izdankov z analcimom bogatih ka-
mnin so tudi žilice, kjer najdemo ob prikamnini yugawaralit z laumontitom nad njim
še analcim in nato laumontit. Zaporedje mineralov v teh žilicah kaže na to, da so bili
procesi zeolitizacije zelo zapleteni.
Nastanek zeolitov v vulkanoklastitih smrekovškega podgorja
Pojav zeolitov v vulkanoklastitih smrekovškega podgorja kaže, da je njihov izvor
predvsem hidrotermalen. Hidrotermalne razmere so ustvarile intruzije andezitne ma-
gme v še nekonsolidirane, z vodo prepojene sedimente. Najmočneje so kamnine spre-
menjene prav v bližini takšnih intruzivnih teles, kjer laumontit, skupaj z albitom in
kremenom, nadomešča tudi vulkansko steklo in drobnozmato tufsko osnovo. Ti in-
truzivi so nastali v sklopu vulkanskega delovanja, s katerim je nastal kompleks stra-
tovulkana pa so bili ali premajhni ali tudi preplitvo ležeči, da bi lahko povzročili
kontaktno metamorfne spremembe večjih, kilometrskih razsežnosti.
Kremen, albit in glineni minerali z zmesno strukturo vrste klorit/montmorillonit
so močno razširjeni v vseh kamninah smrekovškega vulkanskega kompleksa ne glede
na njihovo bolj ali manj debelozmato strukturo, lego ali vsebnost zeolitov. Zato je
verjetno, da so vsaj deloma nastali med zgodnjo diagenezo tonjenja.
Zahvala
Zahvaljujem se Inštitutu za geologijo, geotehniko in geofiziko, ki mi je omogočil
delo na tej problematiki. Raziskave je financiralo Ministrstvo Reublike Slovenije za
znanost in tehnologijo. Prof. dr. Veri Gregorič se iskreno zahvaljujem za mentorstvo
pri magistrskem delu.
Prisrčna zahvala velja prof. dr Josipu Tišljarju iz Univerze v Zagrebu za recenzijo
rokopisa in številne koristne nasvete.
270_Polona Kralj
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272_Polona Kralj
Plate 1 - Tabla 1
Fig. 1. A laumontite veinlet system in andesite from Vranji Vrh
SI. 1. Sistem žilic laumontita v andezitu z Vranjega Vrha
Fig. 2. Analcime (A) replacing laumontite and a plagioclase feldspar grain (F). Crossed niçois,
magnification 66 x
Si. 2. Analcim (A), ki nadomešča laumontit in zrno plagioklaza (F). Pogled med navzkrižnimi
nikoli, povečano 66 x
Fig. 3. Laumontite (L) and prehnite (P) as cementing minerals in a coarse-grained volcaniclastic
rock. Plane polarised light, magnification 53 x
SI. 3. Laumontit (L) in prehnit (P) kot cement v debelozrnati vulkanoklastični kamenini. Pre-
sevna polarizirana svetloba, povečano 53 x
Fig. 4. The same as in the previous photo, under crossed niçois
SI. 4. Enako kot na prejšnji sliki, med navzkrižnimi nikoli
Zeolites in the Smrekovec volcaniclastic rocks
273
274_Polona Kralj
Plate 2 - Tabla 2
Fig. 1. Laumontite infilling a vesicle (larger one) in a lithic fragment. Plane polarised light, ma-
gnification 66 X
SI. 1. Laumontit, ki zapolnjuje votlinico plinskega mehurčka v litičnem drobcu. Presevna pola-
rizirana svetloba, povečano 66 x
Fig. 2. The same as in the previous photo, under crossed niçois
SI. 2. Enako kot na prejšnji sliki, med navzkrižnimi nikoli
Fig. 3. Laumontite (L) and prehnite (P) as interstitial filling. Crossed niçois, magnification 66 x
SI. 3. Laumontit (L) in prehnit (P) kot zapolnitev medzmskega prostora. Pogled med navzkri-
žnimi nikoli, povečano 66 X
Fig. 4. Laumontite (L) and albite (F) replacing a plagioclase feldspar grain. Crossed niçois, ma-
gnification 53 X
SI. 4. Laumontit (L) in albit (F), ki nadomeščata zmo plagioklaza. Pogled med navzkrižnimi ni-
koli, povečano 53 X
Zeolites in the Smrekovec volcaniclastic rocks
275
276_Polona Kralj
Plate 3 - Tabla 3
Fig. 1. Laumontite (L) replacing volcanic glass in a lithic fragment with perlitic texture. Plane
polarised light, magnification 53 X
SI. 1. Laumontit (L), ki nadomešča vulkansko steklo v litičnem drobcu s perlitsko strukturo.
Presevna polarizirana svetloba, povečano 53 x
Fig. 2. The same as in the previous photo, under crossed niçois
SI. 2. Enako kot na prejšnji sliki, med navzkrižnimi nikoli
Fig. 3. Laumontit from a veinlet. Plane polarised light, magnification 66 x
Fig. 3. Laumontit v žilici. Presevna polarizirana svetloba, povečano 66 X
Fig. 4. The same as in the previous photo, under crossed niçois. Analcime (A) replaces laumontite
SI. 4. Enako kot na prejšnji sliki, med navzkrižnimi nikoli. Analcim (A) nadomešča laumontit
Zeolites in the Smrekovec volcaniclastic rocks
277
278_Polona Kralj
Plate 4 - Tabla 4
Fig. 1. Heulandite (H) infilling vesicle in a pumice lapillus and replacing volcanic glass. Dark-
coloured spherical aggregates are composed of montmorillonite. Plane polarised light, magnifi-
cation 53 X
SI. 1. Heulandit (H), ki zapolnjuje votlinice plinskih mehurčkov in nadomešča vulkansko steklo.
Temni kroglasti skupki sestoje iz montmorillonita. Presevna polarizirana svetloba, povečano 53 X
Fig. 2. Heulandite (H) replacing laumontite (L) in a glassy fragment. Crossed niçois, magnifica-
tion 85 X
SI. 2. Heulandit (H), ki nadomešča laumontit (L) v steklastem drobcu. Pogled med navzkrižnimi
nikoli, povečano 85 x
Fig. 3. Heulandite (H) replacing a fine-grained matrix. Plane polarised light, magnification 66 x
SI. 3. Heulandit (H), ki nadomešča drobnozmato osnovo. Presevna polarizirana svetloba, pove-
čano 66 X
Fig. 4. The same as in the previous photo, under crossed niçois
SI. 4. Enako kot na prejšnji sliki, med navzkrižnimi nikoli
Zeolites in the Smrekovec volcaniclastic rocks
279
280_Polona Kralj
Plate 5 - Tabla 5
Fig. 1. A plagioclase grain replaced by analcime (A), prehnite (Pr) and pumpellyite (Pu). Plane
polarised light, magnification 53 x
SI. 1. Zrno plagioklaza, ki ga nadomeščajo analcim (A), prehnit (Pr) in pumpellyit (Pu), poveča-
no 53 X
Fig. 2. The same as in the previous photo, under crossed niçois
SI. 2. Enako kot na prejšnji sliki, med navzkrižnimi nikoli
Fig. 3. Alkali feldspars (Kf) and analcime (A) replacing laumontite. Plane polarised light, ma-
gnification 66 X
SI. 3. Alkalni glinenec (Kf) in analcim (A), ki nadomeščata laumontit. Presevna polarizirana
svetloba, povečano 66 X
Fig. 4. Needles of pumpellyite as a vesicle filling. Plane polarised light, magnification 66 x
SI. 4. Igličast pumpellyit, ki zapolnjuje votlinice plinskih mehurčkov. Presevna polarizirana sve-
tloba, povečano 66 X
Zeolites in the Smrekovec volcaniclastic rocks
281