ACTA CARSOLOGICA	33/1	17	257-264	LJUBLJANA 2004
COBISS:1.01
THE CRYSTALLINE PHASE OF THE CARBONATE MOON-MILK: A TERMINOLOGY APPROACH
KRISTALINSKA FAZA KARBONATNEGA JAMSKEGA MLEKA: TERMINOLOŠKI PRISTOP
MIRONA CHIRIENCO1
1 Department of Mineralogy, "Babes-Bolayi" University, Kogalniceanu 1, 3400 Cluj, Romania E-mail: mchirienco@bioge.ubbcluj.ro
Abstract	UDC: 551.442.4:001.4
Mirona Chirienco: The crystalline phase of the carbonate moonmilk: a terminology approach
The crystals forming the solid phase of moonmilk deposits can grow from various solutions due to inorganic or/and organic processes. The terminology used so far to define the crystalline phase of the moonmilk is vast. However, if one applies for the widely accepted classification and terminology used in sedimentary petrology, this wide range of terms describing particular morphologies of carbonates may be narrowed to the following three categories: fiber crystals, polycrystals, and calcified filaments, and applied whenever the size and shapes of calcite crystals in moonmilk and their mutual interrelationships come into discussion. Key words: moonmilk, calcite crystals, terminology.
Izvleček	UDC: 551.442.4:001.4
Mirona Chirienco: Kristalinska faza karbonatnega jamskega mleka: terminološki pristop
Izvleček: Kristali, ki gradijo trdno fazo jamskega mleka, lahko zrastejo iz različnih raztopin kot posledica anorganskih in/ali organskih procesov. Terminologija, ki se uporablja za definiranje keristalinske faze jamskega mleka je obsežna. Kakorkoli, če se kdo posveti široko sprejeti klasifikaciji in terminologiji uporabljeni v sedimentni petrologiji, se ta široki obseg izrazov, ki opisujejo določeno obliko karbonatov lahko zoži na naslednje tri kategorije: vlaknati kristali, polikristali in kalcitizirana vlakna; uporabljeni izrazi, glede velikosti in oblike kalcitnih kristalov v jamskem mleku ter njihov medsebojni odnos, pa postanejo v tem primeru vprašljivi. Ključne besede: jamsko mleko, kalcitni kristali, terminologija.
INTRODUCTION
The study of moonmilk has been the subject of numerous papers. Several papers have examined the chemical composition (Pobeguin & Geze 1961; Fischer 1987; Hill & Forti 1997), the physical properties (Bernasconi 1973), and the crystallographical aspects (Bernasconi 1975; Onac & Ghergari 1993; Borsato 1995; Richter & Niggemann 1995). Frisia et al. (2000) examined the paleoclimatic significance of moonmilk. Another set of papers was dedicated to the role that microorganisms played in the formation of moonmilk (Caumartin & Renault 1958; Williams 1959; Gradzinski et al. 1997; Northup et al. 1997, 2000).
Moonmilk has a special position among the secondary carbonates precipitated in the cave environment. There was much confusion when the term "moonmilk" was used for defining a speleothem type. In fact, it is a physical texture (Pobeguin & Geze 1961) or a depositional state (Onac 1996), a term used for a large variety of speleothems. About 95% of moonmilk deposits are carbonatic, while the other 5% are represented by sulphates, phosphates, and silicates precipitated in unspecific cave environments. According to Hill and Forti (1997), moonmilk has 79 synonyms in different countries worldwide. This paper will solely concentrate on classifying the crystalline phase composing the carbonatic deposits of moonmilk with the purpose of creating a united terminology.
SIZE AND MORPHOLOGY OF CRYSTALLINE PHASE IN MOONMILK
The microscopic size of the crystals represents an inherent characteristic of moonmilk (Hill & Forti 1997). Seen with the naked eye, moonmilk seems to be just a white or coloured paste. Only the scanning electron microscope (SEM) reveals the different morphologies of the crystals (Fig. 1). From a literature review (Table 1) and from our own observations, it became obvious that the terminology for the crystal morphology of moonmilk does not rigorously follow any of the sedimentary petrology classifications. As a result, for identical crystal shapes different terms are in use. Early papers dealing with the crystallography of the moonmilk deposits were based on observations made through optical and low resolution electronic microscopes. The last two decades, however, have seen significant improvement in the analytical facilities. These analytical improvements may be one of the reasons for the inconsistent terminology.
The vast terminology used to define the crystalline phase of the moonmilk (Table 1) can be simplified if one applies the widely accepted classification used in sedimentary petrology. According to the Folk (1965), Jones & Kahle (1993), and Verrecchia & Verrecchia (1994) classifications for calcite crystals, the terminology of moonmilk's crystalline phase could be the following: • Fibre crystals (s.l.) have a length/width ratio >6:1 (Folk 1965) and are the most common in moonmilk deposits. These crystals have a constant thickness and blunt ends (Jones & Kahle 1993). Within this group, the researchers distinguished two subcategories: nanofibres and microfibres. Nanofibers are pristine crystals, < 1 ^m in width and >1 ^m in length (Borsato et al. 2000) that commonly form mats (Fig. 1n & Fig. 2). The microfibers are 0.5-2 ^m in width and > 10 ^m in length (Fig. 1m & Fig. 3). Two or more parallel-aligned microfibers cemented together are referred to as composite fibres (Fig. 1c & Fig. 4) (Jones & Kahle 1993). Both types of fibre crystals form due to inorganic precipitation. The precipitation is driven by either progressive evaporation or deposition from supersaturated solutions when the relative humidity of the cave atmosphere is 100%. As a result, these crystals are documented to be al-
Fig. 1: Different morphologies of crystals forming moonmilk; n: nanofibres; m: microfibres; c: composite fibres; cf: calcified filaments.
Fig. 2: SEM microphotographs of calcite nanofibres.
most defect-free and to have a unidirectional growth that starts from a single screw dislocation (Givargivoz 1978).
•	Polycrystals as defined by Verrecchia & Verrecchia (1994) and Borsato et al. (2000) are similar to what Jones and Kahle (1993) called rhomb chains. Such crystals were also identified in moonmilk collected from Humpleu Cave (Chirienco 2002). The crystals are 100-500 ^m in width and > 50 ^m in length (Fig. 5).
Different crystal growth mechanisms (e.g., screw dislocation, vapour-liquid-solid) acting alone or in combination are responsible for precipitation of fibres and polycrystals.
•	Calcified filaments. In at least four caves in Romania, Onac (1996) and Chirienco (2002) described moonmilk deposits built up from twisted and anastomosed calcified filaments (Fig. 1cf). Borsato et al. (2000) and Northup et al. (2000) have reported similar morphologies from an Italian cave and from Spider Cave (New Mexico, U.S.A.), respectively. Unlike the Italian cave, in Romanian and U.S. caves such moonmilk deposits were located far from the entrance, within specific cave topoclimatic conditions (e.g. for Humpleu Cave, temperature above 8°C and no ventilation). Filament length varies from 20 to - 100 ^m. The diameter, however, never exceeds 5 ^m.
These calcified filaments were interpreted as being biologically mediated deposits. The microorganism-induced precipitation, however, is hard to be completely detached from products of physicochemical precipitation.
Fig. 3: SEM microphotographs of calcite microfibres.
Fig. 4: SEM microphotographs of composite fibres; the cemented microfibres are marked with
1 and 2 (detail of Fig. 1c).
Fig. 5: SEM image of moonmilk composed of calcite rhomb chains (arrows point the rhombohedra tips).
Table 1: Types of crystals forming the solid phase of moonmilk.
CONCLUSIONS
The crystals forming the solid phase of moonmilk deposits can grow from various solutions with variable chemical composition and saturation levels. The origin of these carbonates has been attributed to inorganic and organic processes (Hill & Forti 1997 and references cited therein). From a wide range of terms describing particular morphologies of carbonates, we propose the following three categories: fibre crystals, polycrystals, and calcified filaments (as described and shown above) to be used whenever the size and shapes of crystals in moonmilk and their mutual interrelationships come into discussion. Although the variety of crystal morphologies composing moonmilk is large, macroscopic and microscopic investigations, as well as comparison with SEM photomicrographs of well-defined crystals morphology will enable one to frame the crystals into one of these categories. Such an approach will allow for a more thorough understanding of the various growth mechanisms controlling crystal habit in moonmilk deposited under different cave settings. Moreover, knowing the cave atmosphere parameters, solution chemistry, and flow velocity in connection with the crystal morphology, one can reconstruct the environment of moonmilk formation and use the whole set of data to inferred potential indications of climate change.
ACKNOWLEDGEMENTS
The author would like to thank Joe Kearns for critically reading and smoothing this text.
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