GEOLOGIJA 36, 211-222 (1993), Ljubljana 1994 UDK 548.4:549.5 = 20 Pyroelectrically caused twisting of quartz crystals Mirjan Žorž LEK Ljubljana, Verovškova 57, 61107 Ljubljana, Slovenia Abstract The twisting of quartz crystals is conditioned by Dauphiné twinning. Each twisted crystal shows morphological characteristics confirming this type of quartz twinning. A linear mathematical correlation based on morphological parameters mea- sured on quartz crystals elongated and twisted along their polar [21.0] a-axis has been deduced. It was demonstrated that the twisting angle is the function of the twisting constant and some of the crystal dimensions. This constant has the same v^ue —4° as the declination of the [21.0] edge of the crystal twisted around the [21.0] a-axis which also corresponds to the surface distribution of the positive charge observed with Dauphiné-twinned quartz crystal on cooling. A similar quartz feature is the twisting of a quartz crystal around the c-axis. A proposed theory describing the reasons for crystal twisting, development of different crystal forms and types of twisted quartz crystals is thus based on the pyroelectrically accelerated growth from slowly cooling and slightly supersaturated quartz-bear- ing solutions. Introduction Quartz crystals elongated and twisted along the [2T.0] a-axis (der Gwindel in German) are typical features of Alpine-type veins that are always associated with quartz crystals of the Friedlaender type. They are considerably rare crystal forms mostly found in the Swiss Alps and are not so frequent in the French, Italian and Austrian Alps. Another regions where they have also been found is the Polar Ural (Dodo and Puiva) in CIS and Corinto, Minas Geraes, Brazil. Two new locations yielding twisted quartz crystals have been discovered in recent years in Bosnia and Hercegovina (Busovača) and in Macedonia (Berovo). Over 300 twisted crystals from all the aforementioned locations are discussed in this study. Their morphological parameters were measured (Figs, la and Id) and some typical features, i. e. morphological hand, colour and form were noted as well. The predo- minant colour is smoky. Colourless crystals are considerably rare. Of all the crystals studied, 83.2% were from Switzerland, 8.7% from CIS, 3.6% from Macedonia, 3.0% from France, 1.2 % from Austria and 0.3 % from Bosnia and Hercegovina. Of all the mentioned crystals, 51.3% were left-handed. 212 Mirjan Žorž Fig. 1. Ra-quartz crystal viewed down its polar [21.0] a-axis with its morphological parameters (a), La-quartz (b) and schematically drawn prism faces of Ra-quartz (c) and La-quartz (d) Pyroelectrically caused twisting of quartz crystals 213 Quartz crystals twisted around c-axis are also typical features of Alpine-type veins and are again associated with quartz crystals of the Friedlaender type. This twisting is not as distinctive as it is in the case of gwindels. Observations and measurements on twisted crystals Since a particular crystal is attached to a matrix from which it grows and since its termination twists - if the crystal is viewed down its [2T.0] a-axis - to the left with left-handed, and to the right with right-handed crystals (Figs, la and lò), the denotations La-quartz or Lc-quartz for left-handed and left-twisted crystals and Ra- quartz or Rc-quartz for right-handed and right-twisted quartz crystals will be used further in the text. Measurements have shown that there is a correlation between twisting angle фа, prism mh height h and prism md diameter d (Figs, la and Id). The equation is: where k is the twisting constant. The linear regression line was calculated from the measured morphological parameters. The final expression is: with a correlation coefficient 0.980. The uncertainty of the /c-value derived from the possible errors of the morphological measurements is about 23%. It can be seen from equation (1) that the twisting angle фа is larger with thinner and higher La- and Ra-quartz. Fig. 2 a shows the dependence of the twisting angle фа from h/d quotient. The limit: shows that twisted crystals whose prism шн height h approaches md diameter d would show only a declination of the [2T.0] edge. The declination angle Qa of this edge between md prisms away from the direction of the c-axis is ~4° and can be observed with La- and Ra-quartz crystals and especially with crystals where the trapezohedron Xh faces are not developed (Figs, la - Id). This edge is declined to the left with La- and to the right with Ra-quartz. The [2Î.0] edge however does not show this declination (Figs. Ic and Id). Discussion Frondel (1978) calculated the twisting period (180° turn) from angle б (Fig. la) of a particular crystal and obtained values between 20 and 600 cm, and twisting degrees between 0.05 and 0.85 °mm-i. Rykart (1989) measured angle б, obtaining twisting degrees between 0.02 and 0.5 °mm-i. It can be seen from equation (1) and 214 Mirjan Žorž Fig. 2a. Data and evaluated linear regression line of measured La- and Ra-quartz crystals from Fig. 2b that twisting degree фа/h is the function of the prism md diameter d. The twisting degree decreases with increasing d. Diameters under 3 and over 25 mm were not observed with the crystals measured in this study. The distribution of d is shown in Fig. 2b. A complete 180° turn would be achieved with a crystal whose height h would be approximately 45 times larger than its diameter d (2). This is not very likely to occur. As the crystal grows it becomes both higher and thicker, which causes a simultaneous decrease in twisting angle фа. That is why the twisting period is not a suitable parameter for the description of the crystal twisting rate. More illustrative is the twisting degree quotient фа/h. La- and Ra-quartz from all the mentioned locations have given the same values for the twisting constant k. This means that twisting is controlled by a mechanism that cannot be ascribed to structural dislocations and temperature only. It is most probable that the pyroelectrical phaenomenon also contributes to the formation of twisted quartz crystals. Linck (1923) and Lang (1974) described quartz's pyroelec- trical properties and showed some illustrations based on Kundt's dust method for determining the charge distribution on a crystal by use of red lead oxide and sulphur. In pictures shown it can be seen that the charge distribution on a Dauphiné-twinned quartz-crystal is such that the lines of charge are declined by ~4° away from the prism edges and the c-axis direction. Frondel (1978) showed that the edge [2T.0] of La- and Ra-quartz crystals acquires a positive charge on cooling. Observations on the Pyroelectrically caused twisting of quartz crystals 215 Fig. 2b. Twisting degree фд/ћ of the La- and Ra-quartz in the dependence of the nid prism diameter d. Standard deviations are given for points of at least five measured crystals. Solid line represents theoretical twisting degree calculated with fc = 3.98°. Distribution of measured m^ prism diame- ters is shown on the abscissa. In this case each 0.2 digit represents twenty crystals La- and Ra-quartz crystals have shown that all of them show morphological charac- teristics typical of Dauphiné twinning, i.e. trapezohedrons or bipyramids in twinning positions and/or an etching pattern on crystal faces confirming this twinning. It can be concluded that the growth of a quartz crystal in the [2T.0] a-axis direction in the Alpine-type vein milieu under the conditions of only slightly supersaturated, slowly cooling solutions is accelerated by this effect. The termination of growing La- or Ra- quartz serves as a positive anode attracting [8104]^- ions from the vein solution. Since the leading, positively charged [2T.0] edge is declined by —4° at any time, a crystal grows most quickly in the direction of polar [2T.0] a-axis, simultaneously turning around it with constant degree. The crystallization rate is thus higher in comparison 216 Mirjan Žorž Pyroelectrically caused twisting of quartz crystals 217 218 Mirjan Žorž Fig. 3. Ra-quartz (18 x 20 mm) of young closed type with e!v ratio of 0.7 from Piz Gendusas in Switzerland (a), slightly opened La-quartz (30 x 32 mm) with e/u ratio of 0.6 from unknown Swiss locality (b), open La-quartz (47 X 47mm) with elv ratio of 0.2 from Piz Gendusas (c), closing Ra-quartz (88 X 78mm) with вЊ ratio of 0.1 from Mont Blanc in France (d) and old closed Ra-quartz (59 x 34mm) with elv ratio of 0.4 from Berovo in Macedonia (e) to one controlled only by diffusion of [SÌ04]''- ions from the solution towards the crystal surfaces under the same conditions. A tabular crystal elongated along the [2T.0] a-axis with an extremely developed mh faces is formed (Figs. Ic, Id and 3a-3e). A prerequisite is the orientation of the seeding Dauphiné-twinned quartz crystal whose [2T.0] edge must be attached in parallel and whose [2T.0] a-axis must be perpendicular to the matrix. The higher growth rate in the [2T.0] a-axis direction (Va) explains the unusually well-developed trapezohedrons Xh and rarely well developed, Dauphiné twinning showing bipyramid Sd faces (Figs, la and lb). The reason is the rate of trapezohedron Xh growth which, in this case, is lower in comparison with that of md, Sd and Xd. The Sd faces are frequently present and are, in fact, as narrow as an edge. The declination of the [2T.0] edge is visually pronounced in the presence of the Sd faces (Figs, la and Ic). Trapezohedron Xd faces with La- and Ra-quartz are only exceptionally developed. The growth of a particular La- or Ra-quartz crystal can be divided into several phases. In the first phase the crystal grows from the matrix along its [2T.0] a-axis and Pyroelectrically caused twisting of quartz crystals 219 is twisted in a particular direction. The crystal growth rate along [2T.0] a-axis (Vg) is higher in comparison with growth rate along its c-axis (Vc), i.e. Va» ^с. Crystals in this phase are tabular with well-developed Xh faces. Neither reentrant angles nor typical sutures on twisted and smoothly developed faces occur. These crystals tend to be relatively small. This is a young closed type. The quotient of the [2T.0] edge length e and crystal length v in the c-axis direction - e/v ratio - is up to 0.9 (Figs, la and 3a). In the next growth ph'ase (Va> Vc) reentrant angles and some sutures appear on the crystal, otherwise the crystal faces are smooth. The e/v ratio decreases to lower values (Fig. 3Ò). Further growth (Va~Vc) causes deepening of the reentrant angles and sutures, making it look as if the crystal were resolved to many »subindividuals«. The e/v ratio decreases to 0.1. This is an open type (Fig. 3c). During the next phase (Va