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The occurrence of sharp clear crystals of primary alpha cristobalite

Last Updated: 21st Oct 2019

By Robert Housley

The occurrence of sharp clear crystals of primary alpha cristobalite in the Santa Monica Mountains of Southern California
ROBERT M. HOUSLEY, MAREK CHORAZEWICZ, AND GEORGE R. ROSSMAN


To our knowledge this is the first report directly recognizing the occurrence of well formed, clear, unfractured crystals of alpha cristobalite deposited directly as such in vugs in volcanic rocks by low temperature hydrothermal solutions. As such it will largely simply describe some of examples that we have recently found and examined in Southern California. All examples occur in vugs less than 1 cm in diameter, while adjacent vugs contain normal low temperature minerals such as zeolites, calcite, or other forms of silica such as quartz, opal, and chalcedony. The first order question at this time seems to be whether Southern California volcanic rocks are unique in this well crystallized low temperatue cristobalite, or whether similar occurrences have been overlooked, or at least underdescribed, worldwide. Comments in this regard will be appreciated.

Introduction.

Cristobalite was collected and first suggested as a separate mineral species by vom Rath in 1884. Subsequently its physical and structural properties, its thermodynamic relationship to other silica polymorphs, and its modes of formation and occurrence have all been studied extensively and the state of knowledge as of about 1960 is summarized in Volume 3 of the Seventh edition of Dana’s System of Mineralogy (Frondel 1962). Cristobalite is only a stable phase above 1470 C at atmospheric pressure and then as the cubic beta form. Hence it can almost never form naturally as a stable phase.
However, despite being unstable, fine grained cristobalite is fairly abundant in nature (Smith 1998). It is thought to form metastably principally via one of two path ways. It can form during the solidification of volcanic rocks or during the devitrification or weathering of siliceous volcanic rocks. It can also form during the diagenesis of opal containing sediments such as diatomaceous earth. It is found in some soils.
Well formed crystals of cristobalite are less common and are generally thought to only form by vapor deposition in siliceous volcanic rocks while they are still at high temperature. These crystals necessarily form in the high temperature cubic beta cristobalite structure. As they cool somewhere in the range from 270 C to 200 C they transform to low temperature tetragonal alpha cristobalite. The large strain associated with this transition causes them to invariably develop a fine scale network of internal cracks (Schmahl 1993). The network of cracks in turn causes the crystals to appear cloudy white instead of clear. This can be seen in the many pictures on Mindat and the Quartz Page (Akhaven 2014).
In January of 1996 one of us (RMH) while doing an SEM study of material collected by the late Dave Yeomans in 1978 and 1979 during a construction project on the NW corner of Reyes Adobe Road and Canwood Street in Los Angeles County encountered 10 um well formed cristobalite octahedra with mordenite and clinoptilolite. These cristobalite crystals formed hollow spheres often suspended on the mordenite inside cavities lined with clinoptilolite. Thus the cristobalite clearly formed from a hydrothermal solution at relatively low temperature. Typical examples of these hollow spheres made up of cristobalite octahedra are shown in images 1 and 2.
09672210015716137292876.jpg
SEM picture of hollow cristobalite ball on clinoptilolite with mordenite.
02579170015716137304598.jpg
SEM image of cristobalite crystals on surface of hollow ball, with mordenite fiber.
09672210015716137292876.jpg
SEM picture of hollow cristobalite ball on clinoptilolite with mordenite.
02579170015716137304598.jpg
SEM image of cristobalite crystals on surface of hollow ball, with mordenite fiber.
09672210015716137292876.jpg
SEM picture of hollow cristobalite ball on clinoptilolite with mordenite.
02579170015716137304598.jpg
SEM image of cristobalite crystals on surface of hollow ball, with mordenite fiber.


A literature search at the time turned up only one report (Van Valkenburg and Buie 1945) describing cristobalite from Ellora Caves in India that had also clearly formed from a low temperature hydrothermal solution. In this case individual crystals or small clusters of cristobalite and/or quartz up to about 500 um in size (From their text it is clear that the scale bar in their figures should read mm rather than cm.) were found suspended on mordenite needles and associated with calcite. From their growth on mordenite the authors inferred that the cristobalite had crystallized below about 300-400 C. However, largely from the apparent spinel law twinning frequently observed the authors also concluded that the crystals had initially grown as the high temperature beta cristobalite polymorph. Optical studies of a number of untwined crystals showed that the c-axis of the tetrahedral alpha cristobalite always lay along one of the apparent 111 axes of the octahedra. A companion paper (Wolfe 1945) reported goniometer measurements of the interfacial angles in untwined crystals. Although they were of poor quality because the crystals faces all showed the effects of etching they again were consistent with octahedral symmetry.
These crystals from Ellora Caves have subsequently been used in all reported studies that required relatively large crystals of alpha cristobalite, and hence appear to have been regarded as unique. The structure of alpha cristobalite was determined using them (Dollase 1965). The nature of the alpha beta transition was studied using temperature dependent single crystal x-ray diffraction (Peacor 1973). This work showed that the transition temperature on cooling was 220 C and that the initial alpha single crystals became a mosaic of differently oriented domains on cooling back through the transition. This proved conclusively that the crystals actually initially grew below 220 C and in the alpha phase. More detailed studies of the alpha beta transition (Schmahl 1993), and of the elastic properties and Poisson’s ratio (Yeganeh-Haeri, Weidner et al. 1992) of alpha cristobalite have also used crystals from Ellora Caves.

Field Observations.

In December of 1999 RMH discovered similarly suspended although much smaller 10 um crystals of cristobalite on mordenite in Tick Canyon, Los Angeles County CA. Also in 1999 RMH found clear 100 um sized cristobalite crystals associated clinoptilolite and calcite in a single large clast from a volcanic breccia at the Saratoga Hills construction site along Kanan Road in Los Angeles Co. CA. From the absence of cracks it was apparent that these larger cristobalite crystals had grown directly as low temperature alpha cristobalite and hence at temperatures below about 200 C.
In 2015 RMH and MC again found cristobalite, this time in Lobo Canyon, Los Angeles County CA that must have crystallized directly in the alpha form. An SEM picture is shown in Figure 3.
00580470015716126332876.jpg
SEM image of cristobalite from Lobo Canyon.

After this we decided to start systematically looking for low temperature cristobalite crystals whenever we were out in the local volcanic hills and we soon found several additional occurrences. An example from Ladyface Mountain, Los Angeles County CA is shown in Figure 4.
02054240015716126334598.jpg
SEM image of cristobalite from Ladyface Mountain.

Implications.

Before considering the possible implications of these finding it seems to us that the first question that must be answered is whether these Southern California volcanics are unique in some way, or whether similar occurrences are widespread, but have remained overlooked, or at least unreported. In the hope of stimulating other workers to help answer this question we have prepared this basic description of some of the occurrences we have found so far. All are in small vugs or vesicles and many occur in association with other forms of silica such as quartz crystals, chalcedony, agate, and opal.


Akhaven, A. C. (2014). from http://www.quartzpage.de/gen_mod.html.
Anonymous. (2017). from https://www.mindat.org/min-1155.html.
Dollase, W. A. (1965). "Reinvestigation of the structure of low cristobalite." Zeitschr. Kristallographie; with German abs. 121(5): 369-377.
Frondel, C. (1962). Danas System of Mineralogy. New York, John Wiley and Sons,Inc.
Peacor, D. R. (1973). "High-temperature single-crystal study of the cristobalite inversion." Zeitschrift fuer Kristallographie 138: 274-298.
Schmahl, W. W. (1993). "Athermal transformation behaviour and thermal hysteresis at the SiO (sub 2) -alpha /beta -cristobalite phase transition." European Journal of Mineralogy 5(2): 377-380.
Smith, D. K. (1998). "Opal, cristobalite, and tridymite: Noncrystallinity versus crystallinity, nomenclature of the silica minerals and bibliography." Powder Diffraction 13(1): 2-19.
Van Valkenburg, A. and B. F. Buie (1945). "OCTAHEDRAL CRISTOBALITE WITH QUARTZ PARAMORPHS FROM ELLORA CAVES, HYDERABAD-STATE, INDIA." American Mineralogist 30(7-8): 526-535.
Wolfe, C. W. (1945). "Crystallography of cristobalite from Ellora caves, India." American Mineralogist 30(7-8): 536-537.
Yeganeh-Haeri, A., D. J. Weidner and J. B. Parise (1992). "Elasticity of alpha -cristobalite; a silicon dioxide with a negative Poisson's ratio." Science 257(5070): 650-652.






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