Eleventh Annual V. M. Goldschmidt Conference (2001) 3430.pdf

A NEW MODEL FOR THE FORMATION OF TIGER'S-EYE. P.J. Heaney1, D.M. Fisher1`, and S. Yang1, 1The Pennsylvania State University (309 Deike, Dept. of Geosciences, University Park, PA 16802, [email protected], [email protected], and [email protected])

Introduction: Tiger's-eye is a striped golden- quartz grain textures but appear to represent random brown gemstone with chatoyant reflections that vary infiltration of oxidizing fluids. Pseudomorphism of in attitude from band to band within a single specimen. hematite after crocidolite is commonplace in the Mined principally from quartzose schists in regions of brown tiger's-eye, and some replaced fibers Griqualand-West, South Africa, tiger's-eye commonly are hollow with hematite crusts when viewed in cross- is cut as cabochons or carved as small ornamental section. figures. For more than a century, scientists have Conclusion: The textures in tiger's-eye are most attributed the cat's-eye effect to quartz elegantly explained by a crack-seal mechanism for pseudomorphism after the fibrous amphibole quartz vein evolution [3-5]. Cracking of a crocidolitic crocidolite [1]. Frondel [2] reports that "partial or host rock was followed by antitaxial deposition of complete silicification" of cross-fiber crocidolite veins columnar quartz from silica-saturated fluids. often is accompanied by oxidation of the crocidolite, Curvaceous quartz crystals thus reflect growth during transforming the color of the gem from blue to brown. host-rock rotation and not post-crystallization We have examined 10 tiger's-eye specimens from deformation. Crocidolite inclusion trails were initiated the PSU Earth and Mineral Sciences Museum by light by syntaxial overgrowth of crocidolite grains on the optical petrography, X-ray diffraction, and opposing vein wall. The crocidolite overgrowths then transmission electron microscopy (TEM). We believe were enveloped by columnar quartz, and both the that the formation of tiger's-eye is not primarily the quartz and its included crocidolite separated from the result of pseudomorphism of quartz after crocidolite. vein wall during subsequent cracking. These Rather, we propose that tiger's-eye is the product of 2 crocidolite inclusion trails trace the true host-rock distinct reaction processes: 1) a series of antitaxial displacement and may be used for monitoring crack-seal episodes that generated quartz veins with displacement histories. oriented inclusions of crocidolite; and 2) a later References: [1] Dana E.S. (1892) System of infiltration of oxidizing fluids that altered the Mineralogy. Wiley, New York. [2] Frondel C. (1962) crocidolite to iron oxyhydroxides and hematite. System of Mineralogy, 7th Ed. Vol. III. Silica Minerals. Results: Our interpretation is based on the Wiley, New York. [3] Ramsay J.G. (1980) Nature. following observations: 284:135-139. [4] Cox S.F. (1987) J Structur Geol 1) The quartz that constitutes tiger's-eye typically 9:779-787. [5] Fisher D. and Brantley, S.L. (1992) J occurs as columnar crystals measuring 0.5 to 1.0 mm Geophys Res 97:20,043-20,063. in diameter and 5 to 10 mm in length. Long axes often are oriented nearly normal to the banding planes, but in some instances the columnar grains are extremely curvaceous. X-ray pole figures of the grains and their length-slow optical character indicate that the quartz fibers are strongly correlated crystallographically, with the column lengths parallel to the c-axis. 2) TEM analysis of the quartz reveals extremely low dislocation and Brazil twin plane densities, and little undulosity is observed in cross-polarized transmitted light when the crystals are rotated to extinction. These observations suggest that the crystals are relatively unstrained. 3) Crocidolite fibers often are oriented parallel to the long axes of the quartz columns, but in some instances the fiber axes transect the quartz grain boundaries at angles up to 30˚. The crocidolite crystals often appear as repetitively interrupted sequences of short, colinear fibers, and neighboring crocidolite fibers are always oriented parallel to each other. 4) The boundaries between the blue hawk's-eye zones and the brown tiger's-eye are not geometrically correlated with the banding planes or the columnar