Microscopic cordierite in fused Torridonian arkose

Abstract

Progressive thermal metamorphism of Torridonian arkose by a picrite sill in Soay (Hebrides) has culminated in extensive fusion. Microlites of cordierite were precipitated from the liquid as colorless, six-sided prisms (maximum dimension 0.06 mm.) with abundant magnetite inclusions. Refractive indices of the cordierite lie between 1.529 and 1.541, and (-) 2V is small. The cordierite is frequently twinned on (110), and rare cruciform twins are also present. The latter have (021) or (101) as the probable twin plane. Only (110) and (130) have been reported previously as cordierite twin planes. The distortion index, Δ (degree of distortion from hexagonal symmetry, measured by x-ray powder diffraction), is 0.08. From independent evidence, the maximum P_(H2O) was 430 kg/cm^2, and the minimum temperature interval of cordierite crystallization was 1025° C. to 935° C. If P_(H2O) were less than 430 kg/cm^2, the estimated crystallization temperatures would be higher. A useful temperature scale may be established by: (1) correlation of the structural state of cordierite with estimated temperature and pressure of water vapor in buchites, and (2) extension of hydrothermal studies on synthetic cordierites. Both approaches are required because the crystallization history of cordierite may affect its structural state. It is probable that the crystallization histories of cordierites in most buchites followed similar patterns.

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