Observations on Heat-Treated Authigenic Microcline and Albite Crystals

As part of a general study of authigenic feldspars, microcline and albite crystals of authigenic origin were heat-treated at temperatures near the melting point of albite. Within 28 days the microcline crystals were converted by a diffusive disordering transformation to sanidine. Untwinned albite cleavage fragments transformed to analbite within 10 days, and a measurable change in the reciprocal lattice angle γ was observed in one day. The transformation from low-temperature albite to analbite is gradational in character, and it is evident that the crystals pass through all intermediate states; it is thus similar to the micro-cline-sanidine transformation. During the heating period the albite crystals became mosaic and produced smeared X-ray reflections. The mosaic structure appears to be related to the formation of the unquenchable monoclinic form of NaAlSi₃O₈, the existence of which is deduced from fine albite and pericline twinning. After the crystals had completely transformed to monoclinic NaAlSi₃O₈ the smeared X-ray reflections became sharp once again.