A Study of Authigenic Feldspars

Abstract

Samples from forty localities were examined and include monoclinic potash feldspar, microcline, albite, and the first reported example of naturally occurring soda feldspar structurally intermediate between albite and analbite. Potash feldspar was observed in carbonates, sandstones, siltstones, and shales; soda feldspar was found only in carbonate rocks. All of the carbonate rocks that contain authigenic feldspars are partly or wholly recrystallized or dolomitized. Authigenic monoclinic potash feldspars crystallize as tiny rhombohedron-like crystals commonly referred to as the adularia habit. The authigenic microcline crystals have a distinctive triclinic morphology, which is almost certain evidence of primary crystallization in the triclinic system. The refractive indices and optic angle of authigenic potash and soda feldspars are similar to those of "non-authigenic" feldspars of equivalent composition. The authigenic microcline crystals are fourlings, twinned after the albite and pericline laws; some of the authigenic albites are Roc Tourné fourlings, twinned after the albite and carlsbad laws. Authigenic feldspars are nonperthitic and have chemical compositions harmonious with crystallization at low temperatures. Spectrographic analyses show that the content of authigenic K-feldspars rarely exceeds 0.3 per cent and the and CaO content of the soda feldspars rarely exceeds 0.4 per cent and 0.2 per cent, respectively. The geometrical features of authigenic feldspars are not significantly different from those of "non-authigenic" feldspars. An epigenetic or replacement origin is proposed for most authigenic feldspars, based primarily on the textural interrelationships between the feldspar and matrix rock crystals. A hydrothermal origin in its broader sense is ruled out for most authigenic feldspars.