Lawsonite and pumpellyite in glaucophane schist, North Berkeley Hills, California, with notes on the X-ray crystallography of lawsonite, by A. Pabst

Abstract

A previously undescribed lawsonite-pumpellyite locality in the North Berkeley Hills is characterized by exceptional development of lawsonite as veins and as lithologic layers in glaucophane schist. The glaucophane schist outcrop, a tectonic block in serpentinite, is believed to have been derived at least in part by retrogressive metamorphism of eclogite. Lawsonite-rich layers in the schists and the vein minerals lawsonite and pumpellyite are regarded as products of metamorphic differentiation under conditions of glaucophanitic metamorphism. Optical and morphological data for lawsonite and pumpellyite are presented together with a new lawsonite chemical analysis and a semi-quantitative spectroscopic analysis of pumpellyite. A description is given of lawsonite-pumpellyite intergrowths and pumpellyite twinning. The cell dimensions of lawsonite from 3 California localities and from Cuba have been determined by means of calibrated Weissenberg patterns. All values obtained lie within the limits 8.788+ or -0.008 Aa for a _o , 13.129+ or -0.010 for b _o , and 5.846+ or -0.006 for c _o . A powder diffraction pattern giving calculated and observed d values to 1.46 Aa has been tabulated. The thermal transformation of lawsonite to metastable hexagonal CaAl ₂ Si ₂ O ₈ has been studied by means of single crystal diffraction. The change can be detected after prolonged heating at temperatures as low as 410 degrees C. In the neighborhood of 700 degrees C. it is complete in an hour or less. At intermediate temperatures partly changed lawsonite may persist for long periods. Single crystal patterns are more sensitive than powder patterns for detecting the beginning and end of the transformation. Lawsonite that had been heated under a bonfire was found to be partly transformed.