Epitaxies of Ca-sulfates on calcite (CaCO3) I. Gypsum {010} on the calcite {10.4} form: epi-twins of gypsum induced by the calcite substrate

Abstract

3D-epitaxy of the {010} pinacoid of deposited gypsum (CaSO₄·2H₂O) on the substrate {10.4} calcite rhombohedron is described, in order to give a theoretical background to the replacement of calcite with gypsum both in the laboratory and by weathering in nature. 2D-lattice coincidences are geometrically obtained from the matching of periodic bond chains (PBCs) running within the elementary slices facing towards the substrate/deposit interfaces. The epitaxy in the third dimension, perpendicular to the interfaces, has been verified as well, to establish if ad/absorption occurs (anomalous mixed crystals) at the calcite/gypsum contact. This research represents the reticular part of a wider program extended to the epi-interaction among the gypsum/calcite, gypsum/bassanite (CaSO₄·1/2H₂O) and gypsum/anhydrite (CaSO₄) couples.