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 by...

Epitaxies of Ca sulfates on calcite. II. The main {010}, {001} and {100} forms of bassanite epi-deposited on the {10.4} substrate form of calcite

“…Further, swallow-tail twins do theoretically occur in all three cases of the epitaxies of bassanite on (10.4) Cc . In brief, these new 2D twin laws have been found for bassanite, promoted by the calcite substrate; together with the (10.4) Cc / (010) Gypsum coupling examined earlier (Aquilano et al, 2022), a new way of thinking is being developed in detail about the epitaxy between different species. This can be particularly useful when a new mineralogical species tends to replace another, as in the case of Ca sulfates (gypsum, bassanite, anhydrite) replacing calcite, in nature and/or the laboratory (Ruiz-Agudo et al, 2016).…”

“…We recently investigated all compatible 3D and 2D lattice coincidences (herein 3D-and/or 2D-LCs) that can occur at the interface between the {10.4} form of cleaved calcite (substrate) and the {010} pinacoid of gypsum (deposit) (Aquilano et al, 2022). Working on calcium sulfates deposited on gypsum, we knew that when gypsum is heated above $150 C in the dry state (or at 75 C in methanol-water solutions), a part of the crystalline water is removed and CaSO 4 Á0.5H 2 O is formed (Maslyk et al, 2022).…”

“…It is therefore a matter of starting from scratch, slowly moving in crystallographic morphology: to do this, we began with the surface profiles of all the involved forms, obtained through a strict application of the Hartman-Perdok method (Hartman, 1973). As we recently treated the {10.4}-calcite/{010}-gypsum epitaxy (Aquilano et al, 2022) in the same way, we will complete our program in a subsequent study dealing with the {10.4} Cc / anhydrite (CaSO 4 ) epitaxy, starting from crystallographic experience (Aquilano et al, 1992) acquired many years ago. recollect the related PBC strength (the end chain energy, erg ion À1 Â 10 10 ), i.e.…”

Epitaxies of Ca sulfates on calcite (CaCO3). III. {010}, {001}, {100} and {011} growth forms of anhydrite (CaSO4) deposited on the {10.4} rhombohedron of calcite

“…the interaction energy between the atoms contained in the d 10.4 slice, and found that E 10:4 slice = 0.222 erg  10 À10 ion À1 corresponds to 94.32% of the calcite total crystallization energy. The shape of growth (or dissolution) of {10.4} patterns (spirals and/or 2D nuclei) is theoretically limited (Aquilano et al, 2022a) by the dominant border steps h441i, followed by the h421i vertical and h010i horizontal directions, as can be appreciated in Fig. 1.…”

101 contact twins in gypsum experimentally obtained from calcium carbonate enriched solutions: mineralogical implications for natural gypsum deposits

“…An epitaxial relationship between the {010} pinacoid of gypsum and the {10.4} rhombohedron of calcite (Ruiz-Agudo et al, 2016;Aquilano et al, 2022) has been demonstrated. Therefore, the effect of carbonate anions -ubiquitous in evaporitic environments -on the habit of gypsum crystals deserves further investigation.…”