Calcite

Calcite Group. Calcite-Rhodochrosite Series.

A very common and widespread mineral with highly variable forms and colours. Calcite is best recognized by its relatively low Mohs hardness (3) and its high reactivity with even weak acids, such as vinegar, plus its prominent rhombohedral cleavage in most varieties.

NOTE on the unit cell and the Miller indices: Before the advent of X-ray crystallography, the axial ratios were determined by measuring the interfacial angles and looking for the smallest numbers that fitted, assuming that the largest faces were the lowest order. Remarkably the classical crystallographers usually got it right, confirmed by indexing the X-ray diffraction patterns. However, for calcite and the calcite group they got it wrong. The old morphological calcite cell had c/a = 0.8543. The structural cell has c four times as large as would have been calculated for the morphological cell, now c/a = 3.419 (Maslen et al. 1993). Most of the Miller indices here are from ancient times and the c value should, therefore, be multiplied by 4. E.g. the rhombohedral cleavage is given as "Perfect on {1011}" in the old setting and should be "Perfect on {1014}" in the correct structural setting.

An important biomineral. As such, it forms from metastable vaterite via dissolution-reprecipitation process, preceded by vaterite formation via dissolution of its precursor, that is Unnamed (Amorphous Calcium Carbonate) (Bots et al., 2012; Sugiura et al., 2016). Calcite biomineralization may be induced by bacteria. A very recent review on fungal calcite biomineralization is given by Bindschedler et al. (2016). Vaterite/calcite precipitates are known in the Cladosporium fungus, too (Ye et al., 2023).

Aquilano et al. (2023) suggest a re-examination of the polymorphic calcite-aragonite system due to (overlooked) homo-epitaxy.