Instead of a specific proprium, this stereo-chemical structure could perhaps be interpreted as the Essence itself of the crystal concerned. The stereo-chemical structure is equivalent to The Chemical Composition + Space Group (The Space Group describes the total symmetry of the crystal lattice, furnished with (chemical) motifs. The Chemical Composition imposes upon the symmetry requirement, as set by the Space Group, an actual, real and specific lattice structure).
Accordingly we could, for example regarding the mineral Aragonite, let the genus refer to : Calcium carbonate (CaCO3), and this genus then is, in the case of Aragonite, further specified (determined) as having a lattice symmetry, described by the Space Group Pmcn, and this then represents the differentia specifica of the species Aragonite. Another species of the genus Calcium carbonate is the mineral Calcite. This crystallizes in the Space Group R3*c, and this is then the differentia specifica, now of the species Calcite (In the crystallographical literature the " * " in the Space Group Symbol is indicated by a horizontal score above the previous numeral).
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