Abstract

A crystal-growth model is proposed to reproduce the principal crystallographic properties of saddle dolomite, i.e., tabular shape, curved surfaces, and sweeping extinction. Beginning with a central rhombohedron, crystal growth occurs by rhombohedral faces that advance outwards and by nucleation and growth of new rhombohedra at the six lateral vertices. Continued growth and repeated nucleation produced chains of rhombohedra which emanate from the central rhombohedron, and which alternately rise and fall at 15 degrees from the horizontal. Simultaneous infilling by additional nucleation and growth produces a tabular macro-crystal with curved upper and lower surfaces. Slight and progressive misorientation of nuclei along the length of a chain gives rise to numerous blocky sub-crystals and to sweeping extinction. Some support for this model is found in crystals from Newfoundland in which the Ca/Mg ratio does not exceed unity, rhombobedral offshoots are occasionally seen in thin-sections, and small faces (possible nucleation sites) are visible in SEM images.

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