Dolomite
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Dolomite from Black Rock, Arkansas, U.S.A., measuring 70 × 57 × 14 mm and weighing 50 grams. A flattish plate of well-formed crystals from a state better known among collectors for its quartz than its carbonates.
Dolomite is calcium magnesium carbonate, CaMg(CO3)2, trigonal in crystal system and often showing the same curved, saddle-shaped rhombohedral faces that make the mineral easy to recognise in hand. Northern Arkansas sits within the Ozark region's carbonate-hosted, Mississippi Valley-type lead-zinc province, where Dolomite is both an abundant host rock and a common later-stage vein and cavity mineral, forming as mineralising fluids moved through the same limestone sequences that elsewhere in the district carried Sphalerite and Galena.
A clean, well-formed Dolomite plate like this is a good, affordable way to represent an Arkansas locality outside the state's famous quartz crystal belt. This piece is from the David Vince Collection.