An extremely rare black grain of an intergrown mixture of grimaldiite and mcconnellite from the type locality for both species: Merume River, Kamakusa, Potaro-Siparuni Region, Guyana. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY 4.0
Grimaldiite
Grimaldiite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula CrO(OH).
It crystallizes in the hexagonal crystal system (space group R-3m).
Key properties: specific gravity 4.11–4.12.
IMA symbol Gmd, IMA status: approved, named in 1967.
Found in fine-grained intergrowth with other chromium oxide-hydroxide minerals in alluvial gravels (Merume River, Guyana); in cracks and crevices in penroseite from a hydrothermal mineral deposit (Hiaco, Bolivia).
It is found associated with Bracewellite, eskolaite, guyanaite, mcconnellite (Merume River, Guyana); penroseite, barite (Hiaco, Bolivia).
The name refers to Dr. Frank Saverio Grimaldi (1915–), formerly Chief Chemist, U.S. Geological Survey.