Gaudefroyite

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Formula: Ca₄Mn³⁺₃(BO₃)₃(CO₃)O₃
Monoborate, manganese-bearing mineral
Crystal System: Hexagonal
Specific gravity: 3.35 to 3.50 measured, 3.49 calculated
Hardness: 6
Streak: Brown
Colour: Black
Environments

Hydrothermal environments

Gaudefroyite occurs as a hydrothermal mineral in manganese deposits (Mindat). It is stable between 25°C and 486°C but that at higher temperatures a second phase exsolves from the original mineral structure (R&M 97.5.424-433).

Localities

At the type locality, the Tachgagalt Mine, Tansifite Caïdat, Agdz Cercle, Zagora Province, Drâa-Tafilalet Region, Morocco, gaudefroyite, the first known borate-carbonate of calcium and manganese, was found on the mine dumps. It occurs in a gangue of white, non-manganiferous calcite, quartz, and locally brucite. Associated minerals include pyrolusite, marokite, braunite, hausmannite, crednerite and polianite (as an alteration product along the gaudefroyite cleavage). Crystals of gaudefroyite are black, acicular hexagonal prisms up to 5 cm long, with pyramidal terminations (AM 50,806-807, Mindat). Hausmannite and braunite are sometimes present as microscopic inclusions in the gaudefroyite (R&M 97.5.424-433).

At the Kuruman district, Kalahari manganese field, Northern Cape, South Africa, fine specimens of gaudefroyite have been found as shiny to dull black hexagonal crystals, often doubly terminated, with some reaching 5 cm in length, although most are smaller. Some of the matrix consists of drusy, translucent red-brown, deltoidal trapezohedral grossular crystals. Other associated minerals include andradite, baryte, calcite, clinochlore, ettringite, gypsum, hausmannite, hematite and saponite; the gypsum, hematite and calcite all postdate the gaudefroyite, with the elongate black crystals of gaudefroyite occasionally protruding through the hematite. Some of the most aesthetic specimens consist of bright red andradite with black gaudefroyite.
Gaudefroyite is also a minor, but widespread, component of the solid manganese ore; other boron-bearing species also present in the ore include gowerite, inyoite, sussexite, datolite and oyelite (R&M 97.5.424-433).

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