GAUDEFROYITE
Ca4Mn3+3-x(BO3)3(CO3)(O,OH)3

Gaudefroyite (black), barite, andradite from KMF.
specimen and photo – J de Jongh
Crystal system: Hexagonal
Hardness: 6 Density: 3.35 – 3.50 Cleavage: None, brittle.
Streak: Brown Colour: Black, charcoal, bronze
Occurrence: An uncommon hydrothermal
mineral in manganese deposits.
Habitat: Needle-like to stubby
hexagonal crystals up to 5 cm long, with pyramidal terminations.
Composition: Borate comprising of calcium,
manganese, boron, hydrogen, carbon and oxygen.
The
Tsumeb mine in Namibia is famous for its variety of green minerals that are
difficult to distinguish from one another.
Likewise, the Kalahari Manganese Field near Kuruman, is well known for
its various black minerals that can also be very tricky to identify
accurately. One such mineral is
gaudefroyite. Unfortunately, fairly
limited information is available on this rare mineral. In the early 1960s Pascal Gallo, chief miner
at the manganese mine at Tachgagalt 17 km (some sources indicate 17 miles)
south-southwest of Quarzazate, Morocco, noticed an association of unusual
minerals in Vein No. 2 of the mine. He
presented a specimen of the material to Georges Jouravsky, chief geologist,
“Division de la Geologie du Maroc”, who in turn passed it along to Abbe
Christophe Gaudefroy (1888-1971), a French mineralogist who worked in
Morocco. Marokite was the first new
mineral to be described from this mine.
Soon thereafter gaudefroyite, named after the abovementioned French
mineralogist, was identified in 1964 from a sample apparently found on the
dumps of the same mine.
The
Kalahari Manganese Field is only the second reported occurrence in the world of
this rare mineral. It was discovered in
the early 1990s at N’Chwaning II and Wessels mines. Not only was it found as the best crystals,
but also as the biggest crystals of the species in the world.
Underground,
the black crystals of gaudefroyite are difficult to distinguish from the
manganese ore. It is also often mistaken
for vesuvianite from the Wessels mine.
Of interest to mineral collectors are those crystals up to 5 cm long and
1,5 cm in diameter, pitch black and in many cases doubly terminated, with well
developed pyramidal faces which came from the N’Chwaning II mine in 1995. The majority of these crystals were floaters
or embedded in gypsum or calcite.
Associated minerals are sturmanite, calcite,
hematite, andradite, todorokite, bixbyite, chlorite and barite. Fine needle-like gaudefroyite from N’Chwaning
II have been reported, occurring as cavity fillings in hausmannite. These needles, only a few mm long, are bronze
in colour.
Article posted with permission from Cape Town Gem & Mineral Club