Bultfonteinite

Bultfonteinite
Bultfonteinite from N’Chwaning mine, South Africa
General
Category	Nesosilicates
Formula (repeating unit)	Ca2SiO2(OH,F)4
Strunz classification	9.AG.80
Dana classification	52.4.7.2
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 10.99 Å, b = 8.18 Å c = 5.67 Å, α = 93.95° β = 91.32°, γ = 89.85°; Z = 4
Identification
Color	Colorless, pink
Twinning	Interpenetrating on {100} and {010}; polysynthetic
Cleavage	Good on {100} and {010}
Fracture	Conchoidal
Mohs scale hardness	4.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent
Optical properties	Biaxial (+)
Refractive index	nα = 1.587 nβ = 1.590 nγ = 1.597
Birefringence	δ = 0.010
2V angle	70° (measured)
Dispersion	r > v; barely perceptible
Solubility	Soluble in hydrochloric acid
References

Bultfonteinite, originally dutoitspanite, is a pink to colorless mineral with chemical formula Ca₂SiO₂(OH,F)₄. It was discovered in 1903 or 1904 in the Bultfontein mine in South Africa, for which the mineral is named, and described in 1932.

Bultfonteinite is transparent and pale pink to colorless. The mineral occurs as radiating prismatic acicular crystals and radial spherules up to 2 cm (0.79 in).

The crystal structure of bultfonteinite consists of strips of [Ca₄Si₂O₄]⁸⁺, that run along the 5.67 Å c-axis, held together by Ca–O–Ca, Ca–F–Ca, Ca–H₂O–Ca, and Ca–O–Si bonds. Silicon atoms occur in isolated tetrahedra and the calcium atoms have seven-fold coordination, derived from a triangular prism with a seventh atom present on one of the square faces.

In either 1903 or 1904, a miner discovered the first specimen of bultfonteinite on the 480-foot level of the Bultfontein mine in Kimberley, South Africa. The mineral occurred in a several-hundred-foot-tall horse of kimberlite-enclosed dolerite and shale fragments. The specimen, mistakenly thought to be natrolite, was given to Alpheus F. Williams. Several years later, additional samples were found by C. E. Adams in the nearby Dutoitspan mine and given to the MacGregor Museum in Kimberley. Shortly before 1932, the mineral was found about 100 miles (160 km) to the southeast of Kimberley at the Jagersfontein Mine in Orange River Colony.

After John Parry and F. E. Wright described the mineral afwillite in 1925, Williams recognized that his samples of bultfonteinite were not natrolite, but were likely a new mineral species. Chemical analysis by John Parry and crystallographic and optical determination by Wright proved it to be a new mineral. The mineral was described by Parry, Williams, and Wright in 1932 and named bultfonteinite. Their original description does not explicitly state the origin of the name, but it is presumably named after the mine in which it was discovered. Earlier that year in his book The Genesis of the Diamond, Williams had called the mineral dutoitspanite, a name which was "apparently discarded". When the International Mineralogical Association was founded, bultfonteinite was grandfathered as a valid mineral species.