Sturmanite

Sturmanite is a rare sulfate mineral with the chemical formula Ca₆Fe³⁺₂(SO₄)_2.5(B(OH)₄)(OH)₁₂ · 25 H₂O.^[2][3][4] It crystallises in the tetragonal system and it has a Moh's hardness of 2.5. Sturmanite has a bright yellow to amber colour and falls in the ettringite group. It was named after Bozidar Darko Sturman (born 1937), Croatian-Canadian mineralogist and Curator Emeritus of Mineralogy, Royal Ontario Museum.^[5]

Sturmanite
Distinctive sturmanite on matrix from the N'Chwaning Mines. Size: 5.4 x 4.2 x 1.7 cm. Ex.Charlie Key Stock
General
Category	Sulfate minerals
Formula	Ca6Fe3+2(SO4)2.5[B(OH)4](OH)12 · 25 H2O
IMA symbol	Strm[1]
Strunz classification	7.DG.15
Dana classification	32.4.4.2
Crystal system	Trigonal
Crystal class	3m - Ditrigonal Pyramidal
Space group	P31c (no. 159)
Unit cell	a = 11.188(9) Å, c = 21.91(7) Å
Identification
Color	Bright yellow to amber
Crystal habit	Hexagonal, pyramidal, prismatic
Cleavage	Perfect {1010}
Tenacity	Brittle
Mohs scale hardness	2.5
Luster	Vitreous, greasy
Streak	Pale yellow, greenish yellow, brownish orange
Diaphaneity	Transparent to translucent
Specific gravity	1.847 (measured) 1.855 (calculated)
Optical properties	Uniaxial (+/-)
Birefringence	δ = 0.002
Ultraviolet fluorescence	none

Occurrence

Sturmanite was first identified in 1983 and approved by the IMA in the same year.^[4][5] It was first found in the Black Rock Mine, Black Rock, Kalahari manganese field, Northern Cape Province, South Africa. It is found as flattened dipyramidal crystals on hematite and baryte.^[5] Sturmanite has also been identified in mines near the Black Rock Mine, such as the Wessel's and Perth mines, in the N'Chwaning mines, and near Lakargi Mountain in Russia.^[2] It is found as a rare secondary mineral embedded in manganese deposits and is associated with baryte, manganite, hausmannite, and hematite.^[3]

Crystal structure

The crystal structure of sturmanite shows two distinct features: one being columns of iron-octahedra and calcium polyhedra, the other being the SO₄⁻ and B(OH)₄⁻ tetrahedra surrounding these columns. These two structures are linked together through a dense and complex network of hydrogen bonds.^[6]