Felsőbányaite

Felsőbányaite or basaluminite is a hydrated aluminium sulfate mineral with formula: Al₄(SO₄)(OH)₁₀·4H₂O. It is a rare white to pale yellow mineral which typically occurs as globular masses and incrustations or as minute rhombic crystals. It crystallizes in the monoclinic crystal system.^[3][2]

Felsőbányaite
Felsöbanyaite as white spherules. Roadcut near Sant Marti dels Castells (Lérida) Spain
General
Category	Sulfate mineral
Formula	Al4(SO4)(OH)10·4H2O
IMA symbol	Fsb[1]
Strunz classification	7.DD.05
Crystal system	Monoclinic
Crystal class	Sphenoidal (2) (same H-M symbol)
Space group	P21
Unit cell	a = 13.026 Å, b = 10.015 Å, c = 11.115 Å; β = 104.34°; Z = 4
Identification
Color	White to pale yellow, pale brown
Crystal habit	Globular masses, minute rhombic crystals
Cleavage	Distinct to good on {010} and {100}
Mohs scale hardness	1.5
Luster	Vitreous, pearly on cleavage surfaces
Diaphaneity	Semitransparent
Specific gravity	2.33
Optical properties	Biaxial (+)
Refractive index	n = 1.515–1.540
References	[2][3]

It occurs as a weathering product under acidic conditions associated with pyrite or marcasite decomposition. Associated minerals include hydrobasaluminite, hydroargillite, meta-aluminite, allophane, gibbsite, gypsum and aragonite.^[3]

Felsőbányaite was first described in 1853 for an occurrence in the Baia Sprie mine, Baia Sprie (Felsőbánya), Maramureș County, Romania, and named for the locality.^[2] The mineral name basaluminite was used for an occurrence of the same mineral in England in 1948 and discredited by the International Mineralogical Association (IMA) in 2006.^[4]