Beraunite
Crystalline beraunite on green kidwellite, Coon Creek Mine, Polk County, Arkansas, US. Size: 3.4 × 3.3 × 2.8 cm
General
Category	Phosphate minerals
Formula (repeating unit)	Fe2+ Fe3+5(OH)5(PO4)4·4H2O
IMA symbol	Bru[1]
Strunz classification	8.DC.27
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Unit cell	a = 20.953(8) Å, b = 5.171(1) Å, c = 19.266(4) Å; β = 93.34°; Z = 4
Identification
Color	Dull greenish to greenish brown when fresh, may be color banded; reddish brown to hyacinth-red, blood-red on exposure
Crystal habit	Tabular crystals common, may be in coarse radially fibrous aggregates, globular or discoidal, and in crusts
Twinning	On {100}, may be interpenetrant
Cleavage	On {100}, good
Mohs scale hardness	3–4
Luster	Vitreous, pearly on cleavages, resinous on fractures
Streak	Olive-drab when fresh; yellow, brownish yellow on exposure
Diaphaneity	Translucent
Specific gravity	2.8–3.08 (measured); 2.894 (calculated)
Optical properties	Biaxial (+)
Refractive index	nα = 1.775 nβ = 1.786 nγ = 1.815
Birefringence	δ = 0.040
Pleochroism	X = pale flesh-pink, yellow, blue-green; Y = pale flesh-pink, yellow, pale olive-green; Z = carnelian-red, reddish brown, olive-green.
2V angle	Measured: 30° to 60°, Calculated: 66°
References	[2][3][4]

Beraunite is an iron phosphate mineral. It was first described by August Breithaupt for an occurrence in Beraun currently in the Czech Republic. Beraunite occurs as a secondary mineral in iron ore deposits, and as an alteration product of primary phosphate minerals in granite pegmatites.

Beraunite crystallizes in the monoclinic crystal system with point group 2/m. Beraunite's formula is Fe²⁺ Fe³⁺₅(OH)₅(PO₄)₄·4H₂O. Aluminium and zinc may substitute in the structure.

Occurrence

Beraunite occurs as a secondary mineral in iron ore deposits and as an alteration product of primary phosphates minerals in granite pegmatites. It is found as an alteration product of triphylite at the Big Chief and Hesnard mines, Keystone, Pennington County, South Dakota, New Hampshire, in Arkansas, New Jersey and Pennsylvania in the United States. It is also found in Ireland, Germany, and the Czech Republic.

Structure

It occurs as fibrous greenish-black to brown nodules. The chemical analysis of beraunite often show small and irrational amounts of ferrous iron. The chain segments in Beraunite are one and two octahedral in length. Beraunite has two different types which are Zn-rich and Al-rich Beraunites, those two types have variations in data between those taken by Krsano in the X-ray powder diffraction experiment and those in the literature. Phosphate minerals have special characteristics about their chemical composition. The polyatomic complex of iron-oxygen octahedral face is sharing triplets corner to four other octahedral in the arrangement of atoms of the basic iron phosphates like dufrenite, rockbridgeite, and Beraunite. This complex is linked together by the phosphate tetrahedra.