Corkite

Corkite is a phosphate mineral in the beudantite subgroup of the alunite group. Corkite is the phosphate analogue of beudantite and with it, a complete solid solution range exists. Corkite will also form a solid solution with kintoreite.

Corkite
Corkite on a goethite matrix
General
Category	Phosphate minerals
Formula	PbFe3[(OH)6:SO4:PO4]
IMA symbol	Cok[1]
Strunz classification	8.BL.05
Dana classification	43.4.1.2
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3/m) (same H-M symbol)
Space group	R3m
Unit cell	a = 7.3065(5) Å, c = 16.897(2) Å; V = 781.2 Å³; Z = 3
Identification
Formula mass	667.82 g/mol
Color	Brown to light yellowish brown, pale yellow, yellowish green to dark green
Crystal habit	Crystals pseudocubic rhombohedral with prominent {1011}. Commonly in crusts and massive
Cleavage	Perfect on {0001}
Mohs scale hardness	3.5–4.5
Luster	Vitreous, resinous
Diaphaneity	transparent
Specific gravity	4.295 (measured), 4.31 (calculated)
Optical properties	Uniaxial (−), may appear anomalously biaxial
Refractive index	nω = 1.930 nε = 1.930 n = 1.93 – 1.96
Birefringence	δ = 0.000
Other characteristics	Readily soluble in warm HCl
References	[2][3][4][5][6]

Corkite is named after County Cork, Ireland; the location where the first notable amount was discovered in 1869.^[3] Like many of the other minerals in the beudantite group, corkite is a relatively uncommon, secondary mineral that occurs in oxidation zones near hydrothermal base metal deposits.^[3] It occurs associated with pyromorphite, malachite, plumbojarosite, limonite and quartz.^[6]