Putnisite

Putnisite is a mineral composed of strontium, calcium, chromium, sulfur, carbon, oxygen and hydrogen.[3] It was discovered on the Polar Bear Peninsula in Shire of Dundas, Western Australia in 2007 during mining activity.[3][4] Following identification and recognition by the IMA in 2012[4] the mineral was named after mineralogists Andrew and Christine Putnis.[2]

Putnisite
Putnisite, from Polar Bear Peninsula, Western Australia, Australia
General
Category	Mineral
Formula (repeating unit)	SrCa4Cr83+(CO3)8(SO4)(OH)16·25 H2O
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pnma
Unit cell	a = 15.351 Å, b = 20.421 Å, c = 18.270 Å; Z = 4
Identification
Color	Purple
Crystal habit	Pseudocubic crystals
Cleavage	[100], [010] and [001] good
Fracture	Brittle - uneven
Mohs scale hardness	1.5-2
Luster	Vitreous
Streak	Pink
Diaphaneity	Translucent
Specific gravity	2.20
Optical properties	Biaxial (–)
Refractive index	nα = 1.552, nβ = 1.583 and nγ = 1.599
Pleochroism	Distinct: X pale bluish grey, Y pale purple, Z pale purple
References	[1][2][3][4]

Putnisite has unique chemical and structural properties, and does not appear to be related to any of the existing mineralogical families.[2] Crystals are translucent purple, but show distinct pleochroism (from pale purple to pale bluish grey,[3] depending on the angle of observation) and leave pink streaks when rubbed on a flat surface.[3]

Putnisite occurs as small (< 0.5 mm) cube-like crystals in volcanic rock.[2] The mineral formed during the oxidation environment within komatiite to dioritic bodies containing sulfide minerals.[3]