Oldhamite
Oldhamite (field of view: 1.5 cm)
General
Category	Sulfide mineral
Formula (repeating unit)	(Ca,Mg)S
IMA symbol	Old[1]
Strunz classification	2.CD.10
Crystal system	Cubic
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fm3m
Unit cell	a = 5.69 Å; Z = 4
Identification
Color	Pale chestnut-brown
Crystal habit	Crystal nodules, anhedral grains
Cleavage	Good on {001}
Mohs scale hardness	4
Luster	Sub-metallic
Diaphaneity	Transparent
Specific gravity	2.58
Optical properties	Isotropic
Refractive index	n = 2.137
Fusibility	2450 °C
Alters to	Tarnishes on exposure to moist air
References	[2][3][4]

Oldhamite is a calcium magnesium sulfide mineral with the chemical formula (Ca,Mg)S.^[2][3] Ferrous iron may also be present in the mineral resulting in the chemical formula (Ca,Mg,Fe)S.^[4] It is a pale to dark brown accessory mineral in meteorites. It crystallizes in the cubic crystal system, but typically occurs as anhedral grains between other minerals.

Discovery and occurrence

It was first described in 1862 for an occurrence in the Bustee meteorite, Gorakhpur, Uttar Pradesh, India. It was named for Irish geologist Thomas Oldham (1816–1878), the Director of the Indian Geological Survey.^[2][3]

It occurs as an interstitial mineral phase between silicate minerals in enstatite chondrite and achondrite meteorites.^[2][4] It occurs in association with enstatite, augite, niningerite, osbornite, troilite, gypsum and calcite.^[2] It has been reported from a variety of meteorite locations around the world including the Allan Hills 84001 meteorite of Antarctica. It has also been reported from a slag occurrence in France and a coal deposit in Poland.^[3]