Åkermanite

Åkermanite (Ca₂Mg[Si₂O₇]) is a melilite mineral of the sorosilicate group, containing calcium, magnesium, silicon, and oxygen. It is a product of contact metamorphism of siliceous limestones and dolomites, and rocks of sanidinite facies. Sanidinite facies represent the highest conditions of temperature of contact metamorphism and are characterized by the absence of hydrous minerals. It has a density of 2.944 g/cm³.[3][2] Åkermanite ranks a 5 or 6 on the Mohs scale of mineral hardness, and can be found gray, green, brown, or colorless. It has a white streak and a vitreous or resinous luster. It has a tetragonal crystal system and a good, or distinct, cleavage. It is the end member in a solid solution series beginning with gehlenite (Ca₂Al[AlSiO₇]).[1]

Åkermanite
Åkermanite (yellowish brown), calcite (blue), foshagite (hillebrandite variety, fibrous), tilleyite (creamy)
General
Category	Sorosilicates
Formula (repeating unit)	Ca2Mg(Si2O7)
Strunz classification	9.BB.10
Dana classification	55.4.1.1 (8 ed)
Crystal system	Tetragonal
Crystal class	Scalenohedral (42m) H-M symbol: (4 2m)
Space group	P421m
Unit cell	a = 7.8288(8) c = 5.0052(2) [Å]; Z = 2
Identification
Color	Colorless, yellowish gray, green, brown
Cleavage	Distinct on {001}, poor on {110}
Fracture	Irregular/ uneven, conchoidal
Tenacity	Brittle
Mohs scale hardness	5 - 6
Luster	Vitreous, resinous
Streak	White
Diaphaneity	Transparent, translucent
Specific gravity	2.944
Optical properties	Uniaxial (+)
Refractive index	nω = 1.632 nε = 1.640
Birefringence	δ = 0.008
References	[1][2]

The mineral is named for Anders Richard Åkerman (1837–1922), a Swedish metallurgist. It has been found at Monte Somma and Vesuvius, and Monte Cavalluccio near Rome. It was "grandfathered" in as a species of mineral because it was described prior to 1959, before the founding of the International Mineralogical Association.[2]