åkermanite

{{Infobox mineral | name = Åkermanite | image = Åkermanite, Calcite, Hillebrandite, Tilleyite - Mineralogisches Museum Bonn1.jpg | imagesize = 260px | alt = | caption = Åkermanite (yellowish brown), calcite (blue), foshagite (hillebrandite variety, fibrous), tilleyite (creamy) | category = Sorosilicates | formula = Ca2Mg(Si2O7) |IMAsymbol=Åk | molweight = | strunz = 9.BB.10 | dana = 55.4.1.1 (8 ed) | system = Tetragonal | class = Scalenohedral (2m) H-M symbol: ( 2m) | symmetry = P21m | unit cell = a = 7.8288(8) c = 5.0052(2) [Å]; Z = 2 | color = Colorless, yellowish gray, green, brown | colour = | habit = | twinning = | cleavage = Distinct on {001}, poor on {110} | fracture = Irregular/ uneven, conchoidal | tenacity = Brittle | mohs = 5–6 | luster = Vitreous, resinous | streak = White | diaphaneity = Transparent, translucent | gravity = 2.944 | density = | polish = | opticalprop = Uniaxial (+) | refractive = nω = 1.632 nε = 1.640 | birefringence = δ = 0.008 | pleochroism = none | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | prop1 = | prop1text = | references = }}

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. 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]).