fayalite

{{Infobox mineral | name = Fayalite | category = Nesosilicate | image = Fayalite crystal group - Ochtendung, Eifel, Germany.jpg | imagesize = 260px | caption = Fayalite crystals on substrate. Sample collected from Ochtendung, Eifel, Germany | formula = Fe2SiO4 | IMAsymbol = Fa | molweight = | strunz = 9.AC.05 | system = Orthorhombic | class = Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) | symmetry = Pbnm (no. 62) | unit cell = a = 4.8211, b = 10.4779 c = 6.0889 [Å]; Z = 4 | color = Greenish yellow, yellow-brown, brown; pale yellow to amber in thin section | habit = Commonly granular, compact, or massive | twinning = On [100]; also on [031], as trillings | cleavage = {010} moderate, {100} imperfect | fracture = Conchoidal | mohs = 6.5–7.0 | luster = Vitreous to resinous on fractures | refractive = nα = 1.731–1.824, nβ = 1.760–1.864, nγ = 1.773–1.875 | opticalprop = Biaxial (−) | birefringence = δ = 0.042–0.051 | pleochroism = Faint | 2V = Measured: 74° to 47°, calculated: 54° to 66° | streak = White | gravity = 4.392 | density = | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = Transparent | other = | references = }}

Fayalite (, commonly abbreviated to Fa) is the iron-rich end-member of the olivine solid-solution series. In common with all minerals in the olivine group, fayalite crystallizes in the orthorhombic system (space group Pbnm) with cell parameters a = 4.82 Å, b = 10.48 Å and c = 6.09 Å.