bertrandite

{{Infobox mineral | name = Bertrandite | image = Bertrandite-38545.jpg | alt = | caption = Bertrandite from the Golconda pegmatite, Minas Gerais, Brazil | category = Sorosilicate | formula = Be4Si2O7(OH)2 | IMAsymbol=Btd | strunz = 9.BD.05 | dana = | system = Orthorhombic | class = Pyramidal (mm2) H-M symbol: (mm2) | symmetry = Ccm21 | unit cell = a = 8.7135(4) Å, b = 15.268(1) Å, c = 4.5683(3) Å; Z = 4 | color = Colorless to pale yellow | habit = Thin tabular, prismatic to needle-like crystals commonly in radial clusters | twinning = Common on {011} or {021} forming heart- or V-shaped twins | cleavage = Perfect on {001}; distinct on {100}, {010} and {110} | fracture = | tenacity = | mohs = 6–7 | luster = Vitreous, pearly on cleavage surfaces | streak = | diaphaneity = Transparent | gravity = 2.59–2.60 | density = | polish = | opticalprop = Biaxial (−) | refractive = nα = 1.591 nβ = 1.605 nγ = 1.614 | birefringence = δ = 0.023 | pleochroism = | 2V = Measured: 73° to 81° | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | references = }}

Bertrandite is a beryllium sorosilicate hydroxide mineral with composition: Be4Si2O7(OH)2. Bertrandite is a colorless to pale yellow orthorhombic mineral with a hardness of 6–7.