herderite

{{infobox mineral | name = Herderite | image = Herderite-gem7-08a.jpg | alt = | caption = | category = Phosphate mineral | formula = CaBe(PO4)(F,OH) | IMAsymbol = Hrd | molweight = | strunz = 8.BA.10 | dana = | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = P21/a | unit cell = a = 4.81, b = 7.7, c = 9.82 [Å]; β = 90.1°; Z = 4 | color = | colour = Colourless, pale yellow, greenish-white | habit = Occurs as prismatic tabular crystals, pseudo-orthorhombic or pseudo-hexagonal; fibrous botryoidal to spheroidal aggregates | twinning = On {100} or {001} as fishtail contact | cleavage = Indistinct on {110} | fracture = Subconchoidal | tenacity = | mohs = 5 - 5.5 | lustre = Vitreous | streak = | diaphaneity = Transparent - translucent | gravity = 3.02 | density = | polish = | opticalprop = Biaxial (-) | refractive = nα = 1.556 - 1.592 nβ = 1.578 - 1.610 nγ = 1.589 - 1.620 | birefringence = δ = 0.033 | pleochroism = | 2V = Calculated: 70° | dispersion = | extinction = | length fast/slow = | fluorescence = Fluoresces violet under UV; cathodoluminesces and phosphoresces pinkish orange under X-rays | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | references = }} Herderite is a phosphate mineral belonging to the apatite, phosphate group, with formula CaBe(PO4)(F,OH). It forms monoclinic crystals, often twinned and variable in colour from colourless through yellow to green. It forms a series with the more common hydroxylherderite, which has more hydroxyl ion than fluoride.

It is found in many parts of the world, often in pegmatites and associated with other apatite minerals.