robertsite

{{Infobox mineral | name = Robertsite | boxwidth = | boxbgcolor = | image = Robertsite-Beraunite-220952.jpg | imagesize = 260px | alt = | caption = Robertsite spheres (reddish brown) on beraunite | category = Phosphate minerals | formula = Ca3(Mn3+)4[(OH)3| (PO4)2]2·3H2O | IMAsymbol = Rbt | molweight = | strunz = 8.DH.30 | dana = 42.08.04.02 Mitridatite group | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = A2/a | unit cell = a = 17.36 Å, b = 19.53 Å c = 11.30 Å; β = 96.0°; Z = 12 | color = Red, red-brown, deep red, bronzy brown, black | colour = | habit = laty to wedge-shaped, pseudorhombohedral crystals; fibrous, in botryoidal to feathery aggregates | twinning = Common perpendicular to {100} | cleavage = Very good {100} | fracture = Micaceous | tenacity = | mohs = | luster = Vitreous to resinous or waxy | streak = Chocolate-brown | diaphaneity = Translucent | gravity = 3.13 - 3.17 | density = | polish = | opticalprop = Biaxial (-) | refractive = nα = 1.775 nβ = 1.820 nγ = 1.820 | birefringence = δ = 0.045 | pleochroism = Strong, X pale red to pink Y,Z deep red brown | 2V = Measured: 8° | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | prop1 = | prop1text = | references = }} Robertsite, Ca3(Mn3+)4[(OH)3| (PO4)2]2·3(H2O) (alternatively formulated Ca2(Mn3(PO4)3O2)(H2O)3), is a secondary phosphate mineral named for Willard Lincoln Roberts (1923–1987), mineralogist and professor at South Dakota School of Mines in Rapid City, South Dakota.

The type locality for Robertsite is the Tip Top mine, Custer County, South Dakota, US. Robertsite occurs at the Tip Top Mine as minute crystal aggregates and crusts found on quartz associated with triphylite. It is a dark reddish brown to black monoclinic mineral.