alstonite

{{infobox mineral | name = Alstonite | image = Alstonite-Witherite-oldeuro-107d.jpg | imagesize = 260px | alt = | caption = | category = Carbonate mineral | formula = BaCa(CO3)2 |IMAsymbol=Asn | molweight = 297.42 g/mol | strunz = 5.AB.35 | dana = 14.02.05.01 | system = Triclinic Unknown space group | unit cell = a = 17.38, b = 14.40 c = 6.123 [Å]; α = 90.35° β = 90.12°, γ = 120.08°; Z = 24 | color = | colour = Colourless to snow-white; also pale gray, pale cream, pink to pale rose-red | habit = Steep pseudohexagonal dipyramids, pseudo-orthorhombic | twinning = Common on pseudo-orthorhombic {10} and {310} | cleavage = Imperfect on pseudo-orthorhombic {110} | fracture = Uneven | tenacity = | mohs = 4 to 4.5 | lustre = Vitreous | streak = White | diaphaneity = Transparent to translucent | gravity = 3.70 | density = | polish = | opticalprop = Biaxial (−) | refractive = nα = 1.526 nβ = 1.671 nγ = 1.672 | birefringence = δ = 0.146 | pleochroism = None | 2V = Measured 6°, calculated 8° | dispersion = Weak, r > v | extinction = | length fast/slow = | fluorescence = Weak yellow under LW and SW | absorption = | melt = | fusibility = | diagnostic = | solubility = Soluble in dilute HCl | impurities = | alteration = Colour may fade on exposure to light | other = Not radioactive | references = }} Alstonite, also known as bromlite, is a low temperature hydrothermal mineral that is a rare double carbonate of calcium and barium with the formula , sometimes with some strontium. Barytocalcite and paralstonite have the same formula but different structures, so these three minerals are said to be trimorphous. Alstonite is triclinic but barytocalcite is monoclinic and paralstonite is trigonal. The species was named Bromlite by Thomas Thomson in 1837 after the Bromley-Hill mine, and alstonite by August Breithaupt of the Freiberg Mining Academy in 1841, after Alston, Cumbria, the base of operations of the mineral dealer from whom the first samples were obtained by Thomson in 1834. Both of these names have been in common use.

==Structure== Alstonite is triclinic, but appears pseudo-orthorhombic because of twinning. The space group is P1 or P. Alstonite appears to have a superstructure based on paralstonite without long range order of the metal cations or the CO3 groups. The structure of paralstonite is similar to that of other double carbonates.