Erssonite, CaMg (sub 7) Fe (super 3+) (sub 2) (OH) (sub 18) (SO (sub 4) ) (sub 2) .12H (sub 2) O, a new hydrotalcite-supergroup mineral from Langban, Sweden

The new wermlandite-group mineral erssonite, ideally CaMg (sub 7) Fe (super 3+) (sub 2) (OH) (sub 18) (SO (sub 4) ) (sub 2) .12H (sub 2) O (or [Mg (sub 7) Fe (super 3+) (sub 2) (OH) (sub 18) ][Ca(SO (sub 4) ) (sub 2) ].12H (sub 2) O), was discovered in a late-stage, low-temperature assemblage in cavities of a magnetite-bearing dolomitic rock from the Langban deposit, Varmland county, Bergslagen ore province, Sweden. The associated minerals are dolomite, calcite, members of the magnetite-magnesioferrite solid-solution series, phlogopite, chrysotile, pyroaurite and norbergite. Erssonite has a vitreous lustre and forms colourless, platy hexagonal crystals flattened on [0001], up to 0.5 mm across and up to 10 mu m thick, occurring mainly as aggregates in cavities of dolomitic rock. Erssonite is malleable; separate crystals are flexible and non-elastic, with a perfect, mica-like cleavage on {0001}. The calculated density is equal to 2.02 g.cm (super -3) . Raman spectroscopy shows the presence of typical bands for S-O bonds attributed to intercalated SO (sub 4) (super 2-) anions and structural OH- anions together with the absence of C-O bonds, attributed to carbonate anions. The chemical composition is (wt.%, electron microprobe, H (sub 2) O content is calculated from structure data): MgO 28.67, CaO 2.76, Al (sub 2) O (sub 3) 0.23, Cr (sub 2) O (sub 3) 0.23, Fe (sub 2) O (sub 3) 16.00, SiO (sub 2) 0.48, SO (sub 3) 14.80, H (sub 2) O 35.58, total 98.75. The empirical formula based on 38 O atoms is H (sub 41.48) Ca (sub 0.52) Mg (sub 7.47) Fe (super 3+) (sub 2.11) Al (sub 0.05) Cr (sub 0.03) S (sub 1.94) Si (sub 0.08) O38. The ideal formula is CaMg (sub 7) Fe (super 3+) (sub 2) (OH) (sub 18) (SO (sub 4) ) (sub 2) .12H (sub 2) O or {Mg (sub 7) Fe (super 3+) (sub 2) (OH) (sub 18) }{[Ca(H (sub 2) O) (sub 6) ](SO (sub 4) ) (sub 2) (H (sub 2) O) (sub 6) }. The crystal structure was determined using single-crystal X-ray diffraction data and refined to R=0.093. Erssonite is trigonal, P31, with a=9.3550(6), c=22.5462(14) Aa, V=1708.8(2) Aa (super 3) and Z=2. The strongest lines of the powder X-ray diffraction pattern [d, Aa(I,%)(hkl)] are: 11.22 (90)(002), 5.63 (64)(004), 4.670 (100)(110, 104, 014), 2.626 (64)(032, 302), 2.435 (66)(034, 304) and 1.951 (45)(038, 308). The mineral is named in honour of the Swedish amateur mineralogist Dr. Anders Ersson (b. 1971).