Caryinite revisited

Abstract

AbstractCaryinite from Långban, Sweden is associated with barite, richterite, manganberzeliite, hedyphane, sarkinite or eveite, and an exsolved fermorite-like mineral. The crystal structure of caryinite, space group I2/a, a 6.855(2), b 13.147(3), c 11.479 (4) Å, β 98.97°, V 1022.0(5) Å3, has been refined to a conventional R = 3.5% using 1295 observed [F > 3α(F)] reflections. Caryinite is isostructural with alluaudite and shows the following key features: (1) Pb is ordered at X2, Mg is ordered at M2, Mn is disordered over M1 and M2, Ca is disordered over M1 and X1 and Na is disordered over X1 and X2, and (2) the X1 and 02 atoms of the X1 polyhedron show positional disorder. With the above site preferences taken into consideration, the assignment rules of Moore and Ito (1979) can now be used to accurately predict site occupancies for caryinites and arseniopleites using chemical data only. On the basis of the general structural formula X2X1M1M22[AsO4]3 (Z = 4), caryinite is (Na, Pb) (Ca, Na) Ca (Mn, Mg)2 [ASO4]3; the analogous formula for arseniopleite is Na(Ca,Na)Mn(Mn,Mg)2[AsO4]3. Carinite has Ca dominant at M1; by analogy, arseniopleite should have Mn dominant at M1.