Instalment of the margarosanite group, and data on walstromite–margarosanite solid solutions from the Jakobsberg Mn–Fe deposit, Värmland, Sweden

Abstract

AbstractThe margarosanite group (now officially confirmed by IMA-CNMNC) consists of triclinic Ca-(Ba, Pb) cyclosilicates with three-membered [Si3O9]6–rings (3R), with the general formulaAB2Si3O9, whereA= Pb, Ba and Ca andB= Ca. A closest-packed arrangement of O atoms parallel to (101) hosts Si andBcations in interstitial sites in alternating layers. The 3R layer has three independent Si sites in each ring. Divalent cations occupy three independent sites: Ca inBoccupies two nonequivalent sites,Ca1 (8-fold coordinated), andCa2 (6-fold coordinated).A(=Ca3) is occupied by Pb2+(or Ba2+) in 6+4 coordination, or 6+1 when occupied by Ca; this third site occurs within the 3R-layer in a peripheral position. Three minerals belong to this group: margarosanite (ideally PbCa2Si3O9), walstromite (BaCa2Si3O9) and breyite (CaCa2Si3O9). So far, no solid solutions involving theCa1 andCa2 sites have been described. Therefore, root names depend on the composition of theCa3 site only. Isomorphic replacement at theCa3 sites has been noted. We here report data on a skarn sample from the Jakobsberg Mn–Fe oxide deposit, in Värmland, Sweden, representing intermediate compositions on the walstromite–margarosanite binary, in the rangeca.50–70% mol.% BaCa2Si3O9. The Pb-rich walstromite is associated closely with celsian, phlogopite, andradite, vesuvianite, diopside and nasonite. A crystal-structure refinement (R1= 4.8%) confirmed the structure type, and showed that theCa3 (Ba, Pb) site is split into two positions separated by 0.39 Å, with the Ba atoms found slightly more peripheral to the 3R-layers.