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

Dan Holtstam, Fernando Cámara, Andreas Karlsson
2021 Mineralogical magazine  
The margarosanite group (now officially confirmed by IMA-CNMNC) consists of triclinic Ca-(Ba, Pb) cyclosilicates with three-membered [Si 3 O 9 ] 6rings (3R), with the general formula AB 2 Si 3 O 9 , where A = Pb, Ba and Ca and B = Ca. A closest-packed arrangement of O atoms parallel to (101) hosts Si and B cations in interstitial sites in alternating layers. The 3R layer has three independent Si sites in each ring. Divalent cations occupy three independent sites: Ca in B occupies two
more » ... nt sites, Ca1 (8-fold coordinated), and Ca2 (6fold coordinated). A (=Ca3) is occupied by Pb 2+ (or Ba 2+ ) 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 PbCa 2 Si 3 O 9 ), walstromite (BaCa 2 Si 3 O 9 ) and breyite (CaCa 2 Si 3 O 9 ). So far, no solid solutions involving the Ca1 and Ca2 sites have been described. Therefore, root names depend on the composition of the Ca3 site only. Isomorphic replacement at the Ca3 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 range ca. 50-70% mol.% BaCa 2 Si 3 O 9 . The Pb-rich walstromite is associated closely with celsian, phlogopite, andradite, vesuvianite, diopside and nasonite. A crystal-structure refinement (R 1 = 4.8%) confirmed the structure type, and showed that the Ca3 (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.
doi:10.1180/mgm.2021.15 fatcat:e35f5u3ltzfhxp6drrtqqkmtcq