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

Abstract

The 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 formula AB2Si3O9, 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 nonequivalent sites, Ca1 (8-fold coordinated), and Ca2 (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 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.% 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 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.

Zoisite-(Pb), a New Orthorhombic Epidote-Related Mineral from the Jakobsberg Mine, Värmland, Sweden, and Its Relationships with Hancockite

“…Walstromite from Jakobsberg is associated with celsian, phlogopite, andradite, vesuvianite, diopside, and nasonite. In [29] it was noticed that celsian associated to walstromite lacks any Pb content, reported as below the EDS detection limit (≤0.2% PbO), in contrast to the relevant Pb contents found both in celsian from Jakobsberg (present study) and from Långban [27].…”

“…Mineral chemistry of our associated phases is strongly related to those found by [26][27][28] on Långban phases. In the recent definition of the margarosanite group [29], the crystal chemical relationships between the lead silicate margarosanite, ideally PbCa 2 Si 3 O 9 , walstromite (BaCa 2 Si 3 O 9 ), and breyite (CaCa 2 Si 3 O 9 ) are described. The occurrence of the lead silicate walstromite in a skarn rock from Jakobsberg is reported, with a complete single-crystal, EPMA, and Raman study of the mineral.…”

Crystal Structure Evolution of CaSiO3 Polymorphs at Earth’s Mantle Pressures

“…CaSiO3 polymorphism is of particular interest for their peculiar crystal chemical features. The geometrical features observed in chain and ring-structured CaSiO3 minerals present in fact clear analogies with the predicted and observed features in tetrahedrallystructured ultra-high-pressure carbonates [8]. In the past, due to analytical difficulties to study the occurrence of phase transformation in silicates at extreme conditions, the investigation of germanates has been adopted as structural analogues of silicates at high pressures [9].…”

“…We performed experiments on pure CaSiO 3 compositions, but we may expect that the observed variations are compatible with compositions other than pure CaSiO 3 . These observations can be likely transferred to possible analogue topologies, such as the predicted and observed high-pressure carbonate structures at deep-mantle conditions, above 80 GPa [8], where the carbon cation changes its coordination from three to four [13,14,56,57]. To date, no systematic investigations have been performed on high-pressure carbonates due to difficulties in synthesis of minerals at such extreme conditions and the limited accuracy of both theoretical and, especially, experimental studies at Mbar conditions.…”

Crystal chemistry and Raman spectroscopy study of bennesherite, Ba2Fe2+Si2O7, and rare accessory Ba minerals from Caspar quarry, Bellerberg volcano, Germany

“…That is related to the mineral association because, in the Jakobsberg deposit, walstromite occurs together with margarosanite. As the authors noted, both minerals contain variable Ba-Pb amounts, indicating a solid solution series between them (Holtstam et al, 2021).…”