Extremely Pb-rich rock-forming silicates including a beryllian scapolite and associated minerals in a skarn from Långban, Värmland, Sweden

Christy, A. G. and Gatedal, K. (2005) Extremely Pb-rich rock-forming silicates including a beryllian scapolite and associated minerals in a skarn from Långban, Värmland, Sweden. Mineralogical Magazine, 69 6: 995-1018. doi:10.1180/0026461056960304


Author Christy, A. G.
Gatedal, K.
Title Extremely Pb-rich rock-forming silicates including a beryllian scapolite and associated minerals in a skarn from Långban, Värmland, Sweden
Journal name Mineralogical Magazine   Check publisher's open access policy
ISSN 0026-461X
1471-8022
Publication date 2005-01-01
Sub-type Article (original research)
DOI 10.1180/0026461056960304
Open Access Status Not yet assessed
Volume 69
Issue 6
Start page 995
End page 1018
Total pages 24
Place of publication Middlesex, United Kingdom
Publisher Mineralogical Society
Language eng
Formatted abstract
We report preliminary petrographic and mineral chemical data for a rock hosting an unusual mineral assemblage from Långban, Värmland, Sweden. The rock is a two feldspar-scapolite-spessartine-romeite skarn. The bulk composition and high degree of enrichment in Pb, Sb and As suggest that the rock was formed by reaction between a pre-existing Mn skarn containing the chalcophiles and a potassic granite, with loss of silica, alkalis and CO2. The alkali feldspar is a Pb-rich hyalophane, averaging Or63Ab19Cs15Pb03, the plagioclase feldspar a Pb-rich labradorite, An48Ab48Or02Pb02, and the scapolite a 'mizzonite' (Ca/(Na+Ca) = 0.66-0.70). These minerals show their highest Pb contents recorded in nature to date: up to a maximum of 5.7 wt.% PbO in the hyalophane, 2. 1 % PbO in the plagioclase, and 5.3% PbO in the scapolite. Laser ablation ICP-MS of a scapolite grain detected substantial Be up to 1.7 wt.% BeO (0.6 Be per 12 tetrahedral cations), as well as Pb up to 7.05 wt.% PbO. The Be is incorporated into scapolite via the coupled exchange [Be(OH)] [A1(CO3, SO4)]-1. This is the first documentation of scapolite as the major repository for Be in a rock. The romeite also contains substantial Pb, and shows extensive solid solution towards end-members containing Fe3+, Ti and Sb3+ .In some analyses, the dominant end-members are Ca2(Fe0.53+Sb1.55+)O6(OH) and its Pb analogue rather than (Ca,Pb)2Sb2O7. Complex exsolution textures are displayed in the hyalophane, by hancockite-epidote, romeite-bindheimite and hedyphane-johnbaumite. Ca-rich scapolite and hancockite appear to be new minerals for the Långban deposit. The mineralogy appears consistent with the regional peak conditions of P = 3 kbar, T > 600°C. Several potential thermobarometers for Mn-rich skarns are identified in this rock.
Keyword Hancockite-epidote
Hedyphane-johnbaumite
Hyalophane (plumboan)
Långban
Plagioclase (plumboan)
Romeite-bindheimite
Scapolite (beryllian and plumboan)
Skarn
Sweden
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Non-UQ

Document type: Journal Article
Sub-type: Article (original research)
Collection: School of Earth and Environmental Sciences
 
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