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

摘要

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)][Al(CO₃,SO₄)]_–1. This is the first documentation of scapolite as the major repository for Be in a rock.