Hyalotekite from Reedmergnerite-Bearing Peralkaline Pegmatite, Dara-i-Pioz, Tajikistan and from Mn skarn, Långban, Värmland, Sweden: A New Look at an Old Mineral

Edward S. Grew, Martin G. Yates, Dimitriy I. Belakovskiy, Roland C. Rouse, Shu-Chun Su and Nicholas Marquez
Department of Geological Sciences, 5711 Boardman Hall, University of Maine, Orono, Maine 04469-5711, U.S.A.
A. E. Fersman Mineralogical Museum, Russian Academy of Sciences, Leninskiy Prospekt, 18(2), Moscow, Russia 117071
Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063, U.S.A.
Hercules Inc. Research Center, Wilmington, Delaware 19894, U.S.A.
Aerospace Corporation, PO Box 92957, Los Angeles, California 90009, U.S.A.

Abstract: In specimens of Mn skarn from the type locality of Långban, hyalotekite, (Ba,Pb,K)4Ca2− (Si,B,Be)12O28F, occurs in a matrix consisting mostly of aegirine (⩽22 mol.% CaMnSi2O6), andradite (⩽27 mol.% Mn3Fe2Si3O12), hematite, pectolite, quartz, calcite, baryte, barylite, and hedyphane. Melanotekite, plumbian taramellite, ferrian K-feldspar (to 6.5 wt.% Fe2O3), rhodonite, a talc-like mineral, apophyllite, and several Pb-As-Sb-O minerals are found in trace amounts. In a single specimen of reedmergnerite-microcline pegmatite from Dara-i-Pioz, hyalotekite occurs in close association with microcline. Other accessory minerals are albite, aegirine, pyrochlore, eudialyte, and polylithionite. The optical constants for hyalotekite from Långban and Dara-i-Pioz are, respectively, α = 1.656, 1.646, β = 1.659–1.660, 1.649, γ = 1.670–1.671, 1.659 (all ± 0.002), 2Vγ (mcas) = 57.2–60.5 ± 0.5°, 57.0 ± 1.1°(λ = 589 nm). Cell parameters of the Dara-i-Pioz hyalotekite for a body-centred triclinic cell are: a = 11.284(2), b = 10.930(I), c = 10.272(8) Å, α = 90.35(2)°, β = 90.11(3)°, γ = 89.98(1)°. Electron and ion microprobe data show that Långban hyalotekite is heterogeneous even within a given sample; the most important substitutions are Pb = Ba, K and coupled B = Si and B = Be. Minor constituents include Mn in the Långban hyalotekite and Na in the Dara-i-Pioz hyalotekite, which also differs in its significantly higher Ba/Pb ratio. Conditions suggested for hyalotekite formation at Långban and Dara-i-Pioz are P ⩽ 4 kbar, T ⩾ 500°C, silica saturation, peralkalinity, and relatively high oxygen fugacities and low sulphur fugacities. These temperatures must have been sufficiently high to allow for miscibility of Pb with Ba (and K) despite the lone pair of electrons of Pb2+.

Keywords: hyalotekite • metamorphism • peralkaline pegmatite • Mn skarn • Långban • Sweden • Dara-i-Pioz • Tajikistan

Mineralogical Magazine; June 1994 v. 58; no. 391; p. 285-297; DOI: 10.1180/minmag.1994.058.391.11
© 1994, The Mineralogical Society
Mineralogical Society (www.minersoc.org)