HYALOTEKITE FROM REEDMERGNERITE-BEARING PERALKALINE PEGMATITE, DARA-I-PIOZ, TAJIKISTAN AND FROM MN SKARN, LANGBAN, VARMLAND, SWEDEN - A NEWLOOK AT AN OLD MINERAL

In specimens of Mn skarn from the type locality of Langban, hyalotekit e, (Ba,Pb,K)4Ca2-(Si,B,Be)12O28F, occurs in a matrix consisting mostly of aegirine (less-than-or-equal-to 22 mol.% CaMnSi2O6), andradite (le ss-than-or-equal-to 27 mol.% Mn3Fe2Si3O12), hematite, pectolite, quart z, calcite, baryte, barylite, and hedyphane. Melanotekite, plumbian ta ramellite, ferrian K-feldspar (to 6.5 wt.% Fe2O3), rhodonite, a talc-l ike mineral, apophyllite, and several Pb-As-Sb-O minerals are found in trace amounts. In a single specimen of reedmergnerite-microcline pegm atite from Dara-i-Pioz, hyalotekite occurs in close association with m icrocline. Other accessory minerals are albite, aegirine, pyrochlore, eudialyte, and polylithionite. The optical constants for hyalotekite f rom Langban and Dara-i-Pioz are, respectively, alpha = 1.656, 1.646, b eta = 1.659-1.660, 1.649, gamma = 1.670-1.671, 1.659 (all +/- 0.002), 2Vgamma (meas) = 57.2-60.5 +/- 0.5-degrees, 57.0 +/- 1.1-degrees (lamb da = 589 nm). Cell parameters of the Dara-i-Pioz hyalotekite for a bod y-centred triclinic cell are: a = 11.284(2), b = 10.930(1), c = 10.272 (8) angstrom, alpha = 90.35(2)-degrees, beta = 90.11(3)-degrees, gamma = 89.98(1)-degrees. Electron and ion microprobe data show that Langba n hyalotekite is heterogeneous even within a given sample; the most im portant substitutions are Pb = Ba, K and coupled B = Si and B = Be. Mi nor constituents include Mn in the Langban hyalotekite and Na in the D ara-i-Pioz hyalotekite, which also differs in its significantly higher Ba/Pb ratio. Conditions suggested for hyalotekite formation at Langba n and Dara-i-Pioz are P less-than-or-equal-to 4 kbar, T greater-than-o r-equal-to 500-degrees-C, silica saturation, peralkalinity, and relati vely high oxygen fugacities and low sulphur fugacities. These temperat ures must have been sufficiently high to allow for miscibility of Pb w ith Ba (and K) despite the lone pair of electrons of Pb2+.