HYALOTEKITE, (BA,PB,K)(4)(CA,Y)(2)SI-8(B,BE)(2)(SI,B)(2)O28F, A TECTOSILICATE RELATED TO SCAPOLITE - NEW STRUCTURE REFINEMENT, PHASE-TRANSITIONS AND A SHORT-RANGE ORDERED 3B SUPERSTRUCTURE

Hyalotekite, a framework silicate of composition (Ba,Pb,K)(4)(Ca,Y)(2) Si-8(B,Be)(2) (Si,B)(2)O28F, is found in relatively high-temperature(g reater than or equal to 500 degrees C) Mn skarns at Langban, Sweden, a nd peralkaline pegmatites at Dara-i-Pioz, Tajikistan. A new paragenesi s at Dara-i-Pioz is pegmatite consisting of the Ba borosilicates leuco sphenite and tienshanite, as well as caesium kupletskite, aegirine, py rochlore, microcline and quartz. Hyalotekite has been partially replac ed by barylite and danburite. This hyalotekite contains 1.29-1.78 wt.% Y2O3, equivalent to 0.172-0.238 Y pfu or 8-11% Y on the Ca site; its Pb/(Pb+Ba) ratio ranges 0.36-0.44. Electron microprobe F contents of L angban and Dara-i-Pioz hyalotekite range 1.04-1.45 wt.%, consistent wi th full occupancy of the F site. A new refinement of the structure fac tor data used in the original structural determination of a Langban hy alotekite resulted in a structural formula, Ba1.86K0.18)Ca-2(B1.76Be0. 24)(Si1.56B0.44)Si8O28F, consistent with chemical data and all cations with positive-definite thermal parameters, although with a slight exc ess of positive charge (+57.14 as opposed to the ideal +57.00). An unu sual feature of the hyalotekite framework is that 4 of 28 oxygens are non-bridging; by merging these 4 oxygens into two, the framework topol ogy of scapolite is obtained. The triclinic symmetry of hyalotekite ob served at room temperature is obtained from a hypothetical tetragonal parent structure via a sequence of displacive phase transitions. Some of these transitions are associated with cation ordering, either Pb-Ba ordering in the large cation sites, or B-Be and Si-B ordering on tetr ahedral sites. Others are largely displacive but affect the coordinati on of the large cations (Pb, Ba, K, Ca). High-resolution electron micr oscopy suggests that the undulatory extinction characteristic of hyalo tekite is due to a fine mosaic microstructure. This suggests that at l east one of these transitions occurs in nature during cooling, and tha t it is first order with a large volume change. A diffuse superstructu re observed by electron diffraction implies the existence of a further stage of short-range cation ordering which probably involves both (Pb ,K)-Ba and (BeSi,BB)-BSi.