PREPUBLICATION: Chemographic exploration of the hyalotekite structure-type

The hyalotekite group has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (memorandum 57-SM/16). The general formula of the minerals of the hyalotekite group may be written as: A(2)B(2)M(2)[Si8T4O28]W where A = Ba2+, Pb2+ or K+; B = Ba2+, Pb2+ or K+; M = Ca2+, Y3+ or REE3+; T = Si4+, B(3+ )or Be2+; and W = F or square (where REE = rare-earth elements and square = vacancy). Four minerals are currently known in this group: hyalotekite, Ba4Ca2[Si8B2(SiB)O-28]F, triclinic, I (1) over bar; khvorovite, Pb42+Ca2[Si8B2(SiB)O-28]F, triclinic I (1) over bar; kapitsaite-(Y), Ba-4(YCa)[Si8B2B2O28]F, triclinic, I (1) over bar; and itsiite Ba4Ca2[Si8B4O28]square, tetragonal, I (4) over bar 2m. We explore the possible end-member compositions within this group by conflating the properties of an end-member with the stoichiometry imposed by the bond topology of the hyalotekite structure-type and the crystal-chemical properties of its known constituents. There are two high-coordination sites in the hyalotekite structure, A and B, and occupancy of each of these sites can be determined only by crystal-structure refinement. If these two sites are considered together. there are 19 end-member compositions of the triclinic structure and six end-member compositions of the tetragonal structure involving A and B = Ba2+, Pb2+, K+; M = Ca2+, Y3+, REE3+; and T = Si4+, B3+, Be2+. There is the possibility for many other hyalotekite-group minerals, and two potential new minerals have been identified from data in the literature.