114 Mineralogy/Crystallography (Posters)

Monday, 20 October 2014
9:00 AM - 6:30 PM
Exhibition Hall C

THE STRUCTURE AND CRYSTAL CHEMISTRY OF HARKERITE AND SAKHAITE

Paper number: 114-17
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Harkerite, c. Ca48Mg16[AlSi4(O,OH)16]4(BO3)16(CO3)16(H2O, HCl)2, and sakhaite, c. Ca48Mg16(BO3)32(CO3)16(H2O, HCl)2, are rare borate-carbonate minerals typically occurring in high-temperature, low-pressure calcareous skarns, but harkerite has also been found in a deep-seated, granulite facies complex. The structure of harkerite and sakhaite is based on a cubic framework of 6-coordinated Mg sites and 8-to-10-coordinated Ca sites. This framework is cross-linked by triangular borate and carbonate groups. In harkerite, up to half of the total borate is replaced by aluminosilicate pentamers, Al(SiO4)4, which replace groups of four disjoint borate triangles, (BO3)4. Substitution of additional CO3 for BO3 of up to 4 per formula unit has been reported in both sakhaite and harkerite. Interstitial H2O or HCl occur within the cavities of some (BO3)4 groups, from 2 per formula unit up to the highest reported of 15 per formula unit. Overall composition of the sakhaite-harkerite series can be described by the general formula

Ca48Mg16[AlSi4(O,OH)16]x(BO3)32–4x–y(CO3)16+y–w(OH)2w(H2O,HCl)z.

Compositions close to end-member sakhaite (up to x = 1.5) have cubic symmetry (space group Fd-3m) with a unit cell size a = 14.7 Å. End-member harkerite (x = 4) has rhombohedral symmetry (space group R-3m) with unit cell a = 10.4 Å, c = 51.3 Å, due to ordering of the Al(SiO4)4 and (BO3)4 groups. The 3-fold rotational axis of the rhombohedral harkerite unit cell corresponds to a doubling of the body diagonal in the cubic sakhaite unit cell. Disorder of the borate, silicate, carbonate and interstitial H2O/HCl groups leads to strong cubic pseudosymmetries in harkerite and intermediate sakhaite-harkerite compositions.

In studying the crystal chemistry of the sakhaite-harkerite series, we intend to characterize symmetry and crystal structures across the series, study the roles of OH, H2O, and HCl molecules in the crystal structure, and fully define harkerite.