THE STRUCTURE AND CRYSTAL CHEMISTRY OF HARKERITE AND SAKHAITE

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.