The högbomite polytypes

Abstract

Summary Single crystal X-ray examination has shown that högbomite forms a series of polytypes, designated nH or nR , with hexagonal or rhombohedral lattices and hexagonal unit-cell dimensions a 5·72 Å, c 4·6 × n Å. The polytypes arise by variation, in a manner as yet undetermined, of the stacking sequence of approximately close-packed oxygen layers with interstitial cations on fourfold and on sixfold sites; the composition of 1/ nt h of a unit-cell may be represented as R 2+ 1.0_1.6 T 4+ 0.2-0.4 R 3+ 3.7-4.3 O 2- 7.6-8.0 (OH) - 0-0.4 ,, where R 2+ = Zn, Fe, Mg, and R 2+ = Fe, Al. The polytypes so far observed are 4 H , 5 H , 6 H , 15 H , 15 R , and 18 R . Minerals structurally related to högbomite are nigerite (3 H ) and taaffeite (4 H ). A new occurrence of högbomite, polytype 5 H , with composition Ti 1·7 Fe 1·6 Mg 6·3 Al 18·8 Si 0·2 O 40 , is described from a spinel-free paragenesis in a magnesian skarn at Mautia Hill, Tanganyika. Another new occurrence in an aluminous xenolith in the Cashel gabbro in Co. Galway, Ireland, is recorded. X-ray powder data are given for two of the polytypes.