The unit-cell of aenigmatite

Abstract

Summary: Redetermination of the unit-cell dimensions as a 10·406 Å, b 10·813 Å, c 8·926 Å, α 104° 56′, β 96° 52′, γ 125° 19′, has shown that the hitherto accepted unit-cell of Gossner and Spielberger (1929) is in error. The morphological unit-cell of Palache (1933) is related to the new unit-cell, to a high degree of precision, by the transformation Palache → Kelsey-MeKie [001∕01½½∕½00]. Fifteen published and two new analyses indicate that the unit-cell contents approach Na⁺₄ [Fe^2-₁₀- Ti⁴⁺₂] Si⁴⁺₁₂O^2-₄₀. The common twin axis constitutes the diad of a pseudo-monoclinic cell, the (010) plane of which is dimensionally closely related to that of pyroxenes and amphiboles. It is suggested that the aenigmatite structure is based on silicate chains of pyroxene type cross-linked by Ti⁴⁺ and Fe²⁺ on distinct 6-fold sites, with Na⁺ on sites of higher coordination number. Aenigmatite from volcanic in contrast to that from plutonic parageneses is often submicroscopically twinned and may be inverted from a high temperature monoclinic polymorph.