Twinning of Tetrahedrite—OD Approach

Emil Makovicky

doi:10.3390/min11020170

Abstract

The common twinning of tetrahedrite and tennantite can be described as an order–disorder (OD) phenomenon. The unit OD layer is a one-tetrahedron-thick (111) layer composed of six-member rings of tetrahedra, with gaps between them filled with Sb(As) coordination pyramids and triangular-coordinated (Cu, Ag). The stacking sequence of six-member rings is ABCABC, which can also be expressed as a sequence of three consecutive tetrahedron configurations, named α, β, and γ. When the orientation of component tetrahedra is uniform, the α, β, γ, α sequence builds the familiar cage structure of tetrahedrite. However, when the tetrahedra of the β layer are rotated by 180° against those in the underlying α configurations and/or when a rotated α configuration follows after the β configuration (instead of γ), twinning is generated. If repeated, this could generate the ABAB sequence which would modify the structure considerably. If the rest of the structure grows as a regular cubic tetrahedrite structure, the single occurrence of the described defect sequences creates a twin.

Highlights

The well-known twinning of tetrahedrite–tennantite (Figure 2) has been repeatedly described by those twin elements which constitute the difference between described by those twin elements which constitute the difference between the holohedral cubic point-group symmetry 4/m −3 2/m and the point group symmetry of the holohedral cubic point-group symmetry 4/m −3 2/m and the point group symmetry of tetrahedrite, which is −43m, as demonstrated by its morphology (Figure 2)
The model of twinning described here isinbased on order–disorder occur180°-rotated version of the ring when thestructure
Tetrahedrite structure grows as a sequence of incremental (111) layers which are one tetrahedron thick

Summary

Introduction

The principal complication of the chemistry of this solid solution, the interplay of Fe3+ and Fe2+ in tetrahedrite and tennatite, has been studied by several authors (e.g., Makovicky et al [5,6]; Andreasen et al [7]; Nasonova et al [8]). It does not alter the crystal structure principles of these minerals. Note filling of truncated-tetrahecavities in the tetrahedral framework by “spinners” composed of triangular co-ordinations.

Twinning of Tetrahedrite

The Untwinned Layer Sequence

Twin Symmetry in OD Description

Penetration Twins

Conclusions