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Figure 4
Schematic presentation of (theoretically) fully developed coplanar and alternating twins in rutile and cassiterite. (a,b) In coplanar twins, the maximum number of subsequent twin boundaries in both minerals is five and the cycle is closed by a non-crystallographic contact (NCC). The co-planar c-axes of twin domains are indicated by arrows below the projections along [010]. (c,d) In an alternating variant of cyclic twins, perfect eightlings can develop in rutile due to the almost exactly 45° angle between edge-on oriented {101} planes of the domains along the [111] zone axis; in cassiterite, however, the equivalent angle is 46.5°, allowing for a maximum of seven subsequent {101} twin boundaries and an additional NCC. In this type of twin, the c-axes of twin domains alternate up and down as shown below the projections along [111].

ISSN: 2052-5206
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