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![[Figure 1]](ei5095fig1mag.jpg)
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Figure 1
Geometry of contact and penetration twins, viewed along the [010] direction of gypsum. Modified from the work of Rubbo et al. (2012a  ,b) with permission from the American Chemical Society. Twin laws: 100, 101, 001, 201 and 101. For each twin law, the re-entrant angle value (θ) and the optical extinction angle value (Δ) have been reported. The extinction angles (Δ) were measured having adopted the structure defined by De Jong & Bouman (1939). The steps followed to measure the extinction angles are reported in Figs. S2 and S3 of the supporting information. Subscripts `P' (parent) and `T' (twinned) identify the two individuals that make up the twin. |
![[Figure 1]](ei5095fig1.jpg)
 | JOURNAL OF APPLIED CRYSTALLOGRAPHY |
ISSN: 1600-5767
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