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Figure 7
Theoretical morphology of the penetration (a) and contact (d) twins of aragonite. The three individuals involved are encoded by color; noncrystallographic limits between individuals are shown as dashed lines. The characteristic angles of these pseudohexagonal prisms are shown in (b) and (e), respectively: φ = 63.824°, χ = 58.088°, ω = 52.352°. Note that penetration twins produce concavities in two opposite faces, and a larger spread of interfacial angle measurements for the pinacoid/prism angle (because the `pinacoid' surface is not a real crystallographic face). In contrast, the contact twin produces protruding spurs (f).

Journal logoJOURNAL OF
APPLIED
CRYSTALLOGRAPHY
ISSN: 1600-5767
Volume 51| Part 4| August 2018| Pages 1050-1058
https://doi.org/10.1107/S1600576718007343
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