Triclinic crystal system
The only crystal forms in the triclinic system, with its extremely low symmetry, are pinacoids and pedions. Common forms and minerals in the triclinic system are illustrated in Figure 4.30 and listed in Table 4.12.
Figure 4.29 Monoclinic crystal forms: (a) front, side, and basal pinacoids, (b) two monoclinic prisms and a side pinacoid.
Table 4.11 Common crystal forms, form indices, and minerals in the monoclinic system.
| Crystal form | Form indices | Form description | Minerals that commonly exhibit crystal form |
|---|---|---|---|
| Monoclinic prisms; first, third, and fourth orders | {011} {0kl} {110} {hk0} {hkl} | Four rectangular faces | Gypsum, staurolite, clinopyroxenes, amphiboles, orthoclase, sanidine, sphene (titanite), borax |
| Pinacoids; front, side, and basal | {001} {010} {001} | Pair of rectangular faces perpendicular to a‐, b‐, or c‐axis | Gypsum, staurolite, sphene (titanite), epidote, micas, clinopyroxenes, amphiboles |
Table 4.12 Common crystal forms, form indices, and minerals in the triclinic system.
| Crystal forms | Form indices | Form description | Minerals that commonly exhibit crystal form |
|---|---|---|---|
| Pinacoids | {001}{010}{001} {0k1} {hk1} and variations | Two parallel faces | Kyanite, plagioclase, microcline, amblygonite, rhodonite, wollastonite |
| Pedions | {hk1} | Single face | Similar |
Figure 4.30 Triclinic crystal forms: (a) front, side, and basal pinacoids, (b) various pinacoids and a pedion to the lower right.
4.7 TWINNED CRYSTALS
Many mineral specimens display twinned crystals that contain two or more related parts called twins. Twins have the following characteristics: (1) they possess different crystallographic orientations, (2) they share a common surface or plane, and (3) they are related by a symmetry operation such as reflection, rotation or inversion (Figure 4.31). Because twins are related by a symmetry operation, twinned crystals are not random intergrowths.
A twin law describes the symmetry operation that produces the twins and the plane (hkl) or axis involved in the operation. For example, swallowtail twins in gypsum (Figure 4.31a) are related by reflection across a plane (001), which is not a mirror plane in single gypsum crystals. Carlsbad twins in potassium feldspar (Figure 4.31f) are related by a twofold axis of rotation that is parallel to the c‐axis (001), which is not a rotational axis in single potassium feldspar crystals.
The surfaces along which twins are joined are called composition surfaces. If the surfaces are planar, they are called composition planes, which may or may not be equivalent to twin planes. Other composition surfaces are less regular. Twins joined along composition planes are called contact twins and do not appear to penetrate one another. Good examples of contact twins are shown in Figure 4.31a and b. Twins joined along less regular composition surfaces are usually related by rotation and are called penetration twins because they appear to penetrate one another. Good examples of penetration twins are shown in Figure 4.31c–f.
Twinned crystals that contain only two twins are called simple twins, whereas multiple twins are twinned crystals that contain more than two twins. If multiple twins are repeated across multiple parallel composition planes, the twins are called polysynthetic twins. Polysynthetic albite twins (Figure 4.31b) are repeated by the albite twin law, reflection across (010), and are very common in plagioclase. They produce small ridges and troughs on the cleavage surfaces of plagioclase, which the eye detects as linear striations – a key to hand‐specimen identification of plagioclase. They also produce the characteristic “zebra stipe” pattern observed in many plagioclases when viewed under crossed polars with a petrographic microscope (Chapter 6).
Figure 4.31 Examples of twinned crystals: (a) swallowtail twins in gypsum; (b) polysynthetic albite twins in plagioclase; (c) penetration twins in galena; (d) penetration twins in pyrite; (e) penetration twins in staurolite; (f) Carlsbad twins in potassium feldspar.
Source: Wenk and Bulakh (2004). © Cambridge University Press.
Most twins are growth twins that form during mineral crystallization. Less commonly, twins result from displacive mineral transformations or from deformation. Deformation twins are called mechanical twins. The common mineral calcite typically develops mechanical twins (102) during deformation, and their development can play a significant role in the deformation of calcitic marbles and other metamorphic rocks, especially at low temperatures. This helps to explain why marbles deform more plastically than many other rocks during lower grades of metamorphism