John O'Brien - Earth Materials

Because of its lower symmetry, the only crystal forms in the monoclinic crystal system are four‐sided prisms, two‐sided domes, sphenoids and pinacoids, and pedions. More complex forms cannot exist in systems with low symmetry in which some crystallographic axes do not all intersect at right angles. Common crystal forms and associated minerals in the monoclinic crystal system are depicted in Figure 4.29and Table 4.11.

Table 4.9 Common trigonal crystal forms and associated minerals.

Crystal forms	Form indices	Form description	Minerals that commonly exhibit crystal form
Rhombohedron	{h0h l}	Six parallelogram faces inclined to c‐axis	Dolomite, calcite, siderite, rhodochrosite, quartz, tourmaline; chabazite
Trigonal Scalenohedron	{hki l} and variations	12 scalene triangle faces inclined to c‐axis	Calcite
Trigonal prism	{hki 0} and variations	Three rectangular faces parallel to c‐axis	Tourmaline, calcite, quartz
Trigonal dipyramid	{hki l} and variations	Six triangular faces; top three separated from bottom three by a mirror plane	Tourmaline

Figure 4.28 Common crystal forms in the orthorhombic crystal system: (a) rhombic dipyramid, (b) front, side, and basal pinacoids, (c) rhombic prism with a pinacoid.

Table 4.10 Common crystal forms, form indices, and minerals in the orthorhombic 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 4.30and 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.

Table 4.12 Common crystal forms, form indices, and minerals in the triclinic system.

Figure 4.30 Triclinic crystal forms: (a) front, side, and basal pinacoids, (b) various pinacoids and a pedion to the lower right.

Many mineral specimens display twinned crystalsthat contain two or more related parts called twins. Twinshave 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 lawdescribes 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 twinsand 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 twinsbecause 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 twinsare 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).