What are the different types of crystal imperfections ?

CRYSTAL IMPERFECTIONS: DEFECTS

A hypothetical perfect crystal has a perfectly ordered atomic structure with all atoms in the correct places, and it contains no elements other than those described by its chemical formula. As pointed out by C. G. Darwin in 1914, such crystals cannot exist. While a crystal may look perfect on the outside, atomic structures always contain some flaws, called defects. 

 In 1921 A. R. Griffiths showed that the low strength of many crystals was due to structural defects. Subsequently, mineral physicists found that plastic deformation of crystals always depends on the presence of defects. During the last few decades, advanced X-ray, transmission electron microscope (TEM), and, most recently, high-resolution transmission electron microscope (HRTM) studies have allowed mineralogists to image such defects (Figure 6). 

FIGURE 6 An image of the atomic structure of crocidolite, an asbestiform amphibole, obtained with a transmission electron microscope. The black and white colors indicate atomic units composed of a small number of atoms. The entire view shows an imperfect grain composed of multiple subgrains with slightly different atomic orientations, as shown by the letters and vectors labeling crystallographic axes. Zipper faults show lines along which the atomic structure is defective. In some places, especially along subgrain boundaries, a coarsening of texture indicates small areas that have atomic structure dissimilar from that of normal crocidolite. Photo courtesy of the Mineralogical Society of America and American Mineralogist from “Microstructures and fiberformation mechanisms of crocidolite asbestos” by Jung Ho Ahn and Peter R. Buseck, American Mineralogist, 76 (1991), 1467–1478.

No mineral is perfectly pure; minerals always contain minor or trace amounts of elements not described by their formula, often at levels that we cannot detect using standard analytical techniques. Perhaps the simplest type of defect is an impurity defect, occurring when a foreign atom is present in a mineral’s atomic structure. The atom may replace one normally in the structure, or it may occupy an interstitial site (Figure 7a). We call both point defects because they occur at one or a few points in the structure. Other types of point defects are Schottky and Frenkel defects (Figure 7b and c). Frenkel defects occur when an atom is displaced from the position it normally occupies to an interstitial site. Frenkel defects may affect both cations and anions, but cation defects are more common because anions are larger and usually more securely bonded in place. Schottky defects occur when an atom is displaced from a structure altogether. Schottky defects involve both cations and anions. To maintain charge neutrality, missing anions must be compensated by missing cations.

Other types of defects include line defects and plane defects (Figure 7d). Line defects, such as edge dislocations and screw dislocations are defects that end at lines in a structure. Plane defects, as their name implies, are planes along which a crystal’s structure is displaced or distorted. On a macroscopic scale, grain boundaries can be considered plane defects. At the atomic level, irregular lineage structures may separate slightly misoriented portions of a crystal structure or, in layered atomic structures, stacking faults may separate layers that are out of order. Because of defects, many crystals consist of domains of slightly different atomic orientation (Figure 6).

FIGURE 7 Diagram illustrating different types of crystal defects: (a) impurity defects occur when a foreign atom (gray) replaces one normally in a crystal structure; (b) Schottky defects occur when atoms are missing from a structure; (c) Frenkel defects occur when atoms are displaced to interstitial sites not normally occupied; (d) edge dislocations occur when a plane of atoms in a structure terminates at a line in the crystal’s interior.

CRYSTAL IMPERFECTIONS: ZONING

Crystallizing magmas may produce uneven mineral distribution (Plate 1.4). On a smaller scale, individual minerals develop compositional zoning if different parts of a mineral have different compositions. Zoning is present in many minerals but often on such a small scale that we have difficulty detecting it. Occasionally, zoning results in distinct color changes, such as those shown in Plates 1.1 and 1.2. In many cases zoning can be seen with a petrographic microscope because zones of different composition have different optical properties. In still other cases detailed chemical analyses are needed to detect its presence. 

Most zoning is an artifact of crystal growth. It may result from changes in pressure or temperature during crystallization. It may also result from changes in magma or fluid composition as crystals grow. The principles of thermodynamics dictate that zoned minerals are unstable, but they are common in nature because diffusion of elements is often not fast enough for growing minerals to remain homogeneous. Most zoning is concentric, representing growth rings about an original crystal seed. In some minerals it is more complex and results in compositional zones that are difficult to explain and interpret.

CRYSTAL IMPERFECTIONS: TWINNING 

In ideal crystals, atoms are in repetitive arrangements that are oriented the same way in all parts of the crystal. Twins result when different domains of a crystal have different atomic orientations. The domains share atoms along a common surface, typically a plane called the composition plane. Twins differ from crystal intergrowthscomposed of crystals that grow next to each other. In a twinned crystal the structure and bonds continue across the composition plane; in intergrowths they are discontinuous. Simple twins comprise two domains that share common planes of atoms that separate them. Complex twinscomprise more than two individual domains. 

Figure 8a–f show some simple twins composed of two individual domains. In some the twins appear to be crystals in contact; we call these contact twins. In others (Figure 8a, d, and e), the crystals appear to have grown through each other. In such twins, called penetration twins, a volume of atoms is shared by both domains, not just a plane. Plate 1.5 shows orthoclase crystals with penetration twins; compare this plate with Figure 8e. Figure 8g shows another type of twinning, polysynthetic twinning,composed of alternating domains. Such twins are common in plagioclase, calcite, and some other minerals but usually cannot be seen without a microscope. In Na-rich plagioclase they may result in fine lamellae on crystal faces that can be seen with the naked eye. The presence of twin lamellae is one way to tell plagioclase from K-feldspar in hand specimen. Figure 8hshows cyclic twinning, which is rare but sometimes spectacular. Some minerals twin to form domains with irregular shapes and boundaries. Quartz is a good example, but the twins in most quartz crystals are difficult tosee. 

Different types of twins in different minerals are given names, referred to as twin laws.The laws refer to the way twin domainsare related. Feldspars exhibit many twin laws. K-feldspars often contain simple twins related by the Carlsbad, Baveno, albite,or pericline laws. Plate 1.5 shows orthoclase twinned according to the Carlsbad Law. Albite and pericline twinning combine to give the characteristic Scotch-plaid pattern that can be seen when microcline is viewed under crossed polars with a petrographic microscope. Plagioclase may twin according to the same laws as orthoclase, and according to the albite law. Albite and pericline twins are often polysynthetic and give plagioclase a characteristic zebra stripe appearance when viewed under crossed polars with a petrographic microscope. 

FIGURE 8 Examples of different types of twinning (and a mineral in which it occurs): (a) penetration twin (fluorite); (b) contact twin (spinel); (c) contact twin (cassiterite); (d) penetration twin (staurolite); (e) penetration twin (orthoclase); (f) contact swallowtail twin (gypsum); (g) polysynthetic twin (plagioclase); and (h) cyclic twin (spinel).

Twinning comes at all scales and may be difficult to detect. Sometimes we can see it with the naked eye, sometimes we can only see it with a microscope, and sometimes we can’t detect it without more sophisticated devices. Identifying twins in hand specimens can be difficult, especially in poorly formed crystals. One diagnostic feature is the presence of reentrant angles, visible in Figure 8a–h. Two crystal faces intersect to form a reentrant angle when they produce an angular concavity that points toward the interior of a crystal instead of the (normal) exterior.