Distinguish between compositional and structural variations in minerals and provide one example of each.
Many people associate the word crystal with delicate wine goblets or glassy objects with smooth sides and gem-like shapes. In geology, the term crystal or crystalline refers to any natural solid with an orderly, repeating internal structure.
The smooth faces and symmetry possessed by well-developed crystals are surface manifestations of the orderly packing of the atoms or ions that constitute a mineral’s internal structure. This highly ordered atomic arrangement within minerals can be illustrated by using spherically shaped atoms held together by ionic, covalent, or metallic bonds. The simplest crystal structures are those of native metals, such as gold and silver, which are composed of only one element. These materials consist of atoms packed together in a rather simple three-dimensional network that minimizes voids. Imagine a group of cannon balls stacked in layers such that the spheres in one layer nestle in the hollows between spheres in the adjacent layers.
The atomic structure of most minerals consists of at least two different ions (often of very different sizes).
Figure 1 illustrates the relative sizes of some of the most common ions found in minerals. Notice that the negative ions, which are atoms that gained electrons, tend to be larger than the positive ions, which lost electrons.
Crystal structures can be considered threedimensional stacks of larger spheres (negative ions) with smaller spheres (positive ions) located in the spaces between them, so that the positive and negative charges cancel each other out.
Consider the mineral halite (NaCl), which has a relatively simple framework composed of an equal number of positively charged sodium ions and negatively charged chlorine ions. Because ions of similar charge repel, they are spaced as far apart from each other as possible. Consequently, in halite, each sodium ion (Na1) is surrounded on all sides by chlorine ions and vice versa (Figure 2). This particular arrangement forms basic building blocks, called unit cells, that have cubic shapes. As shown in Figure 2C, these cubic unit cells combine to form cubeshaped halite crystals, including those that come out of salt shakers.
The shape and symmetry of these building blocks relate to the shape and symmetry of the entire crystal.
It is important to note, however, that two minerals can be constructed of geometrically similar building blocks yet exhibit different external forms. For example, fluorite, magnetite, and garnet are minerals constructed of cubic unit cells, but these unit cells can join to produce crystals of many shapes. Typically, fluorite crystals are cubes, whereas magnetite crystals are octahedrons, and garnets form dodecahedrons built up of many small cubes, as shown in Figure 3. Because the building blocks are so small, the resulting crystal faces are smooth and flat.
Despite the fact that natural crystals are rarely perfect, the angles between equivalent crystal faces of the same mineral are remarkably consistent. This observation was first made by Nicolas Steno in 1669.
Steno found that the angles between adjacent prism faces of quartz crystals are 120 degrees, regardless of sample size, the size of the crystal faces, or where the crystals were collected (Figure 4). This observation is commonly called Steno’s Law, or the Law of Constancy of Interfacial Angles, because it applies to all minerals. Because Steno’s Law holds for all minerals, crystal shape is frequently a valuable tool in mineral identification.
Compositional Variations in Minerals
Mineralogists have determined that the chemical composition of some minerals varies substantially from sample to sample. These compositional variations are possible because ions of similar size can readily substitute for one another without disrupting a mineral’s internal framework.
This is analogous to a wall made of bricks of different colors and materials. As long as the bricks are roughly the same size, the shape of the wall is unaffected; only its composition changes.
Consider the mineral olivine as an example of chemical variability. The chemical formula for olivine is (Mg,Fe)2SiO4—which has the variable components magnesium and iron in parentheses. Magnesium (Mg21) and iron (Fe21) readily substitute for one another because they are nearly the same size and have the same electrical charge. At one extreme, olivine may contain magnesium without any iron or vice versa. Most samples of olivine, however, have some of both of these ions in their structure. Olivine has a range of combinations, from a variety called forsterite (Mg2SiO4) at one end to fayalite (Fe2SiO4) at the other. Nevertheless, all specimens of olivine have the same internal structure and exhibit very similar, but not identical, properties. For example, ironrich olivines have a higher density than magnesium-rich specimens, a reflection of the greater atomic weight of iron as compared to magnesium.
In contrast to olivine, minerals such as quartz (SiO2) and fluorite (CaF2) tend to have chemical compositions that differ very little from their chemical formulas. However, even these minerals often contain tiny amounts of other less common elements, referred to as trace elements.
Although trace elements have little effect on most mineral properties, they can significantly influence a mineral’s color.
Structural Variations in Minerals
It is possible for two minerals with exactly the same chemical composition to have different internal structures and, hence, different external forms. Minerals of this type are called polymorphs (poly 5 many, morph 5 form). Graphite and diamond are particularly good examples of polymorphism because, when pure, they are both made up exclusively of carbon atoms. Graphite is the soft gray mineral from which pencil “lead” is made, whereas diamond is the hardest-known mineral. The differences between these minerals can be attributed to the conditions under which they form. Diamonds form at depths that may exceed 200 kilometers (nearly 125 miles), where extreme pressures and temperatures produce the compact structure shown in Figure 5A. Graphite, on the other hand, forms under comparatively low pressures and consists of sheets of carbon atoms that are widely spaced and weakly bonded (Figure 5B). Because the carbon sheets in graphite easily slide past one another, graphite has a greasy feel and makes an excellent lubricant.
Scientists have learned that by heating graphite under high confining pressures, they can generate synthetic diamonds. Because human-made diamonds often contain flaws, they are generally not gem quality, but due to their hardness, they have many industrial uses.
Further, because diamonds form in environments of extreme pressure and temperature, they are somewhat unstable at Earth’s surface. Fortunately for jewelers, “diamonds are forever” because the rate at which diamonds change to their more stable form, graphite, is infinitesimally slow.
The transformation of one polymorph to another is an example of a phase change. In nature, certain minerals go through phase changes as they move from one environment to another. For example, when a slab of ocean crust composed of olivine-rich basalt is carried to great depths by a subducting plate, olivine changes to a more compact, denser polymorph with the same structure as the mineral spinel.
Recall that oceanic lithosphere sinks because it is colder and more dense than the underlying mantle.
It follows, therefore, that during subduction, the transformation of olivine from its low- to high-density form would contribute to plate subduction. Stated another way, this phase change causes an increase in the overall density of the slab, thereby enhancing its rate of descent.