Day 7
Recall from our first day of class the definition of a mineral. Now, ask yourself the following questions:
The answer to both of these questions is yes - the combinations of chemistry and structure are unique for each mineral, but the individual chemistries, or structures, of different minerals may be similar. This leads us to consider the following special considerations of the crystalline character of minerals:
Isostructuralism: minerals with differing chemistry may have analogous structures. For example, halite (NaCl) and Galena (PbS) have analogous structures - in each, cations and anions are arranged in cubic lattices. Crystals in which the centers of the constituent atoms occupy geometrically similar positions, regardless of the size of the atoms and the absolute dimensions of the structure, are said to belong to the same structure type (K&H, p. 151).
Polymorphism: the ability of a specific composition of elements to crystallize into more than one type of structure. A constant chemical composition may crystallize into more than one structure because different structures may represent different levels of internal (structural) energy, and this energy may be a function of pressure, temperature, or both. For example, there are six polymorphs of SiO2, the most common of which is quartz. Although the composition of each is one silicon to every two oxygen, the structural arrangements among the SiO2 polymorphs differ.
Because differing structures can form from one compound, in response to differing conditions of temperature and pressure, mineral polymorphs are ideal indicators of pressure and temperature conditions in geologic processes. For example, the SiO2 polymorph coesite has a very dense and compact structure, which on Earth forms only at pressures reached at the base of the crust - or within the mantle. Therefore, if coesite is in a rock, a geologist can be certain the rock once experienced very high pressures. In addition to the SiO2 polymorphs, several other common groups of polymorphs are important indicators of pressure and temperature conditions (such as the aluminum silicates, the potassium feldspar polymorphs, and calcite-aragonite).
If a chemical compound crystallizes with a particular structural arrangement under high pressure and temperature conditions, will the structure change as the pressure and temperature changes, or will the original structure persist? If we look at a mineral in a basalt, for example, it is obviously not at the same temperature and pressure at which it crystallized (unless you're holding a very fresh basalt straight from a Hawiian volcano - ouch!). So ... would the minerals in a basalt change or remain unchanged as the rock cooled?
The answer to both is ... sometimes yes, sometimes no. Energy is also required to change the structure from one type to another - and the energy needed may be so great as to prevent the change from taking place. Some polymorphs formed at high temeprature and pressure conditions remain at low (surface) temperatures and pressures for billions of years. However, some polymorphs only occur at high temperatures and pressures, and change instantly to others when T and P are lowered.
Why the difference? It is because polymorphism, and the structural transformations between polymorphs, occur through three, very different mechanisms:
Displacive polymorphism: change or reformation of a crystal structure by rotating components in the structure. Changes from one polymorph to another through a displacive transformation do not require the breaking of chemical bonds. Analogous to a 'bending' of the structure or components in the structure. This type of transformation can occur very rapidly. For example, there is a 'high-quartz' polymorph (see fig. 152) that is only stable above 600 deg. C; the displacive transformation from high to low quartz (the quartz we know and love) is so quick that even in a laboratory, you cannot grow high quartz, quench it, and preserve the high-temperature structure. Stated differently, the kinetics of displacive transformations can be very rapid.
Reconstructive polymorphism: changes to a polymorph's structure through extensive rearrangement of atomic bonds, and reassembly of atoms. This type of structural arrangement requires a lot of energy, and thus only tends to occur at high temperatures or pressures. Once formed, the high P-T polymorphs tend to hang around for a long time. Samples of lunar igneous rocks, billions of years old, contain the high-temperature SiO2 polymorph tridymite.
Order-Disorder Polymorphism: the degree to which elements are ordered ('systematically arranged') among the possible sites of a crystal structure affects the symmetry of that structure, and different structural polymorphs can be formed by different degrees of disorder ('randomizing'). This one is tricky to visualize and appreciate, but it is very significant for some important minerals. As a first step, consider that perfect order, in nature, only exists in crystalline materials at absolute zero (this is the third law of thermodynamics)*. For a hypothetical crystal at absolute zero (0 K), there is only one possible arrangement of the constituent atoms. As minerals are at higher temperatures, elements tend to occur in more random distributions among possible sites.
For example, there are three polymorphs of potassium feldspar (KAlSi3O8): Sanidine, one of the potassium feldspar polymorphs, has a high degree of structural symmetry and a relatively random distribution of Si and Al (both of these elements can fit into sites, or 'holes', surrounded by four oxygens); upon cooling, the structure constricts, and there is a tendency for Al to go into some of the smaller sites, and Si to go into some of the larger ones, i.e., the distribution of aluminum and silicon tends to become more ordered. The low-temperature polymorph formed from sanidine by disorder polymorphism is microcline. The ordering of elements in the sanidine-microcline transition actually lowers the structural symmetry, as sanidine has a 2-fold rotation axis and mirror plane which microcline lacks.
Question: would you consider sanidine or microcline to be more common in slowly-cooled plutonic rocks, such as a granite? The answer is ... you have to think about it.
*In fact, it is thermodynamically impossible for matter to reach absolute zero. Therefore, there are no 'perfect' crystals. "Wait just a minute...", you may say, "I thought you said on Days 1 and 2 that minerals are perfectly ordered materials with infinitely repeating structures - I have it right here in my notes!". Well, er, ah ... we (myself, the authors of your book, and the rest of the 'mineralogy establishment') lied just a bit at first. A certain degree of randomness and disorder exists in all materials, at least on the atomic scale. So there. Like it or not, you can't get away from statistics and probabilities, and given even just a little chance, atoms will have some random variation in their structural arrangements.
Which, by chance, bring up yet another type of polymorphism:
Polytypism: mineral polymorphs may form through differences in the stacking order of idential units. These particular polymorphs are called 'polytypes'. For example, there are numerous polytypes of the micas resulting from different stacking orders of Si:O sheets. The differences in stacking order may result from the statistical probability that structural components may link together in a given way, or from a dependence on temperature and pressure conditions.
Defects are very important in controlling variations in the physical properties of minerals, such as hardness, electrical conductivity, mechanical deformation properties, and color. Consider that one means of 'tempering' is to press a mass of hot steel into a sheet, fold it, press, fold, etc., and then finally quench it (picture the blacksmith beating on iron, then plunging it into a bucket of water). This process introduces defects at high temperature, and prevents them from annealling ('healing'). The process can be reversed - heating tempered steel for a time allows the metal structure to anneal. If you've ever put an axe or hammer head in a fire to remove a broken wood handle, then you probably lowered the hardness of that tool.
There are six basic types of defects pictured on page 163 of your text:
The types of defects discussed above may form during the growth of a mineral, or through later deformation. The processes of radioactive decay may also distrupt a mineral lattice and introduce defects, through metamictization. The energy and particle emissions accompanying radioactive decay, of uranium, for example, can break atomic bonds and disrupt structures sufficiently to alter the mineral's physical characteristics (color, hardness, cleavage, etc.). The degree of metamictization tends to increase with higher concentrations of radioactive isotopes, and with time.
Question: Which would tend to be more metamict - a uranium-bearing zircon crystal in an Archean (>2.8 billion years old) granite, or one in a Cenozoic (< 65 Ma) granite?
Pseudomorphism occurs when one mineral has the outward appearance of another. This does not imply that the internal structure is the same (which be isostructuralism) - only that the outer shape and form is similar. Pseudomorphs also occur when a mineral of one crystal system grows in forms that look similar to those of other crystal systems. For example, aragonite is CaCO3 with a orthorhombic structure. A common habit, or general shape (see pg. 52), of aragonite is as six-sided crystals. In this way aragonite can occur as hexagonal pseudomorphs, and only the careful mineralogy sleuth who measures the angles among six-sided aragonite crystals would note they are not 120°, and thus cannot be truly hexagonal.
Pseudomorphs can also form through replacement of one mineral by another. If one mineral is replaced by another, through a chemical reaction, the new mineral may take the shape of the former one. A common example of this is the oxidation of pyrite, which may result in formation of limonite cubes that are pseudomorphs after the original pyrite.
Twinning refers to the formation of identical, or similar, objects.
From the biology of twinning: Monozygotic twins derive from the division of a single zygote(fertilized ovum) during the first developing stages of the embryo after fertilization, hence the term monozygotic. These separate cell masses become embryos which are genetically identical and will be the same sex. Monozygotic twins are contained within the same chorionic membrane and three-quarters of MZ twins share the same placenta. This also means they have the same blood type. Incomplete or late division of the zygote and subsequent cell masses can result in conjoined or Siamese twins. Monozygotic twins make up about a third of all twin births but their occurence has nothing to do with heredity, unlike dizygotic twinning. MZ twinning occurs randomly in all racial groups and follows no discernable hereditary pattern.
Mineralogical twins are analogous to monozygotic, Siamese twins. Mineral twinning occurs when two (or more) crystals of the same mineral are joined along symmetry elements that are not normally present in individual crystals of the mineral (see pg. 97-103 and 146-149 in text). The twinned crystals are, by definition, compositionally and structurally identical. Twinning occurs for many different minerals, and across many of the chemical and structural groups of minerals. Most mineral twins, at least those visable to the eye, form during crystal nucleation and growth. Mineral twinning during crystal growth may be random, but the particular symmetry operations that join mineral twins are constrained by the crystal structure.
The operations, or twin elements, which may relate a twinned crystal to it's counterpart are rotation (a twin axis), reflection (a twin plane), and inversion (a twin center). Twinning is defined by a twin law, which indicates whether there is a center, axis or plane of twinning, and also defines the crystallographic orientation of the twin element (usually with Miller Indices). Certain twin laws are more common in particular crystal systems, and they are also diagnostic features for many minerals.
There are two main types of twins:
Contact Twins are related by reflection;
Penetration Twins are related by rotation or inversion.
Repeated, or multiple, twins are three or more twins repeated by the same twin law. If the twin law defines a plane, then polysynthetic twinning results (such as the twinning on (010), common in plagioclase and called 'albite law twinning'). If the twin law defines a rotation axis, then cyclic twinning results. Cyclic twinning of aragonite, which has an orthorhombic structure, can result in formation of crystals with a nearly perfect 6-fold rotation axis. The six-sideness of aragonite twins results from twinning on {110} faces, which are nearly 60 degrees apart.
Growth twinning results from the emplacement of atoms on the outside of a growing crystal in such a way that the regular arrangement of the lattice is interrupted. Growth twinning therefore reflects a misstep during crystal nucleation and/or growth, analogous to the formation of siamese twins. Transformation twinning occurs in pre-existing crystals and thus is a secondary form of twinning. Transformation twinning commonly results from structural changes during cooling, as in the formation of intersecting albite- and pericline-law twinning in sanidine (monoclinic) as it changes to microcline (triclinic). Glide twinning (deformation twinning) results in some minerals as they change shape in response to stresses of deformation; calcite crystals readily accomodate changes in shape during deformation by twinning.
Twinned minerals have their own following in the mineral collecting hobby - see other discussion and a list of minerals that commonly form specimen-quality twins in The Mineral Galeries twin page: http://mineral.galleries.com/minerals/twins.htm)