Dr Kendal Martyn (UK)
This article describes several processes producing the shape of crystals. Such processes are illustrated in the most common mineral from the Earth’s surface, quartz. Quartz or “simple” silicon dioxide, is made up of interlocking atoms of silicon and oxygen, arranged into various symmetrical structures depending on pressure and temperature conditions. Such variation in the structure accounts for most of the different minerals discussed below. The presence of these different SiO2 minerals in rocks gives important information about the conditions those rocks were exposed to.
- Minerals are classified by their distinctive structure (atomic arrangement), as well as their chemistry: all the mineralsdescribed below have the same chemical composition but different physical forms, known as polymorphs.
- Different physical structures are favoured by specific conditions of temperature and pressure (Fig. 1).
- If conditions favour a change in structure, the old structure may be preserved or a new structure formed,depending on the amount of energy available, the energy needed to make that change and the time available to make that change. Quenching (very rapid cooling) may “freeze in” the old structure. Kinetics (the physical re- arrangement of atoms and bonds) triumphs over thermodynamics (theoretically the energetically most favourable structure).
Breaking and reforming atomic connections, a radical rearrangement, needs more energy and requires material to diffuse across the crystal. Such reconstructive transitions will only happen if there is enough energy available, even over geological time frames: these transitions are quenchable.
Can you break the rules? No, but you can twist them
- Twisting from one structure to another needs small amounts of energy (no breaking and reforming of bonds) and no diffusion of atoms across the material (no long timescale required). Such displacive transformations always happen, they are generally un-quenchable.
- If transforming the structure just requires twisting, the external shape of the crystal may still show the previous form: there just isn’t enough energy or time to produce the new shape.
- Breaking and reforming atomic connections, a radical rearrangement, needs more energy and requires material to diffuse across the crystal. Such reconstructive transitions will only happen if there is enough energy available, even over geological timeframes: these transitions are quenchable.
The most abundant forms: High- and low quartz
The structure of quartz is made up of linked spirals of silicon and oxygen tetrahedrons. At high temperatures, the energy in the system keeps the spirals in a tidy, hexagonal, six-fold, arrangement when viewed from the top. At lower temperatures, when less energy is available, a less neat and tidy arrangement is found: a twist develops along the spiral reducing the symmetry to a trigonal, threefold, arrangement. To help imagine a simple trigonal crystal, think of the box a Toblerone comes in. Hexagonal crystals are best visualised by stacking six Toblerone boxes together to form a compact six-sided column.
High-quartz (also called β-quartz) is the hexagonal, high temperature form, the “quartz shape” most people know (Figs. 2 and 3). But, low-quartz (also called α-quartz), the lower temperature form, is trigonal. All natural quartz at room temperature is low-quartz. Trigonal is only a twist away from hexagonal and requires little energy or time to make the change.
Extra crystal faces can develop in low-quartz (Figs. 4 and 5), but may not be visible if the crystal originally grew as high-quartz and quenched to low-quartz. However, on an atomic scale, the arrangement of atoms is trigonal, regardless of the visible crystal face configuration. Also, as an extra complication, the twist forming the trigonal shape can go to the left or right, so left and right-handed forms result.
Left- or right-hand twists have important implications when considering optical applications that use quartz lenses: images will be rotated by the twist in the structure.
Other, rarer, SiO2 mineral forms:
- Christobalite is a tetragonal, high-temperature form. Occasionally found as tiny crystals in tektites, infilling cavities in some lavas and obsidian (Fig. 6). If you take christobaliteand cool it quickly, the high-temperature form will be “frozen in”. Even though the favoured structure is low-quartz, there is not enough energy kept in the system to break and reform the required atomic connections.
- Tridymite is a triclinic, very high-temperature form. Tridymiteis also found “frozen” in some lava and tektites (Fig. 7).
- Coesite is a monoclinic high-pressure form. Extremely rare, natural coesiteis only recorded as tiny inclusions in some garnet crystals. (Garnet is a silicate mineral with a stable structure over wide range of temperatures and pressures, which acts as a “paleo-vault” that preserve all sorts of interesting information; Fig. 8), and rocks that have been exposed to nuclear blasts or meteorite impacts (including some meteorites).
- Stishovite isa tetragonal high- pressure form. Stishoviteis rarer than coesite but occurs in similar rock types, although it readily melts during exhumation to the surface, so it rarely survives.
- Seifertite is orthorhombic, and the densest and hardest form yet to be found in nature. Only found in meteorites from Mars, seifertitecould theoretically form in the Earth’s mantle at depths over 1,700km, if any free silica is present. All silica in mantle rocks and under mantle conditions is used up in other silicates, so finding seifertite in the Earth’s mantle is likely to remain theoretical.
It should also be mentioned at this point that most of these other forms also have a low and high form. As with quartz, all these high (β) forms readily transform to the corresponding low (α) forms.
Yet more SiO2 materials
There are many other materials rich in SiO2 that aren’t necessarily minerals in their own right but are still worthy of consideration, giving important information on formation conditions.
- Opal arespheres of SiO2 gel with H2O filling the voids (Fig. 9). They are also amorphous mineral species, having no crystal structure even at x-ray level.
- Lechatelierite is a natural glass (quenched melt with few or no crystals) with no structural grouping (amorphous/non-crystalline).
- Fulgurites contain quartz and are rocks formed by lightning strikes (Fig. 10).
- Tektites are other natural glasses formed by meteorite impact events. Energy from a meteorite impact melts surrounding rocks, causing molten glass blobs to fly some distance through the air where they adopt aero-dynamic shapes as they cool (Fig. 11). Indochinites (black), moldavites (green) and Libyan Desert Glass (yellow) all have high SiO2 contents and trace-element chemical signatures linking them to meteorites. Tektites sometimes have tiny christobalite and/or tridymite inclusions within the glass, which are natural volcanic glasses with high SiO2 contents (Fig. 12). Obsidian is formed by volcanic eruptions that cool quickly enough so as to not produce many crystals, excluding occasional suspended crystals that grew in the high temperature magma chamber. Over time, stable structures do crystallise from the volcanic glass (the “snowflakes” in snowflake obsidian),which is another natural glass, formed under specific conditions by some fault movements, such as earthquakes.
- “Fused silica” is a term that is being confusingly used to cover both man-made and natural glasses, and also synthetic (grown) quartz crystals. Synthetic quartz crystals (manufactured for electronics applications) have a distinctive form (Fig. 13) and should be easy to distinguish from glass, which usually contains bubbles, flow lines and visible impurities. X-ray analysis would confirm the nature of the material with crystalline material imparting distinctive patterns, while glasses would produce no pattern at all.
- Moganite is a meta-stable monoclinic form. Moganite,when mixed with crystals composed of alternating layers of left- and right-hand twist low-quartz, gives flint, chert, chalcedony and agate their distinctive characteristics.
Knowing the conditions that produce and preserve minerals is vital in understanding the “life story” of rocks and what they have been exposed to. Who would have thought that some of the rocks in the Alps and Himalayas have been all the way down to 80km deep and back up again to sit at the surface if it wasn’t for the presence of tell-tale inclusions in garnet. Or that obsidian carries crystals that record the temperature conditions in the magma chamber.
As I said, it’s more than just Quartz.
Introducing Mineralogy, by John Mason, Dunedin Academic Press, Edinburgh (2015), 118 pages (Paperback), ISBN: 978-17=80460-28-4
An Introduction to rock forming minerals, by Deer, Howie and Zussman, Mineralogical Society, London (2013), 510 pages (Paperback), ISBN-10: 978-0903056274.