(I’m behind by a week – my goal is to catch up this week)
As I mentioned earlier, I have a mineral for each week of the semester during mineralogy. Within the class, we talk about the mineral a bit and read a Geology paper in which that mineral plays a staring role. I originally had the minerals organized from most important to slightly less important, but swapped things up to match the minerals with the lecture material for that week. Well, at least as far as I could manage. But the week #1 mineral is also the most important mineral in the Earth’s crust: quartz.
What is quartz? The formula for quartz is SiO2, which is rather simple compared to a number of the other minerals my class is going to learn about. Each silica is bonded to four oxygens (called a silica tetrahedra):
each oxygen is then bonded to two different silica ions:
In itself, the structure is rather simple. Not easy to visualize in two dimensions, but simple.
The bonds between silica and quartz are strong because of two things:
- the bonds between silica and quartz are about 50-50 mix of covalent & ionic characteristics due to the electronegativity (ability of an atom to attract electrons to itself) of both elements; ionic bonds are moderately strong, but can be broken up dissolving them in a polar solvent; covalent bonds are just ridiculously strong; the combination of the two types of bonds makes Si-O a strong bond
- the charge of silica is +4 and the charge on oxygen is -2; to balance charge in the structure, each of the four oxygens surrounding the silica contributes -1 to the central silica (which balances the +4) and has a -1 to share with some other cation; we call this a mesodemic bond according to Pauling rule #2 because its equal charge to balance both cations; in the case of quartz, that other cation is also a silica, so the positive – negative pull on the oxygen is the same towards both cations
Where do we find quartz? The answer to this is just about everywhere within the crust & on the surface of the Earth. The reasons for this is multifold:
- there may be 92 elements found naturally on Earth, but >98% of the crust of our planet is made of just eight (!) elements; top 2? oxygen & silica
- the strong bond between silica & oxygen means that quartz itself is a fairly hard mineral and its difficult to weather
- because the bonds are equal in all directions, quartz doesn’t always break along the same planes (cleavage), but rather randomly (fracture); this also makes quartz less likely to weather
Rock-wise, quartz can be found in:
- the majority of igneous rocks (the exception are silica-undersaturated rocks)
- most types of sedimentary rocks either as a principle component (e.g. arenites, shales) or as an accessory phase (e.g. limestones)
- and because metamorphics were once either igneous or sedimentary, also most metamorphic rocks contain quartz
Loose material on the surface:
- component within soils
- as a principal component in clastic sediments
- as an accessory component in precipitated sediments
Since its found in some many different types of rocks & environments, what is the variation? Quartz can come in just about any color and in a huge variety of sizes & shapes. The color is due to impurities within the structure of the mineral (e.g. amethyst is due to iron, rose quartz due to Mn / Ti / Fe, citrine due to Fe). The size & shape of the mineral is due to how much space the mineral had to grow, quick or slow growth, amount of water present, and whether or not the mineral was deformed post-growth.
I talked about quartz in thin section yesterday and will address the paper we read separately.
Next week? Hematite & magnetite.