## Top 5 Minerals Meme

In response to Callan’s Top 5 Minerals Meme.

I’m organizing around chemical formulas, not names. So, for example, just clays are not a candidate, because there are too many different formulas. However, smectites would be fine, even though that isn’t as specific as, say, quartz.

So, for my list,

• Quartz, $\mathrm{SiO_2}$: extremely physically and chemically stable at Earth surface conditions. Has conchoidal fracture, which sounds cool, looks cool, and has practical applications. It’s what most sand is made of and is a major component of the continental crust ( both due to that whole stability thing). Reasonably interesting to look at in both hand sample and thin section.
• Calcite and Aragonite, $\mathrm{CaCO_4}$: everybody’s heard of limestone. Many organisms have hard parts made of it. An important part of some fairly common geomorphology. Plays fairly significant role in hydrology and oceanography. Can have some pretty neat properties in hand sample (double refraction!) and in thin section (that characteristic change in relief on rotation!).
• Olivine, $\mathrm{\left(Mg,Fe\right)_2SiO_4}$: it’s what the mantle is mostly made of. The mantle is what the Earth is mostly made of. And it’s pretty in both hand sample and in thin section. Case closed!
• Smectite, $\mathrm{~Ca_{0.17}\left(Al,Fe,Mg\right)_2\left(Si,Al\right)_4O_{10}\left(OH\right)_2}\cdot n \mathrm{H_2O}$: One of the major groups of 2:1 clay minerals. Does that whole swelling and contracting thing as it absorbs and releases water, which leads to one of the main reasons I’d want someone unfamiliar with geology to know about it: it can lead to landslides, and screw up the foundation of your house. So, yeah. Nobody should panic about it, but everybody needs to be aware of it. Montmorillonite (a type of smectite) feels cool, and comes from the weathering of bentonites, so that’s pretty neat.
• Apatite, $\mathrm{Ca_5\left(PO_4\right)_3\left(F,OH,Cl\right)}$: It has a hardness of 5. It’s pretty in hand sample, and very cool looking in thin section. Also? It’s a major part of what vertebrates do some of their most important biomineralization with. In other words, it’s your skeleton.