That makes me think of soil components. What's left in soil after those is lots of water and air, and a little bit of organic material, and tiny bits of other minerals. Or, that's what's in the lower layers anyway: the regolith, the saprolite, and the bedrock. The top layer is humus - organic materials in soil that have been decomposed enough to lose their cellular structure are called humus. And humus is a mess, chemically. There are big random molecules like damaged chain-link fences full of rings with random bits hanging off, which is fitting because that's what plenty of non-decomposed organic molecules like lignin also look like. There are also organic acids like fatty acids and phenolic acids and hydroxy acids, and other organic things like terpenoids and alcohols, and ugh. Let's get back to rocks and minerals.
Other rocks: Marble is metamorphosed limestone, which is made of calcite, which is an arrangement of calcium carbonate arranged in the trigonal system, which is a little laborious to describe, but basically the unit cell has a nice little three-dimensional diamond shape with equal side length and some special symmetries. Let's talk about it some other time. Chalk is also a limestone, formed under the ocean from calcium carbonate shells, also having bits of flint, which is quartz. Although sidewalk chalk is usually a different mineral these days, gypsum. We'll talk about that later. The calcium carbonate in shells is mostly tiny crystal of the mineral aragonite: it's the same chemical as calcite, but it's in a different crystal arrangement from calcite; aragonite has in the orthorhombic arrangement, which has a rectangular prism as its unit cell. There's another famous limestone, travertine, which is formed in hot springs. It's a limestone rock made of calcium carbonate minerals, like how marble and chalk are limestone rocks made of calcium carbonate minerals.
Slate, schist, and phyllite are all metamorphosed shale, I think? Probably in different stages of increasing metamorphism. In some other order. Anyway, the base rock, shale is a sedimentary rock made of mud and clay. A mudstone. We don't ask what mud is. Okay, you win, it's humus soil. And clay. But that doesn't make it any less taboo. Humus soil is scary stuff to an inorganic chemist. When the organics in mud become fossilized, they're called kerogen, and kerogen in shale is where a lot of the carbon on earth is located. Petroleum, coal, natural gas, it's all kerogen. And that's why people drill into shale to get fossil fuels. What about less organic rocks? Soapstone is a metamorphic schist (a metamorphized shale) but the shale it comes from is almost all talc, a nice inorganic clay mineral. There's a continuum of sedimentary rocks from mudstones (with mud and clay) to sandstones (with feldspar and quartz and mica and some other stuff). A sandstone that's rich in quartz can be metamorphosed into quartzite, where tiny bits of sand get merged together into larger grains, I think.
Let's talk about gems. They're mostly pure minerals and those are easier to think about. Just a chemical formula and sometimes a crystal arrangement. Amethyst and citrine are colorful impurities in quartz. Alabaster is made of gypsum, which is a new mineral! It's calcium sulfate. It's also called plaster of Paris and drywall. Outside of geology, especially in the ancient middle east and north Africa, alabaster can also refer to calcite. They look very similar. Aquamarine and emeralds are colorful impurities in beryls, another new mineral. It's a beryllium aluminum silicate. And it's a cyclosilicate, which is cool. It's got six silica tetrahedra arranged in a ring, kind of looking like a star of David. There aren't a lot of famous cyclosilicates. Tourmaline is another one.
Jasper and chalcedony are both quartz aggregates. So they're rocks with lots of tiny crystals, rather than one big crystal. Bloodstone and carnelian are both quartz stones with hematite for color, which is iron rust. Cinnabar rocks are their own mineral! Mercury sulfide! Blood red, beautiful, toxic. Where we get our elemental mercury from.
Ruby and sapphire are colored impure versions of corundum, which is aluminum oxide. I don't know what garnets are, and I'm afraid to learn. Jacinth in colored zircon, which is its own mineral: zirconium silicate. Opals are hydrated silica, which is not quite quartz.
I think you've heard of diamonds. Topaz and turquoise are their own minerals. Topaz is a silicate with aluminum and fluorine while turquoise is a hydrated copper aluminum phosphate. Pretty cool. Peridot is gem-quality olivine, which is in the category of magnesium and iron-rich minerals (mafic!) that also includes pyroxenes and amphiboles. The mafic minerals form a sequence of increasing complexity: olivine/peridot have solitary silica tetrahedra, then pyroxenes have single changes of silica tetrahedra, and amphiboles have doubles chains. I remember the order by saying "Opa!" in the Greek fashion. Olivine, pyroxene, amphibole. Some other minerals besides the olivines also have just one or two silica tetrahedra, like garnets and epidote. But they're weird and confusing. There are also silicate minerals that form in sheets like clay and mica. And there are silicate minerals that form three-dimensional frameworks, like quartz and feldspar and weird zeolites. There's a pretty nice orderly system of arranging them.
But back to gems. How about jade? That's a gem, right? Jade can refer to rocks made of either of two silicate minerals: nephrite jade is a silicate with calcium and magnesium, closely related to asbestos, and jadeite jade is a silicate with sodium and aluminum, and it's a pyroxene (a one-chain silicate). Pyroxenes are common in the earth's mantle (between the core and the crust), so they show up in lots of igneous and metamorphic rocks. Most of them are kind of heavy, and contain more magnesium and iron than light minerals which contain more sodium, potassium, and calcium. Jadeite doesn't have iron or magnesium, but oh well.
That is the extent of my knowledge.
That is the extent of my knowledge.
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