Five most important minerals to know

Following up on yesterday's Fifty Great Minerals meme, Callan at NOVA Geoblog asks a related question:

So let me issue a new challenge for my fellow geobloggers... Which five minerals do you think are the most important ones to know, and why? In other words, if you had to introduce a non-geologist to just five of the earth's multitudinous building blocks, which ones would you choose to share, and offer a justification for each.

It's a great question (particularly during the week when I'm covering minerals in my intro course). I'm going to answer it without reading the rest of Callan's post, so I'm not influenced by his answers.

Quartz: It's made of the two most common elements in the crust (silicon and oxygen). It's in every type of rock - igneous, sedimentary, metamorphic. It survives weathering, which makes it a major component of well-traveled sand. It's hard, it's chemically tough, it makes pretty crystals, and it's ubiquitous.

Calcite: Calcium carbonate. Either calcite or its polymorph aragonite makes up the shells of lots of marine organisms, which can then become limestone. It reacts with acids, which means that it's useful to treat an upset stomach or acid rock drainage. It dissolves to make caves, which are beautiful, capable of transporting groundwater (and groundwater pollution) rapidly, and capable of collapsing. It's used to make concrete, and polished slabs in fancy bathroom walls, and (especially as marble) great art. And when it's buried by a continent-continent collision, it carries carbon dioxide away from the atmosphere and into the mantle.

Pyrite: "Fool's gold" is not an ore mineral itself (at least, not most of the time), but it's common in many ore deposits, in slates, and in coal. And when it comes into contact with the water and oxygen, its weathering creates sulfuric acid. It's pretty, it sparkles, and its chemistry creates an important side effect to many kinds of mining.

Clay minerals: This is cheating, because they're a group of minerals, and kaolinite (which has the simplest chemical formula) is one of the less interesting members of the group. They're diverse and chemically complex, but they're one of the major groups of minerals on Earth's surface, because they form from weathering most silicate minerals. They are especially important in soils, because they retain water. Some of them swell when they get wet or dry. Chemical reactions that occur on their surfaces can change the ions dissolved in groundwater. They're important indicator minerals for exploration for ore deposits. And if you heat them to 500 degrees Celsius and 4 kilobars, they turn into really pretty stuff (none of which make my list of Five Most Important Minerals, alas). (You can also make pottery from them at surface conditions, which is also nice.)

And one last one... hmmm.

I'm going to go with olivine, to represent the mantle. I've focused on minerals that are important on Earth's surface, because that's where people live. But most of the Earth is not crust - it's mantle, iron- and magnesium-rich silicate minerals. Olivine is the prettiest and easiest to recognize, and I think the mantle should be proud to have olivine as its spokesmineral.

And if you are one of my students reading this: yes, these are likely to be on a test at some point.