Geology And Geophysics

How to Identify Rocks and Minerals



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"What is the difference between minerals and rocks?" is one of geology's basic questions. It's not really much different from asking, "What is the difference between a brick and a wall?" though, because minerals are the building blocks of rocks, just as bricks are the building blocks of a wall.

The logical next question is, "What is a mineral?" because, though most of us know what bricks are, minerals aren't so easy to define. A dictionary will say that minerals are "naturally occurring substances" that are "of inorganic origin" with "definite chemical composition" and "definite crystal structure." Sounds, errrr, scientific, right? But what does it mean?

Have you ever seen a mineral? Sure, you have, because a very common mineral is in your kitchen right now. It's ice: ice is inorganic, since it isn't grown by plants or animals. Ice has a definite chemical composition, good old H2O like liquid water. And finally, ice has a definite crystal structure, which is why all snowflakes are six-sided. Ice is just one mineral, although a very special one. There are tens of thousands of minerals, each of which can be identified from its chemical makeup and a few physical features.

Other familiar minerals include table salt, a mineral called halite; and quartz, the mineral in play sand. Those two are extremely common, but we've also seen more rare specimens: gems like diamond, ruby, and emerald; and semiprecious stones like garnet and opal. Though priceless gems are much more glamorous, common minerals are quite important to our everyday life. Right now, many of us are surrounded by mineral named gypsum: it is the main ingredient in drywall.

As for the difference between rocks and minerals: rocks are made of mineral grains. Just as there are many minerals, there are many different rocks. Rocks don't have "definite chemical composition," though, or "definite crystal structure" like minerals. Instead, rocks are classified from two things: what minerals are  present, and how the mineral grains are arranged.

Rocks fall into three types based on the how they form. The first is igneous rocks, and includes rocks like granite and basalt. "Igneous" means "formed from fire" (notice the similarity to the word "ignite"), and refers to rocks that crystallize from a super-hot liquid, just like ice freezes. Freezing like this makes the grains of igneous rocks lock together like pieces of a jigsaw puzzle. Geologists subdivide igneous rocks based on the size of the grains and the minerals present.

The second type is rocks that form at the Earth's surface, at the temperatures and pressures at which humans live. These are sedimentary rocks. Most sedimentary mineral grains are eroded pieces of older rocks, and some crystallize from sea water. This includes the only minerals that are of organic origin, which are grown by animals (and some plants) to make shells and other body parts. Sedimentary rocks are divided into ones made from bits of older rocks, such as sandstone; and those made of grains that crystallize at the surface inorganically (rock salt) or from the action of plants and animals (limestone).

The last type of rocks is metamorphic, or rocks that were changed by high temperature and pressure; just not hot enough to melt them. Like igneous rocks, metamorphic rocks are made of interlocking mineral grains, but there is a difference: the mineral grains in some metamorphic rocks are aligned like a stack of playing cards. These mineral grains are usually long, thin, and flat, like playing cards or sheets of paper. Other metamorphic rocks are made almost entirely of one mineral. These are metamorphosed sedimentary rocks, either sandstone (which becomes quartzite) and limestone (which becomes marble).

So if a friend asks, "What's the difference between minerals and rocks?" if you say "Rocks are made of minerals!" you're right - but that's only part of the story. There are many kinds of rock, which form under different conditions and contain different minerals. And there are also many different kinds of minerals, which also form under different conditions. The difference between a mineral and a rock, however, is the same as the difference between a brick and a wall: you need many bricks to make a wall, and you need many mineral grains to make a rock.

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