Misconceptions – Rocks and Minerals

There are many misconceptions about geological concepts. There is a list here, developed by Kent Kirby of the University of Minnesota. This post is to debunk some of those misconceptions. There will be others. Find them here.

Misconceptions:

A. Rocks (and minerals) grow.

B. Coarse-grained rocks are rough, fine-grained rocks are smooth

C. Coarse-grained sedimentary rocks cooled slowly, coarse-grained igneous rocks formed in areas of high depositional energy, etc.

D. With minerals, the term ‘massive’ texture means that the samples are big.

I lump these together because many of these are issues with unclear definitions of words.

A. Rocks (and minerals) grow.

We do use the term ‘grow’ a lot in reference to minerals getting bigger. They do grow in the sense that atoms add themselves to the exterior part of the mineral’s crystal lattice, making the crystals get bigger and bigger. Thus, minerals grow.

Where the real confusion lies is that rocks and minerals are not the same thing.  A mineral is a single crystal. Rocks are aggregations of minerals. That is, many different crystals are joined together to make a rock. Sometimes the minerals are different kinds. If quartz, feldspar, and mica are together in a rock, that rock is called granite. Sometimes a rock is composed of individual crystals of all the same mineral. A sandstone can be a bunch of rounded quartz crystals all cemented together.

Minerals can get bigger, assuming that there is space for them to grow into. Rocks cannot get bigger. They simply form and are the size they are once they’re done forming.

B. Coarse-grained rocks are rough, fine-grained rocks are smooth

Coarse- versus fine-grained in rocks is a general description of the size of the individual minerals that comprise the rock. If the minerals are large, and easy to see with the naked eye, it’s considered ‘coarse.’ If the mineral grains are really small such that a magnifier – or maybe even a microscope – is needed to be able to see the grains then it’s called ‘fine-grained.’

The roughness or smoothness of a rock has more to do with how it broke than the size of the grains that make it. A coarse-grained rock composed completely of quartz can be quite smooth. A fine-grained rock can break into very jagged, sharp pieces making it rough.

C. Coarse-grained sedimentary rocks cooled slowly, coarse-grained igneous rocks formed in areas of high depositional energy, etc.

This is a problem I’ve seen in my own students and is an example of confusing mechanisms of rock formation.

There are three major groups of rocks: Igneous, Metamorphic, and Sedimentary

Igneous rocks form from cooling and crystallization of molten rock, or magma, in the same way that we get ice cubes from water. If the magma cools more slowly, the crystals have the opportunity to grow larger, resulting in a course-grained igneous rock. If the magma cools quickly, the crystals never get very big causing the resulting igneous rock to be fine-grained.

Metamorphic rocks form when pre-existing rock (which could be igneous, metamorphic, or sedimentary) is subjected to intense heat and pressure cause the rock to change its shape (but not melt).

Sedimentary rocks form for the most part from the breakdown of pre-existing rocks (again of any type), then the transport of the broken down bits somewhere else where they are deposited and cemented into a new sedimentary rock. The size of those bits of pre-existing rocks depends upon how far those bits have traveled from the rock they originally came from, as well as the energy of the medium that moved them there (most often water). Large rock bits (coarse-grained) haven’t gotten far from their starting point. Small rock bits (fine-grained) have gone a long way. Bigger rock bits require more energy to move (a stronger river flow, for example), whereas fine grained bits can be carried along by slower streams (like silt in a river).

Though coarse-grained and fine-grained are terms that can be applied to any of the three types of rock, but the causes are of large and small grains are entirely different in the different rock groups. It is important not to confuse the processes that formed the different types.

D. With minerals, the term ‘massive’ texture means that the samples are big.

Massive is a fun word. It can mean a great number of things. To Joe-average, it means ‘huge.’ In geology-speak, massive usually means well-fused with no clear layers or breaks. The different grains making up a massive rock are indistinguishable from one another.

Sometimes minerals grow together in large masses wherein you can’t tell where one crystal ends and the next begins. Such minerals are said to grow with a massive habit (or crystal shape). These minerals don’t look like the crystals that we expect to see.

Massive turquoise (blue) on quartz. Credit: Aramgutang, WikiMedia Commons

There is a nice photo of a mineral with the massive habit, as well as examples of other common habits of minerals on this Wikipedia page.

I hadn’t expected this post to be as long as it is. Oh well. I hope, at least, that it is informative!

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