Sunday, January 1, 2012

Rock Densities

This is an obvious please-do-my-homework question from someone wanting a quick answer. I chose to answer in a way that I hope was more interesting - and not just some unforgettable list of rock densities.


Can you tell me what the heaviest types of rocks are? Could you also tell me their weights in g/cm3 or where I could find that info?
Marcus T.


Let's start with the basics - which means the metric system. Water, by definition, has a density of 1 gram per cubic centimeter, generally abbreviated g/cc. This is where part of our metric system actually came from - the definition of a gram, at least. To get that, however, also requires a volume definition, and this came from the size of the Earth itself. The meter length comes from the distance between the equator and the pole being divided into 10,000,000 units - meters - or 10,000 kilometers. "Kilo" comes from the Latin for "thousand." A centimeter is 1/100ths of a meter - it uses the Latin word "cent" for "hundred" - so you now understand where the word "century" came from. How much water fits into a cubic centimeter then became the definition of a gram, and 1,000 grams is then a kilogram. Note that grams per cubic centimeter is the main (but not only) scientific density standard.

On my back deck I have a sample of chromite ore: it's a nearly black chunk of pure iron-chromium-oxide. This chunk is roughly the volume and shape of a pair of small running shoes sitting side by side, but it's density is about 7.1 grams per cubic centimeter - it would take over seven times that volume of water to weigh this much. I sometimes challenge visiting guests to pick it up, and they are often surprised when they can't. Though relatively small, it weighs about 30 kilograms. Imagine trying to run wearing a pair of shoes like that!

Here are some typical rock-types that you may have encountered in the world around you, along with a density-range next to them:

Coal                  1.1-1.4 g/cc
Sandstone         2.2-2.8 g/cc
Granite              2.6-2.7 g/cc
Gabbro             2.7-3.3 g/cc
Gneiss               2.6-2.9 g/cc
Peridotite          3.1-3.4 g/cc
Sand/gravel       1.6-2.2 g/cc

Why a density range instead of a simple density? For starts, Elberton granite from northern Georgia - used to make elegant uniformly gray gravestones found all over the southeastern United States - has different minerals than a granite from virtually anywhere else. For seconds, each rock has a definite porosity - spaces between the grains where air can exist, or which is filled with water if the rock is below the water table. Some of this space is simply where angular grains leave space between themselves as they collect (for instance sandstone), and some is caused by fractures or even weathering in the rock.

This leads us to porosity. Some rocks are "looser" than others - sandstone is a particularly good example that can have porosities as high as 30% by volume; if sitting on a shelf, this rock is 30% air! Note also the broad density range of sandstone in that table above. The difference is simply air, though different mineral grains will also be more or less dense than others. Some rocks are "tighter" than others - note the narrow range of gneiss, which has been heated and partially melted, sealing off much of its original porosity under heat and pressure from overlying rock when it was deeply buried. If you see a gneiss on the ground surface, you can be pretty sure that it was buried at least 5 - 7 kilometers at one time in its history. It actually looks squished and stewed.

Granites have less porosity (and thus a narrower density range) because the individual grains grew together in a crystal mush as the original intrusive magma body slowly cooled. Note also the different density ranges of granite and gabbro; both are used as kitchen counter-top surfaces. However, granite is typically a much lighter color - this is due to an abundance of quartz and aluminum-based feldspar minerals in it. Gabbro, on the other hand, is almost always much darker because it has quite a bit more iron and magnesium and calcium minerals instead of the quartz and sodium feldspars.

What about the coal? It is mostly made up of long carbon with hydrogen chain molecules; each of these atoms has a lower mass or proton-count than iron, aluminum, and magnesium. Coal, oil, and natural gas really make up a continuum of these long-chain carbon molecules - the so-called "organic chemistry" that people can spend their entire lives studying. However, oil density is treated in a very different manner: It is rated by its American Petroleum Institute gravity. If the API gravity number is greater than 10, it floats on water. If less than 10, then it will sink in water. You've all seen how Canola oil floats on water when you cook spaghetti? API gravity for Canola is thus greater than 10. If coal was a liquid, it would have an API gravity below 10. Denser, high-carbon-chain organic material is what makes up heavy crude and tar-sands. The shorter the carbon chain, the lower the density - until you get to the higher gravity oils and eventually to the "volatiles" - carbon chain compounds that can evaporate, like the constituents of gasoline. The extreme end of this carbon chain continuum is methane - a gas with just a single carbon atom per molecule.

Now, consider peridotite. This lustrous green mineral is found only in rocks believed to come from great depths in the Mantle. It has been compressed until the molecular structure actually changes and becomes smaller and more dense. A famous type of meteorite is called a "Pallasite" - it looks like steel with blobs of peridotite crystals in it. It's one of the more exotic products of the early formation of our Solar System.

Finally, consider the difference between graphite and diamond. Both are pure carbon. The graphite consists of sheets of carbon-carbon links, relatively weak molecular bonds, and these sheets can slide over each other easily - this is why graphite feels "greasy" and is used to lubricate locks and hinges. But compress the carbon sheets - and we're talking about pressures and heat created by 150 - 200 kilometers of overlying rocks in the crust - and the carbon links change structure into a closely-packed hexagonal form that is much denser. As a graduate student, I did experiments with different molecules under great to extreme pressures - we would watch different minerals change their physical character as they were compressed into different structures. In the case of graphite and diamond, the density shifts from about 2.2 g/cc to 3.15-3.53 g/cc.

So density is not a permanent physical property of any given rock. With time rocks will fracture and weather, some of the minerals converting to clays, and eventually becoming the soils that blanket our planet and as dust are now even significant constituents of the air we breath.

Talk about getting your vitamins and minerals!


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