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Earth Science 101: Earth Science23 chapters | 178 video lessons
John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.
Have you ever been fooled by the question, 'Which weighs more, a pound of feathers or a pound of bricks?' Well obviously, they have the same weight. Well, how about this question, 'Which weighs more, a liter of water or a liter of bricks?' The answer to this question is not so obvious. It requires an understanding of density, the amount of mass in a certain volume of material.
As brick has more mass per unit volume, it has a greater density. As the brick is more dense, it will sink in water. Mineralogists have often used a similar property called specific gravity to describe the density of a mineral. Specific gravity is simply a ratio of the mineral's mass to the mass of an equal volume of water. Different minerals have different densities and thus different specific gravities. This lesson will describe how density and specific gravity are used to differentiate between minerals.
Let's pretend you find an unknown specimen in your backyard. It's a heavy specimen, and you want to know what it is. As a novice mineralogist, you head to the laboratory to measure its density. Well, what to do first?
You need to determine the mass of the sample, so you weigh it on a scale and find its mass to be 150 grams. Now that you've identified the mass, you need to determine how much water the sample displaces. Fortunately, your sample is small enough to put in a beaker of water that's calibrated on the side. Before you place the sample in the water, you note the level at 100 milliliters, or 100 cubic centimeters (cc). After you place the sample in the water, you notice the level rises to 120 cc; therefore, 20 cc of water is displaced. This displaced volume of water is equal to the volume of the sample; therefore, your sample has a volume of 20 cc.
Now you're ready to calculate density. Simply divide the mass of the sample by the volume of the water displaced. Remember, the volume of the water displaced is equal to the volume of the sample.
So, density = mass / volume of water displaced. In our case, density = 150 grams / 20 cc of water. Density = 7.5 grams/cc.
Checking your mineral density handbook, you see that galena, the common form of lead, has a mineral density that ranges from 7.2 to 7.6. While the density does not provide definitive identification, you can use other properties to determine its identity definitely. For example, the sample has a metallic luster, which is characteristic of galena.
Now that we have determined the density of our sample, it is a piece of cake to determine its specific gravity. Recall that specific gravity is simply the ratio of the sample mass to the mass of an equal volume of water. The density of our sample is 7.5 grams/cc. Our sample has a specific gravity of 7.5. Well, how do we know this? Recall that 20 cc of water was displaced by the sample.
Now as we've learned in previous lessons, each cubic centimeter, or milliliter of water, has a density of 1 gram/cc; therefore, 20 cc of water has a mass of 20 grams. To determine the specific gravity of our sample, we simply divide the mass of the sample (that is, 150 grams) by the mass of the same volume of water, which would be 20 grams.
So, specific gravity = 150 grams of the sample / 20 grams of water. Specific gravity = 7.5.
Now, as the units are the same in the numerator and the denominator; that is, grams, they cancel each other out, and this gives us a value with no units.
As it turns out, most common rock-forming minerals have a specific gravity between two and three. For example, quartz has a specific gravity of 2.65. Our sample in this discussion, along with other metallic minerals, are more than twice as dense. Gold is more dense, with 24-karat gold coming in around 20. Platinum has a specific gravity of about 22. Now that's dense! Graphite, used to make pencil lead, and gypsum, used to make wallboard, are less dense minerals with a specific gravity just over two.
In summary, density and specific gravity are properties used to help identify minerals. Density is a measure of the mass of a certain volume of the sample. Specific gravity is a unitless measure, and it is the ratio of the mass of a substance to the mass of an equal volume of water. Most rock-forming minerals have a specific gravity between two and three; for example, quartz has a specific gravity of 2.65. Metallic minerals are far more dense; for example, galena, as we've talked about in our discussion, has a specific gravity of 7.5, and platinum has a specific gravity around 22.
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