Identifying Minerals in Rocks

Mr. Keys
Nov 12, 2015
3 min read

KWBAT Identify the minerals found in rock samples using density

A box with more particles in it will be more dense than the same box with fewer particles

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Today we will do a lab to identify minerals found in rocks using density.

"Density describes how compact or concentrated something is. For example, suppose you have two boxes, one large and one small. However, they both weigh the same. That means the small box has a higher density than the large box"

Step 1: Read the Summary below to provide you with background information about the importance of density in science

An Introduction to Density

"Density is the mass of an object divided by its volume.

Density often has units of grams per cubic centimeter (g/cm3). Remember, grams is a mass and cubic centimeters is a volume (the same volume as 1 milliliter).

Density is a fundamental concept in the sciences; you will see it throughout your studies. It is used quite often in identifying rocks and minerals since the density of substances rarely changes significantly. For example, gold will always have a density of 19.3 g/cm3; if a mineral has a density other than that, it isn't gold.

You probably have an intuitive feeling for density in the materials you use often. For example, sponges are low in density; they have a low mass per unit volume. You are not surprised when a large sponge is easy to lift. In contrast, iron is dense. If you pick up an iron skillet, you expect it to be heavy.

Students, and even teachers, often confuse mass and density. The words heavy and light on their own refer to mass, and not density. A very large sponge may weigh a lot (have a high mass), but its density is low because it still weighs very little per unit of volume. For density, you also need to consider the size, or volume, of the object

How do I calculate Density

Density is not something that is directly measured. Typically if you want to know the density of something you will weigh it and then measure its volume.

Another tricky thing about density is that you can't add densities. If I have a rock that is made up of two minerals, one with a density of 2.8 g/cm3, and one with a density of 3.5 g/cm3, the rock will have a densitybetween 3.5 and 2.8 g/cm3, not a density of 6.3 g/cm3. This is because both the mass and the volume of the two minerals will be added, and so when they are divided to get the density the result will be between the two.

Typical densities for gasses are on the order of thousandths of grams per cubic centimeter. Liquids often have densities of about 1.0 g/cm3, and indeed, fresh water has a density of 1.0 g/cm3. Rocks often have a density around 3 g/cm3, and metals often have densities above 6 or 7 g/cm3.

Where is density used in the geosciences?

Isostasy - determining how high continents will sit on the mantle
Plate tectonics - mechanisms that drive plate tectonics
Minerals - determining the name of a mineral through its density
Rocks - determining the name and composition of a rock by its density
The hypsometric curve - examining the causes of elevation variation on Earth
Oceanography - some ocean currents and ocean circulation is controlled by density