Objective: To be familiar with the common minerals found in igneous rocks; to learn which minerals are most often associated together in rocks.
1. What are the most common chemical
elements in the earth's crust?
2. Where are silicate minerals found?
3. What distinguishes silicate
minerals from other minerals?
4. What are ferromagnesian (mafic)
5. How can you determine if a mineral
ROCK-FORMING SILICATE MINERALS
Molten rock (called lava when on the surface, and magma when below the surface) is a solution of various chemical constituents, but contains primarily silicon and oxygen. When molten rock cools and solidifies (crystallizes), the minerals formed are consequently chiefly compounds of silicon and oxygen with other elements, and are called silicates. The most common rock formed by the cooling of lava is basalt, and the most common rock formed by the cooling of magma is granite. These rocks, which belong to a group known as igneous rocks, make up the major proportion of the earth's crust.
Rocks are aggregates of minerals, and the minerals which make up most of the igneous rocks are known as rock-forming silicates. Surprisingly, there are but a few of great importance, and you should become very familiar with these, because you will then be able to identify most of the common igneous rocks. The rock-forming silicate minerals are as follows:
FELDSPARS: Potassium Feldspars:
Orthoclase/Microcline (potassium aluminum silicates)
Plagioclase feldspars (sodium & calcium aluminum silicates)
QUARTZ: Silicon dioxide
OLIVINE: Iron-magnesium silicate
PYROXENES: Single-chain silicates, most common variety is augite (a complex silicate of calcium, magnesium, iron, and aluminum)
AMPHIBOLES: Double-chain silicates, most common variety is hornblende (a complex silicate of sodium, calcium, magnesium, iron, and aluminum)
MICAS: Layer silicates, most common varieties are:
Muscovite (potassium aluminum silicate)
Biotite (iron-magnesium potassium aluminum silicate)
Of the above minerals, the feldspars are by far the most abundant minerals among the igneous rocks. The potassium feldspars and quartz are generally light-colored (white, pale gray, pink, or red) and are called felsic. The silicate minerals containing iron and magnesium are generally dark-colored (dark green, gray, or black) and are referred to as mafic (or ferromagnesian) minerals. Plagioclase feldspars may range from pale-colored sodium-rich varieties (such as albite), to darker grey, calcium-rich varieties (such as labradorite).
You will be given examples of each of the rock-forming silicates mentioned on Page 1. In the following table record the physical properties of each unknown, and use the mineral identification tables to identify the specimens. Note that there may be more than one variety of the same mineral name. (Tip: Fill out the table vertically in columns, not across in rows - this way, you will practice your laboratory technique for each type of test.)