Rocks and Minerals
- Naturally occurring
- Minerals have a definite chemical composition
- Minerals have an orderly internal crystal structure
Minerals are the building blocks of rocks. Each mineral has different physical and chemical properties which allow it to be identified. Physical properties you will use to identify the minerals include color, hardness, luster, cleavage, magnetism, reaction to acid, etc.
An aggregate of one or more minerals. Rocks are the building blocks of the Earth’s crust. The Earth’s continental crust is dominated by granite, and the oceanic crust is dominated by basalt. Both of these are igneous rocks.
There are three basic categories of rocks:
- Igneous (or crystallized from hot lava or magma) – ex. granite, basalt
- Sedimentary (or fragments laid down by water or wind) – ex. sandstone, shale, limestone
- Metamorphic (or rocks changed by heat and or pressure) – ex. gneiss, schist, slalte, marble
Physical Properties of Minerals
Color - The color of the mineral as it appears in reflected light to the naked eye.
Luster - The character of the light reflected from the mineral. A mineral may have a metallic luster (in other words, you would call it a metal), or a non-metallic luster. Non-metallic lusters may be described in more detail as:
Hardness – The resistanceof a mineral to scratching. Hardness is measured on a scale of 1-10 called Mohs Hardness Scale. In lab, we express hardness in comparison to common objects (fingernail, copper penny, nail, glass).
Cleavage - the tendency of a mineral to break along flat surfaces related to planes of weakness in its crystal structure. Minerals can be identified by the number of cleavage planes they exhibit, and the angles between them. For example, some minerals tend to cleave or break into flat sheets (the micas: muscovite and biotite), other break into cubes (halite), or into rhombs (calcite and dolomite). Other minerals have different types of cleavage. For purposes of this lab, it will be necessary to recognize whether or not minerals have cleavage, and to tell if the cleavage is one of the types mentioned above
Fracture – irregular breakage not related to planes of weakness in the mineral. Some minerals, such as quartz and olivine do not have cleavage. They have instead a special type of breakage called conchoidal fracture. Conchoidal fracture produces curved breakage surfaces, such as would be seen on arrowheads or chipped glass.
Magnetism – A few minerals are magnetic – that is, they are attracted to a magnet, or they act as a natural magnet, attracting small steel objects such as paperclips. The only magnetic mineral we may see in lab is magnetite.
Reaction to acid – The carbonate minerals react with dilute hydrochloric acid (HCl) by fizzing, producing bubbles of carbon dioxide gas (the same type of harmless gas bubbles that are found in carbonated beverages). Calcite fizzes readily in hydrochloric acid. Dolomite will fizz if it is first scratched and powdered. You may use a nail or steel needle probe to scratch the specimen to try this test.