Types of Rocks • Geologists place rocks into three groups according to how they form: igneous, sedimentary and metamorphic.
Igneous Rocks • Igneous rocks were originally hot, fluid magma within the Earth. Igneous get their name from the Latin word, ignis, which means “fire”.
Sedimentary Rock • Most sedimentary rocks are formed from particles that have been carried along and deposited by wind and water. These particles, sediments or detritus, include bits of rock in the form of mud, sand or pebbles. Sediments also include shells, bones, leaves, stems and other remains of living things. Over time they are pressed together to form rocks.
Metamorphic Rocks • Metamorphic rocks are formed when chemical reactions, tremendous heat and great pressure change existing rocks into new kinds of rocks. These new rocks have chemical and physical properties usually quite different from the original rocks.
Properties of Minerals Geologists use characteristics to tell one mineral from another
Mineral Properties we will study Luster Hardness Cleavage Streak
Luster • Refers to the way light reflects from the surface of the mineral. • There are two types of luster, • Metallic: looks like polished metal. • Nonmetallic: does not look like polished metal. • Nonmetallic can be shiny or dull.
Hardness • Is measured by how easy it is to scratch. • Geologists order the hardness by… • Scratched by a fingernail. • Scratched by a penny. • Scratched by a nail. • Scratched by a diamond. These are not all of the tools geologists use, but it will work for our experiment.
Calcite is soft, but a little harder because it cannot be scratched by a fingernail, but it can be scratched by a penny.
Fluorite is harder. It can be scratched by a nail, but not a penny or fingernail.
Cleavage • Not all minerals have cleavage. • Some minerals split easily along a flat surface. • The number of lines that are created when a mineral is split will be the number of cleavage lines.
Streak • Red chalk on a chalk board makes red marks. White chalk makes white marks. • Not all minerals work this way. When some minerals are scratched along a ceramic streak plate, it creates a different color.
Gold • When gold is run across a streak plate it makes a yellowish-gold color. • That makes sense.
Pyrite or “Fool’s Gold” • When pyrite is run across a streak plate, it has a black or dark green streak. • Pyrite is not worth much money, while gold is worth a lot. They look alike, so miners call it fool’s gold.
Hematite • Hematite’s color is grey, but its streak is red. • Hema means blood. • The mineral was named hematite because it looked like it was bleeding when it was taken across a streak plate.
Many minerals can be the same color. Below are gold colored minerals. Which one is gold?
The answer… None of them were real gold.
Just like with people… Outside color does not tell you much about the important characteristics.