Rocks

Rocks are naturally formed, consolidated material composed of grains of one or more minerals. Geologists group rocks into three categories depending on their origin: igneous, sedimentary and metamorphic.

A mineral is defined as a naturally occurring, crystalline solid of definite chemical composition and a characteristic crystal structure.

Igneous rocks are formed from solidification of molten material. Sedimentary rocks are formed by the accumulation of fragmental material derived from preexisting rocks of any origin as well as the accumulation of organic material or precipitated material. Metamorphic rocks occur as a result of high pressure, high temperature and the chemical activity of fluids changing the texture and (or) mineralogy of preexisting rocks.

Classification

Types of Rocks

Igneous

Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens. Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes (in this case, it is called lava). When lava cools very quickly, no crystals form and the rock looks shiny and glasslike. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock.

Sedimentary

Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock. Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock, and it is usually the only type that contains fossils.

Metamorphic

Metamorphic rocks are formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure (squeezing). The rocks that result from these processes often have ribbon-like layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface.

The Rock Cycle

The Rock Cycle is a group of changes. Igneous rock can change into sedimentary rock or into metamorphic rock. Sedimentary rock can change into metamorphic rock or into igneous rock. Metamorphic rock can change into igneous or sedimentary rock.

Igneous rock forms when magma cools and makes crystals. Magma is a hot liquid made of melted minerals. The minerals can form crystals when they cool. Igneous rock can form underground, where the magma cools slowly. Or, igneous rock can form above ground, where the magma cools quickly.

When it pours out on Earth's surface, magma is called lava. Yes, the same liquid rock matter that you see coming out of volcanoes.

On Earth's surface, wind and water can break rock into pieces. They can also carry rock pieces to another place. Usually, the rock pieces, called sediments, drop from the wind or water to make a layer. The layer can be buried under other layers of sediments. After a long time the sediments can be cemented together to make sedimentary rock. In this way, igneous rock can become sedimentary rock.

All rock can be heated. But where does the heat come from? Inside Earth there is heat from pressure (push your hands together very hard and feel the heat). There is heat from friction (rub your hands together and feel the heat). There is also heat from radioactive decay (the process that gives us nuclear power plants that make electricity).

So, what does the heat do to the rock? It bakes the rock.

Baked rock does not melt, but it does change. It forms crystals. If it has crystals already, it forms larger crystals. Because this rock changes, it is called metamorphic. Remember that a caterpillar changes to become a butterfly. That change is called metamorphosis. Metamorphosis can occur in rock when they are heated to 300 to 700 degrees Celsius.

When Earth's tectonic plates move around, they produce heat. When they collide, they build mountains and metamorphose (met-ah-MORE-foes) the rock.

The rock cycle continues. Mountains made of metamorphic rocks can be broken up and washed away by streams. New sediments from these mountains can make new sedimentary rock.

The rock cycle never stops.

Minerals

A mineral is a naturally occurring inorganic solid, with a definite chemical composition, and an ordered atomic arrangement. This may seem a bit of a mouthful, but if you break it down it becomes simpler.

Minerals are naturally occurring. They are not made by humans

Minerals are inorganic.They have never been alive and are not made up from plants or animals

Minerals are solids. They are not liquids (like water), or gases (like the air around you)
Minerals have a definite chemical composition. Each one is made of a particular mix of chemical elements
Minerals have an ordered atomic arrangement. The chemical elements that make up each mineral are arranged in a particular way - this is why minerals 'grow' as crystals.

Minerals are very important. We study them because they play a major role in our economy, industry, and health. We use hundreds of products everyday that are made up of them.

The following are examples of common minerals that are categorized by type. Each has a thumbnail picture of the specimen and a brief description of it. Clicking on the name of the individual specimen will take you to a page with a larger picture and a detailed description.

But before we get to individual specimens lets see if we can agree on what these things are. A scientific definition says that it is something that:

occurs naturally.
Is solid at room temperatures
has a regular structure (its atoms have a definite arrangement).
Has a chemical formula

There are a few things that don’t seem to fit even though they seem to belong. See mineraloids.

The Physical Properties of a Mineral

Cleavage and Structure

Cleavage and fracture are descriptions of how a mineral breaks into pieces. Cleavage describes how a mineral breaks into flat surfaces (usually one, two, three or four surfaces). Fracture describes how a mineral breaks into forms or shapes other than flat surfaces.

Hardness

The hardness of a mineral is a way of describing how easy or difficult it is to scratch the mineral. It is used, in combination with the other physical properties, to help identify a mineral specimen.

Luster

Luster refers to the general appearance of a mineral surface to reflected light. Two general types of luster are metallic and non-metallic

Specific Gravity

Specific Gravity is a measure of the density of a mineral compared to the density of an equal volume of water.

The Silicates

Amazonite

It is a beautiful green variety of microcline feldspar. Its chemical formula is KAlSi3O8 , potassium aluminum silicate.

Tanzanite

It is a variety of Zoisite. It is valued as a gem stone and has the chemical formula Ca₂Al₃Si₃O₁₂(OH).

Sodalite

It is a deep royal blue silicate. The deep color and the fact that it will take a high polish makes it popular in jewelry and other decorative items. The chemical formula is Na₈Al₆Si₆O₂₄Cl₂.

Biotite

It is a member of the mica branch of the silicate mineral group. It is common as a rock-forming mineral and is present in all three rock types: igneous, metamorphic, and sedimentary.

The Quartz Family

Quartz is one of the most common of all minerals that make up the continental crust. It is found in igneous, metamorphic, and sedimentary rocks. It is associated with many valuable ore deposits as well.

The Mineraloids

Mineraloid is the term used for those substances that do not fit neatly into one of these eight classes. Opal, jet, amber, and mother of pearl all belong to the mineraloids.

Moh's Scale of Hardness


Scale	Description
1	Talc
2	Gypsum
3	Calcite
4	Fluorite
5	Apatite
6	Feldspar
7	Quartz
8	Topaz
9	Corundum
10	Diamond

References

http://imnh.isu.edu/digitalatlas/geo/basics/rock.htm

http://www.oum.ox.ac.uk/thezone/minerals/define/

http://www.rocksandminerals4u.com/minerals.html

http://www.cotf.edu/ete/modules/msese/earthsysflr/rock.html