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Chapter 4: Minerals

Ch. 4.1 What is a Mineral?

Minerals are naturally occurring, solid, inorganic compounds or elements.

Mineral Characteristics

Earth's crust is composed of about 3000 minerals. Minerals form rocks and shape Earth's surface. Mineral- a naturally occurring, inorganic solid, with a specific chemical composition and definite crystalline structure.

Naturally Occurring and Inorganic

Formed my natural processes and are inorganic; not alive or ever were alive.

Definite Crystalline Structure

The atoms are arranged in regular geometric patterns that are repeated, forming a crystal. Crystal- a solid in which atoms are arranged in repeating patterns. In open spaces minerals will grow into a large crystal; in a restricted space so atomic arrangement is not apparent.

Solids With Specific Compositions

Minerals are solids with definite shapes and volumes. Chemical composition is unique to the mineral. Few minerals are a single element (copper, silver, sulfur) most are compounds.

Variations in Composition

Some minerals chemical composition can vary slightly depending on the temperature at which the mineral crystallizes. Differences in appearances depend on the temperatures that allow different chemical composition and growth patterns.

Rock-Forming Minerals

Although about 3000 minerals make up Earth's crust, only about 30 are common.

Minerals from Magma

Molten material that forms and accumulates below Earth's surface is magma. Less dense so rises upward to cooler layers of interior and crystallizes. Element types and quantities in magma determines the mineral that forms. Rate of cooling determines size of crystals. Small crystals form from rapid cooling, large crystals from slow cooling magma.

Minerals from Solutions

Minerals are often dissolved in water. Saturated is when a liquid is full of a dissolved substance and can dissolve no more. if more solute is added, the solution is called supersaturated and minerals can form. Atoms can bond together and crystals precipitate.

Minerals can also form when a saturated solution evaporates. These are called evaporites.

Identifying Minerals

Geologists rely on several simple tests to identify minerals.

Crystal Form: Some minerals form distinct crystal shapes that they are can be identified. Halite forms cubes, Quartz has six sides and double pointed ends; but crystals can't always form.

Luster: the way a mineral reflects light. Two types: metallic and nonmetallic. Metallic is shiny and reflects light, but not always a "metal". Using luster is subjective and should be used in combination with other physical characteristics.

Dependent on chemical compositions create differences in luster: dull, pearly, waxy, silky, or earthy.

Hardness: a measure of how easily a mineral can be scratched. Mohs scale compares minerals to ten known minerals that are readily available. Sometimes a sample may have more than one mineral so a sample should be tested in more than one area. Image of scale

Cleavage and Fracture: atomic arrangement determines planes of weakness in which minerals will break.

Cleavage- a mineral that splits relatively easily and evenly along one or more flat planes. Geologists count the number of cleaved planes and study the angle or angles between them.

Fracture- minerals that break with rough or jagged edges. Conchoidal: fracture with arclike patterns

Streak: the color of a mineral when in powder form. Most useful in identifying metallic minerals. Only useful if mineral is softer than a porcelain plate.

Color: one of the most noticeable characteristics but least reliable to identify. Color can be caused by trace elements.

Special Properties: some minerals have unique properties: magnetism, iridescence, double refraction

Texture: how a mineral feels to the touch; is subjective: smooth, rough, ragged, greasy, or soapy. Used with other tests.

Density and Specific Gravity: Density is mass per unit of volume (mass divided by volume). Minerals of same size my have different density. Specific gravity: the ratio of the mass of a substance to the mass of an equal volume of water at 4ºC

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Ch. 4.2 Types of Minerals

Minerals are classified by their chemical properties and characteristics.

Mineral Groups

Elements combine in many different ways and proportions. Each mineral group has distinct chemical natures and characteristics.

Silicates

Oxygen and silicon are two most abundant elements in Earth's crust. Silicates- minerals that contain silicon and oxygen and usually one or more additional elements. Silicates make up 96% of the minerals of the crust. Two most common minerals are feldspar and quartz.

Building block is the tetrahedron. Tetrahedron- a geometric solid having four sides that are equilateral triangles, resembling a pyramid. Silicon has 4 valence electrons so bonds with 4 oxygen. Tetrahedron can combine in many ways.

Can form: sheets, chains, and complex three-dimensional structures.

Carbonates

Minerals composed of one or more metallic elements and the carbonate ion CO3. Primary minerals in rocks such as limestone and marble. Examples: dolomite, calcite, rhodochrosite

Oxides

Compounds of oxygen and metal. Examples: hematite, magnetite

Other Groups

Sulfides: sulfur and one or more elements (pyrite), sulfates: sulfate ion and elements (anhydrite), halides: chloride or fluoride with calcium, sodium, or potassium, Native element: made of one element only (silver, copper)

Economic Minerals

Minerals are virtually everywhere.

Ores

Minerals are ore if it contains a valuable substance that can be mined at a profit. Minerals that are classified as an ore can change with a change in demand.

Mines

Open pit mines collect ore on the surface. Underground mines collect ore deep in Earth's crust. Gangue: unwanted rock and minerals. If separating overburden from ore is too costly, it can change classification of mineral as ore.

Gems

Gems- valuable minerals that are prized for their rarity and beauty. Gems are very hard and scratch resistant. Rarity influences the value, as well as the presence of trace minerals. Example: Amethyst is gem form of quartz.

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