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What is a Mineral?

What is a Mineral?
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Published Date:22-07-2017
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What is a Mineral? A naturally occurring, inorganic solid that has a definite chemical composition and/or crystal structure Formation & Composition • Minerals come from cooled magma • 2500 kinds of minerals; some are easy to find and others are rare. • Mineral Properties Each type of mineral has its own combination of properties that identify it. Some of the properties are Color Many minerals come in a wide variety of colors. Different minerals can be the same color. It is difficult to use just color to identify a mineral. Streak Streak is the color of a mineral in powdered form. You can see a mineral’s streak by rubbing a sample across an unglazed ceramic plate and observing the powder left behind. Sometimes a mineral’s streak is very different from the color of the sample. Hardness Hardness is the resistance of a mineral to scratching. Geologists use the Mohs' hardness scale to seriate and compare mineral hardness. Specific gravity Specific gravity is the ratio of the weight of a given volume of a mineral to the weight of an equal volume of water. Higher specific gravity means the mineral is heavier. Cleavage Cleavage is how a mineral breaks. Some minerals break in smooth, flat surfaces at identifiable angles, such as calcite. Others fracture and produce no flat surfaces, such as quartz. Fracture Fracture is how a mineral breaks when no cleavage surfaces form. For example, quartz breaks in a pattern known as conchoidal fracture. Conchoidal fracture looks like smooth, curved surfaces. Luster Luster is how a mineral reflects light or how it shines. Some ways to describe luster include glassy or vitreous, metallic, dull, and pearly. Crystal form Crystal form describes the geometric shape of a crystal. There are seven main groups of crystal shapes, including cubic, hexagonal, and tetrahedral. Transparency Transparency describes how a mineral transmits light. Some minerals are transparent (you can see through them); others are translucent (some light passes through a sample) or opaque (no light passes through a sample). Magnetism Magnetism is a special property of some minerals, especially magnetite. Samples are attracted by a magnet. Lodestone, a special form of magnetite, is a magnet itself. Reaction to acid Some minerals react to acid. Calcite especially will fizz and bubble when it comes in contact with an acid such as hydrochloric acid at room temperature. Identifying Minerals • Physical Properties are used to identify minerals: – Color- always a reliable way to identify minerals. – However, color can change do to pollution, heat, and cold Types • Idiochromatics:-Fairly constant colour(Copper Group of minerals) • Allochromatic: Variable colour due to impurities(Mineral Like Quartz, Calcite, Fluorite) • Pseudochromatic: Showing a false colour Simultaneous reflection and refraction(Diamond) LUSTER: • The way a • mineral reflects • light from • its surface • Non- metallic luster: – Don’t have much of a reflection; known as glassy, pearly, and dull Factors affects Lustre ` The refractive index of a mineral • The absorption (of light) capacity of a minerals • The nature of reflecting surface   Metallic: Shine resembles as of Metals-Galena  Vitreous: Lustre of Glass-Quartz   Pearly: Shine resembles as of Pearl- Labradorite   Silky: Shine resembles as of Pure silk-Gypsum   Resinous: oily shine, waxy or greasy-Nepheline   Dull: Shine is almost absent-Chalk. Clay , Bauxite Streak • Color of powder scraped off when it is rubbed against • a hard, rough surface • Streak may be a different color than the mineral itself. Streak: colour of the fine powdered mineral Determined by using a streak plate Magnetite and chromite are almost black in colour where as streak of magnetite is black and that of Magnetite is brown Hardness • The ability to resist being scratched • Most useful properties for identifying a mineral • Numbered 1-10. – 1-Talc-softest – 10-Diamond- hardneest Cleavage & Fracture • Describe how the mineral breaks along flat surfaces. Ex: Halite • Most minerals break along a rough or jagged surface. Ex: Quartz Fracture Splintry: Rough • Even: Broken srface is • woody smooth-Chert fracture-Kyanite Uneven: Broken surface Hackly:Irregular sharp • • fine projections- is irregular surface- Native Fluorite copper • Conchoidal: Broken • Earthy: Surface is surface is having smooth and soft and concentric rings with porous-Chalk faint outline-Quartz Tenacity (Behaviour towards Break, bend, cut or crush) • Sectile: Cut with a knife • Malleable: Can be flattened by a hammer • Brittleness: Change to fine grain or powder under a knife or a hammer Crystal Shape • Minerals have a crystal shape that results from the way the atoms or molecules come together as the mineral is forming Structure of a Mineral (Physical make up of a mineral) • Tabular: Mineral in the form of flattened, square, rectangle (Calcite, orthoclase, barite) • Elongated: Mineral is in the form of a thin or thick elongated, column-like crystals. (Beryl, Quartz, Hornblende) • Bladed: Mineral appears to be composed of thin, flat , bladed like overlapping. (Kyanite) • Lamellar: Mineral made up of relatively thick, flexible, leaf-like sheets.(Vermiculite) • Foliated: similar to lamellar in boarder sense but individual sheets are thin. (Muscovite) • Fibrous: Mineral composed of Fibres, generally separable easily or with a little difficulty. (Asbestos & Gypsum) • Radiated: needle like crystal appears to originating from a common point. (iron pyrite) • Granular: densely packed mass of small crystal grains(Chromite) • Globular: Botroiydal, rounded, bulb-like overlapping, (Hematite) • Reniform: Globular similar to human kidney shape(H) • Mammillary: Globular form, conspicuous overlapping with each other(malachite)