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SGES 1302INTRODUCTION TO EARTH SYSTEM

LECTURE 13: Minerals

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Lecture 13: Introduction to Minerals

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Minerals Almost every manufactured product contains minerals. They are essential

for the development of a modern society. All geological processes on earth such as volcanic eruption, earthquake,

landslide, and erosion involve earth materials (and minerals are the building blocks of earth materials). Basic knowledge of earth materials is essential to the understanding of all geological phenomena.

Geological defination of mineral: any naturally occurring inorganic solid that possess an orderly crystalline structure and definite chemical composition. Naturally ocurring – formed by natural geologic processes. Exclude all synthetic

materials produced in the lab. Solid within the temperature ranges normally experienced at Earth’s surface. Ice

is a mineral but water is not. Orderly crystalline structure – atoms are arranged in an orderly, repetitive

manner. Forming regularly shaped objects : crystals. Volcanic glass is not considered as mineral.

Definate chemical composition. Most minerals are chemical compounds made up of 2 or more elements. Quartz = SiO2; Calcite = CaCO3; Gold = Au; Sulphur = S

Generally inorganic. Sugar = crystalline organic compound is not a mineral. Organic compounds such as those in shells and coral reefs are generally considered minerals.

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Example of a mineral: salt or NaCl

Sodium and chloride ions are arranged in an orderly and repetitive manner into a building block having a cubic shape.

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Rock Defination: any solid mass of mineral, or mineral-like matter that occur

naturally as part of the Earth. Most rocks occur as aggregate of several types of minerals

(eg. granite quartz, K-feldspar, plagioclase, etc.) Some rocks are composed of only 1 mineral (eg. limestone calcite) Few rocks are composed of non-mineral matter

(eg. pumice volcanic glass; coal organic matter)

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Properties of Minerals Each mineral has a definite chemical composition and crystalline structure,

which give it a unique set of physical properties. These properties are used in the identification of the minerals.

Primary properties Crystal form Lustre Colour Streak Hardness Cleavage Fracture Specific gravity

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Crystal form The internal orderly arrangement of the atoms of a mineral is reflected in its

crystal form When a mineral forms without space restriction it will develop individual

crystals with well-formed crystal faces Crystal growth is often severly constrained, stunted due to competition for

space, forming small intergrowth of crystals or aggregates of crystals

Properties of Minerals

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Lustre: The appearance or quality of light reflected from the surface of a mineral

Metallic appearance of metals – metallic lustre, opaque Submetallic lustre – imperfect metallic lustre Adamantine lustre – diamond Vitreous lustre – appearance of a broken glass (most common in minerals) Resinous lustre – resin-like Greasy lustre – perfectly smooth appearance Pearly lustre – pearl-like due to reflection of light Silky lustre – due to reflection off small parallel fibers Earthy lustre – dull, lack of lustre

Properties of Minerals

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Colour – Not a reliable diagnostic property

Properties of Minerals

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Streak – colour of a mineral in its powdered form. It is much more reliable indication of colour.

It is obtained by rubbing the mineral across a piece of hard, unglazed porceline called a streak plate

Even if a mineral occurs in more than one colour, its streak usually shows the same colour

Minerals with metallic lustre have a dense dark streak

Properties of Minerals

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Properties of MineralsHardness: a measure of the resistance of a mineral to abrasion or scratching. One of the most useful diagnostic properties. Determined by rubbing the

mineral to be identified against another mineral of know hardness. Standard hardness scale: Mohs scale (10:hardest – 1:softest)

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Properties of MineralsCleavage - is breakage along planar surfaces, which are parallel to possible

external faces on the crystal. This is because bonds between layers of atoms aligned in certain directions are weaker than bonds between different layers.

In some minerals, a single direction of weakness exists, but in others, two, three, four, or as many as six may be present. Where more than one direction of cleavage is present, it is important to determine the angular relation between the resulting cleavage surfaces: are they perpendicular to each other (right angle), or do they meet at an acute or obtuse angle?

Mica is a good example – breaking along very closely spaced flat planes that yield thin "sheets."

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Properties of MineralsFracture: When a mineral without cleavage breaks, the fracture surfaces can

be used for their identification Conchoidal fracture is smooth curved surface like broken glass Subconchoidal fracture is similar to conchoidal, just not as curved, but still

smooth Splintery fracture is a fracture type that occurs in fibrous or finely acicular

minerals Earthy fracture is a fracture that produces a texture similar to broken clay Jagged or hackly fracture has sharp points or edges that catch on a finger

that's rubbed across the surface.

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Properties of MineralsSpecific gravity (G) is defined as the ratio between the weight of a substance

and the weight of an equal volume of water at 4° C. Thus a mineral with a specific gravity of 2 weighs twice as much as the same volume of water. Since it is a ratio, specific gravity has no units.

The specific gravity of a mineral depends on the atomic weights of all its constituent elements and the manner the atoms are arranged.

Most minerals, including all the common rock-forming minerals, have a specific gravity of 2.5 - 3.5. Metallic minerals have higher specific gravity.

Other properties: fluorescence (response to ultraviolet light), magnetism, radioactivity, tenacity (response to mechanical induced changes of shape or form), piezoelectricity and reactivity to dilute acids.

Mineral G

Quartz (SiO2) 2.65

Calcite (CaCO3) 2.7

Pyrite (FeS2) 5

Copper (Cu) 8.9

Gold (Au) 19.3

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Mineral Groups There about 4,000 minerals described. <100 are abundant, and they make up most of the rocks in the Earth’s crust. They are

classified as rock-forming minerals. The bulk of these minerals are made up of 8 elements. The most common rock-forming minerals are silicates (90%): formed by the

combination of oxygen, silicon and one or more metals. Other minerals are grouped under nonsiliates. Many are important economically.

Elemental Composition of the Earth’s crust

Element Wt % Vol %

Oxygen, O 46.6 93.77

Silicon, Si 27.7 0.86

Aluminium, Al 8.13 0.47

Iron, Fe 5.00 0.43

Magnesium, Mg 2.09 0.29

Calcium, Ca 3.63 1.03

Sodium, Na 2.83 1.32

Potassium, K 2.59 1.82

Others (trace) 1.43 0.01

Abundance of minerals on Earth’s surface

Mineral Area %

Feldspars 30

Quartz 28

Clay minerals & mica 18

Calcite 9

Iron oxides 4

Others 11

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Mineral Group Anion or Anionic Complex Representative Minerals

native elements  - sulfur, gold, silver, copper, diamond, graphite

sulfides S-2 pyrite, galena, sphalerite, chalcopyrite

oxides O-2 hematite, magnetite, chromite

halides Cl -1, F-1 halite, flourite

sulfates  (SO4)-2  anhydrite, gypsum, barite

carbonates  (CO3)-2 calcite, dolomite

phosphates  (PO4)-3  apatite

silicates  (SiO4)-2 quartz, feldspar

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Silicate Minerals Silicate minerals are the most common rock-forming minerals. Their atomic structure

is based on silica tetrahedron (SiO4-2), in which four oxygen atoms are bond to each

silicon (Si) atom. The mineral structures are constructed by sharing of oxygen between Si atoms

producing linkages of tetrahedra. This sharing of oxygen between Si tetrahedra produces chains and other 3D Si tetrahedra structures, which are themselves linked together through bonds between O and other atoms (e.g., Al, Mg and Fe).

Chemically, silicate minerals can be separated into two major types: ferromagnesian (iron/magnesian) - olivine, pyroxenes, amphiboles, biotite felsic (silica/aluminum) - quartz, plagioclase, potassium feldspars

Silicate minerals, however, are typically classified on the basis of their silica tetrahedra polymerization. The simplest silicate mineral structures have isolated Si tetrahedra linked together through bonds between oxygen and cations other than silicon. More complicated structures involve tetrahedra linked together to form rings (beryl), single chains (pyroxenes), double chains (amphiboles), sheets (micas and clay minerals), and 3D frameworks or networks (quartz and feldspars).

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