Welcome To Student Notes: complete notes of minerals(geology)

Composition of the Crust

Eight elements make up 99 per cent of the crust. Note how rare many industrial metals are.

Minerals are the Chemicals that make up the Earth

NATURALLY-OCCURRING
INORGANIC
CHEMICAL COMPOUNDS

ABOUT 3000 KNOWN
200 COMMON
20 ROCK-FORMING

MINERALOIDS

Term for mineral-like materials that don't quite fit the full definition of minerals, including:

Non-crystalline materials (opal, some hydrocarbons)
Organic rock materials
Substances that are mixtures of several minerals on a microscopic scale (limonite, bauxite)

Atomic Bonding

1. IONS

Normal Configurations: Sodium: 11 p+, e-; Chlorine: 17 p+, e-

You may have heard that the pictures above are not really accurate. If you want to know more, you can visit

What Do Atoms Really "Look Like?" (optional)

ELECTRON SHELL STRUCTURE
Most stable arrangement: 8 e- in outermost shell

Noble gases (Ar, Ne, etc.) have that arrangement naturally and rarely combine with anything else.
Sodium: Loses electron, has 11p+, 10e-, charge +1: becomes a cation
Chlorine: Gains electron, has 17p+, 18e-, charge -1: becomes an anion
Many elements would have to gain or lose too many electrons and settle for other electron structures instead.

2. ELECTRICAL NEUTRALITY

(+) and (-) Cancel Out

3. BONDING (SATISFY 1 & 2)

Ionic (NaCl)
Covalent (O2)
Metallic (Cu, Al, Fe)
Hydrogen (in water)

Ionic and Covalent Bonding

Ionic Bonding
- Some atoms gain electrons to become anions
- Others lose electrons to become cations
- Ions are attracted by their opposing charges
- Electrical Neutrality Maintained
- Most Important Bonding in Rocks and Minerals
Covalent Bonding
- Electrons share electrons to fill incomplete shells
- Most Important Bonding in Organic Materials (and Organisms)

Metallic Bonding

A. Outermost electrons wander freely through metal. Metal consists of cations held together by negatively-charged electron "glue."

B. Free electrons can move rapidly in response to electric fields, hence metals are a good conductor of electricity.

C. Free electrons can transmit kinetic energy rapidly, hence metals are good conductors of heat.

D. The layers of atoms in metal are hard to pull apart because of the electrons holding them together, hence metals are tough. But individual atoms are not held to any other specific atoms, hence atoms slip easily past one another. Thus metals are ductile. Metallic Bonding is the basis of our industrial civilization.

Hydrogen Bonding

Hydrogen Bonding is Geologically Important

A. Water molecules are asymmetrical. The positively-charged portions of one are attracted to the negatively-charged parts of another. It takes a lot of energy to pull them apart. Hence:

Water melts and boils at unusually high temperatures for such a light molecule.
Water has a high heat capacity.
It takes a lot of energy to melt ice and vaporize water.
Thus water is the principal heat reservoir on the Earth.

B. The asymmetrical charge distribution on a water molecule makes it very effective in dissolving ionically-bonded materials. However, it is not an effective solvent of covalently bonded materials (oil and water don't mix). Hence:

Water is very effective at weathering rocks and minerals. It is the closest thing to a universal solvent.
Water is very effective at transporting ions and dissolved nutrients in the human body.
Water is not an effective solvent of organic molecules. Thus we do not dissolve in our own cell fluids. Nifty feature.

C. When water freezes, it assumes a very open structure and actually expands. Most materials shrink when they freeze and sink in their liquid phases. Implications:

If ice sank like most frozen solids, it would accumulate at the bottoms of frozen lakes and seas. Most of the world's water would be ice.
Expansion of ice in rocks is a powerful weathering agent.

Summary of Bonding

Ionic bonding holds rocks and minerals together
Covalent bonding holds people and other organisms together
Metallic bonding holds civilization together
Hydrogen bonding gives water its heat-retaining and solvent properties

4. LATTICE

Usually anions are bigger (They form framework and cations fill in spaces between). Thus it is often possible to remove one cation and replace it with another. Below, both halite (NaCl) and sylvite (KCl) have identical atomic structures and similar physical properties. They can be distinguished by their taste - sylvite is very bitter, somewhat like licking a belt sander. Those of you who have used so-called "light salt" know.

At right, a rubidium atom has substituted for potassium. Some elements, like rubidium, have no minerals of their own and occur in nature almost entirely by substituting for more common elements. In many minerals, this substitution occurs to such an extent that the mineral can be considered to consist of mixture of two or more ideal compounds. Such mixtures are called solid solutions.

5. RADICALS

Radicals are groups of atoms that behave as single units. Three of the most common are shown at left.

NAMING MINERALS

COLOR

Albite (Latin: Albus = White)
Rhodonite (Greek: Rhodon = Rose)
Glauconite (Greek: Glaucos = Blue-green)
Azurite

OTHER PROPERTIES, USES

Magnetite
Orthoclase (Straight + Break)
Microcline (Small + Angle)
Pyrophyllite (Fire + Leaf)
Fluorite (Latin: Fluere - to Flow)
Graphite (Greek: Graphos - Writing)

COMPONENTS

Magnesite
Chromite
Cuprite
Siderite (Greek: Sideros - Iron)
Calcite

PLACES

Aragonite
Muscovite (Moscow)
Turquoise (Turkey)
Andalusite
Labradorite

PEOPLE

Scheelite
Sillimanite
Biotite
Cordierite
Wollastonite

CHEMICALS (AND MINERALS) ARE CLASSIFIED BY THEIR ANIONS

For Example: Iron Compounds Have Little in Common

Fe: Gray, Metallic
FeCl2: Light Green, Water Soluble
FeSO4: Light Green, Water Soluble
FeCO3: Brown, Fizzes in Acid
FeS2: Dense, Brittle, Metallic, Cubic Crystals

On the Other Hand, Sulfides have Many Properties in Common

FeS2
CuS2
PbS2
ZnS2
All are Dense, Brittle, Metallic, have Cubic Crystals

IDENTIFYING MINERALS

COLOR -Sometimes Distinctive
- Often Unreliable
- Affected By
  - Chemical Impurities
  - Surface Coating
  - Grain Size
  - Weathering
HARDNESS
- Resistance to Scratching
- Directly related to relative strength of atomic bonds
DENSITY
- Directly related to masses of component atoms and their spacing
- Usually very consistent
LUSTER
- Metallic or Nonmetallic is the most important distinction.
- Resinous, waxy, silky, etc. are self-explanatory.
- Vitreous is often used for glassy luster.
CLEAVAGE
- Tendency to split along smooth planes between atoms in crystal
- Thus directly related to atomic structure
- Related to Crystal Form
- Every cleavage face is a possible crystal face
- Not every crystal face is a cleavage face. Quartz commonly forms crystals but lacks cleavage.
CRYSTAL FORM
- Takes Luck & Practice
- Well-formed crystals are uncommon
- Crystal Classification is somewhat subtle
FRACTURE
GEOLOGIC SETTING
- Some minerals occur in all geologic settings: quartz, feldspar, pyrite
- Some minerals occur mostly in sedimentary settings: calcite, dolomite
- Some minerals occur mostly in igneous settings: olivine
- Some minerals occur mostly in metamorphic settings: garnet, kyanite
SPECIAL PROPERTIES
- Taste, Magnetism, Etc.
- Don't try on every mineral, but will quickly identify or rule out specific minerals.
EXPERIENCE AND READING
PROFESSIONAL METHODS
- Chemical Analysis
- X-Ray Studies
- Thin Section

HARDNESS

Scratch Test (MOHS)
Indentation Test (KNOOP) - a more accurate scale used by metallurgists and engineers

Common Errors due to

Weathering, Chalk' marks
Breaking vs. Scratching

MOHS vs. KNOOP Scales

Talc: very small
Gypsum, Fingernail: 30
Calcite, Penny: 135
Fluorite: 163
Apatite: 430
Feldspar, Glass: 560
Quartz: 820
Topaz: 1340
Corundum: 2100
Diamond: 7000

DENSITY - gm/cm3

weight relative to water

Air: 0.001
Wood - Balsa: 0.1, Pine: 0.5, Oak: 0.6-0.9
Gasoline: 0.7, Motor Oil: 0.9
Ice: 0.92
Water: 1.00
Sugar: 1.59
Halite: 2.18
Quartz: 2.65
Most Major Minerals: 2.6-3.0
Aluminum: 2.7
Pyrite, Hematite, Magnetite: 5.0
Galena: 7.5
Iron: 7.9
Copper: 9
Lead: 11.4
Mercury: 13.6
Uranium: 19
Gold: 19.3
Platinum: 21.4
Iridium: 22.4 (densest material on Earth)

MAJOR MINERAL SUITES

ELEMENTS

Metallic:Au, Ag, Cu
Not Al, Pb, Zn, Fe, etc.
Nonmetallic: C - Diamond, Graphite
Sulfur

SULFIDES

Dense, Usually Metallic
Many Major Ores

Pyrite FeS₂
Chalcopyrite CuFeS₂
Galena PbS₂
Sphalerite ZnS₂
Molybdenite MoS₂

HALIDES

Usually Soft, Often Soluble

Halite NaCl
Fluorite CaF₂

SULFATES

Soft, Light Color

Gypsum CaSO₄
Barite BaSO₄

OXIDES

Often Variable, Some Ores

Hematite Fe2O3
Bauxite Al (OH)3 (a hydroxide)
Corundum Al2O3 (Ruby, Sapphire)

CARBONATES

Fizz in Acid, Give off CO2

Calcite CaCO3
Dolomite CaMg (CO3)2

MOST IMPORTANT MINERAL SUITE:
The Silicate Minerals

Si + O = 75% of Crust
Silicates make up 95% + of all Rocks
SiO4: -4 charge
Link Corner-To-Corner by Sharing Oxygen atoms

Nesosilicates - Isolated Tetrahedra

In the sketches, the O's represent oxygen atoms. The tetrahedra are viewed from above, and the Si atom would be below the central O atom. These are schematic only, the actual three-dimensional arrangement is more complex.

Red circles denote other cations between the tetrahedra. The silica unit behaves like any other radical. The clue to a nesosilicate is SiO₄ in the chemical formula. Representatives:

Olivine
Garnet
Kyanite

Sorosilicates - Paired Tetrahedra

Epidote is the most common mineral of this type. The pair of tetrahedra has the formula Si₂O₇

Cyclosilicates - Rings

Minerals with three, four, and six-sided rings are known. Examples of the rare three- and four-sided rings are at top. Six-sided rings are most common. The ring unit has the formula Si(x)O(3x) where x is the number of tetrahedra in the ring. Six sided rings thus have the formula Si₆O₁₈. The most common are:

Beryl (Emerald - bottom left)
Tourmaline (bottom right)

Inosilicates - Chains

Single Chains

The ratio of silicon to oxygen is 1:3, so these minerals have formulas with SiO₃ or some multiple. Pyroxenes have this structure. Related minerals, called pyroxenoids, have single, but twisted, chains

Double Chains

These minerals, the amphiboles, have Si₄O₁₁in their formula. A few triple-chain minerals are also known.

Phyllosilicates - Sheets

These minerals have Si₂O₅ in their formulas. The silica sheets are sandwiched with layers of magnesium and aluminum hydroxide, water, and other cations. There are many possible structures formed by the various layering possibilities but the main groups are:

Micas
Clay minerals
Serpentine (asbestos) minerals

Tectosilicates - Three-Dimensional Networks

These include Quartz and the Feldspars

	One of the simplest tectosilicate structures: tridymite, a high-temperature form of silica.
	Quartz has a more complex structure, with spiral chains of tetrahedra. In this diagram they are colored differently to distinguish neighboring chains.
	The structure of the feldspars. The red atoms are potassium, sodium, or calcium. Since these atoms are cations, some of the tetrahedra contain aluminum (+3) instead of silicon (+4) to maintain charge balance.

The Problem of Crystals

Unit Cells

Repeating patterns, whether flowers on wallpaper, or atoms in a crystal, can all be described in terms ofUnit Cells. A unit cell is an imaginary box that contains the basic pattern. Repeating the unit cells recreates the whole pattern. There are FIVE basic unit cells for two-dimensional patterns:

Parallelogram (green)
Rectangle (purple)
Rhombus (yellow)
Square (blue)
Hexagonal (red)

Note that the rhombus pattern can be considered either as made up of rhombuses or as a rectangle with an extra point in the center. Crystallographers prefer the latter, because it makes the rectangular nature of the pattern clearer. Bricks in a wall have this pattern.

The hexagonal pattern can be described by rhombuses oriented in one of three ways. Two are in red and the third is outlined but not colored. The three unit cells lead to identical descriptions of the pattern.

Unit Cells in Three Dimensions

We can modify a cube by shaving off the edges, as shown in the top row. If we shave away the faces completely, the end result is a shape called a dodecahedron (Greek dodeka = twelve, hedron = side). The bottom left shows how cubic cells can be stacked to create this shape. The lower right shows the dodecahedron. In an actual crystal, the unit cells are so tiny the faces appear perfectly smooth.

We can modify a cube by shaving off the corners, as shown in the top row. If we shave away the corners completely, the end result is a shape called an octahedron (Greek okta = eight, hedron = side). The bottom left shows how cubic cells can be stacked to create this shape. The lower right shows the octahedron.

Below, we see some of the forms that can be made just from combining the three simple ways of stacking cubes above. Clearly, memorizing all the possible shapes is out of the question.

Animation showing how the forms below are related

Symmetry

What crystallographers look for is the rules behind the shapes, called symmetry. All the shapes above can be cut in half in many ways to make mirror-image halves. This is called reflection symmetry.

They can also be rotated in various ways to positions where they look the same as their original orientation. This is called rotational symmetry.

All together, there are 32 kinds of symmetry crystals can have, grouped into six classes according to the shapes of the unit cells in the crystal.

Animation of Symmetry

Upper Left: several possible ways of cutting the crystal into mirror-image halves are shown. There are numerous others not shown.

Upper Right: Note that the crystal looks the same four times during a complete rotation. We refer to this rotation axis as a four-fold symmetry axis.

Lower Left: The crystal looks the same three times during a complete rotation. We refer to this rotation axis as a three-fold symmetry axis.

Lower Right: The crystal looks the same twice during a complete rotation. We refer to this rotation axis as a two-fold symmetry axis.

Note (for the passionately interested only) that something irregular with no symmetry will only look the same once during a 360-degree rotation. Thus crystallographers say something with no symmetry has one-fold symmetry. It sounds convoluted, but all the mathematical formulas (yes, there is math in geology!) that are used to describe symmetry work perfectly.

The Crystal Classes

Just as plane patterns can be described in terms of five unit cells, three dimensional patterns can be thought of as belonging to one of six classes. Just as there are two kinds of rectangular plane patterns, there are several types of three-dimensional pattern for each of the six crystal classes

	ISOMETRIC or CUBIC All edges equal, all angles 90 degrees Halite, Fluorite, Pyrite Galena, Garnet, Magnetite Gold, Copper, Diamond
	TETRAGONAL Two edges equal, all angles 90 degrees. Square cross-section but different third dimension. Zircon Chalcopyrite
	ORTHORHOMBIC No edges equal, all angles 90 degrees. Like the shape of a cereal carton. Olivine, Andalusite, Sillimanite Some Amphiboles and Pyroxenes Topaz, Sulfur
	MONOCLINIC No edges equal, two angles 90 degrees. The shape obtained by knocking the ends out of a carton and skewing it. Some Amphiboles and Pyroxenes Micas Gypsum, Epidote Sugar also belongs to this crystal class.
	TRICLINIC No edges equal, no angles 90 degrees Most Feldspars Kyanite Clay Minerals What if you have one 90 degree angle, or two equal edges? It turns out that these contribute no extra symmetry and the crystal is still triclinic.
	HEXAGONAL Angles of 60, 90, and 120 degrees. Ice (snowflakes) Quartz, Beryl Corundum, Hematite Calcite, Dolomite

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Sunday 18 September 2011

complete notes of minerals(geology)