Minerals: Physical Properties and Crystal Forms From: .

Minerals: Physical Properties and Crystal Forms

From: http://webmineral.com/data/Rhodochrosite.shtml

Minerals: the building blocks of rocks

• Definition of a Mineral: naturally occurring inorganic solid characteristic crystalline structure definite chemical composition

• Definition of a Rock:• A solid aggregate (mixture) of minerals

Mineral characteristics

• Definition of a Mineral:1. naturally occurring

2. inorganic

3. solid

4. characteristic crystalline structure

5. definite chemical composition

steel plastic sugar table salt mercury ice coal

basalt obsidian mica gold paper chalk coral

no, #1 no, #1 no, #1,2 YES! no, #3 YES! no, #2

no, #5 no, #4 YES! YES! no, #1,2 no, #2 no, #2


• Naturally formed– No substance created artificially is a mineral. examples: plastic, steel, sugar, paper

• Inorganic– Anything formed by a living organism and

containing organic materials is not a mineral. examples: wood, plants, shells, coal

• Solid– Liquids and gases are not minerals. examples: water, petroleum, lava, oxygen

Inorganic CalciteInorganic Calcite

CaCO3 calcium carbonate

Calcium carbonate, or CaCO3, comprises more than 4% of the earth’s crust and is found throughout the world. Its most common natural forms are chalk, limestone, and marble, produced by the sedimentation of the shells of small fossilized snails, shellfish, and coral over millions of years. Although all three forms are identical in chemical terms, they differ in many other respects, including purity, whiteness, thickness and homogeneity. Calcium carbonate is one of the most useful and versatile materials known to man.


• Characteristic crystalline structure– must have an ordered arrangement of atoms

– displays repetitive geometric patterns in 3-D

glass not a mineral (no internal crystalline structure)

• Definite chemical composition– must have consistent chemical formulaexamples: gold (Au), quartz (SiO2), orthoclase (KAlSi3O8)

basalt (like many other rocks) contains variable ratios of different minerals; thus, has no consistent formula

Minerals are …Minerals are …

• Composed of ordered arrangements of atoms….

– Repeated patterns

– Uniform spacing

e.g., Atomic Structure of Diamond

–atoms arranged in an orderly, repeating,

3-D array.

– Only ~30 occur commonly

– Why not more?• Some combinations are chemically impossible

• Relative abundances of elements don’t allow more

How many minerals are there?

• Nearly 4,000 types of minerals

Mineral Formation• Crystallization from a magma

– quartz, feldspar, mica in granite

• Crystal growth in the solid state– mica, garnet, feldspar in schist

• Precipitation from solution– calcite in marine organism shells

– silica in agate

Quartz Crystals

Element abundances in the crust

All others: 1.5%

Ions• When an atom loses or gains an electron to or

from another atom it is called an ion.

• Positively charged ions (loss of electron) are called cations.

• Negatively charged ions (gain of electron) are called anions.

• Ions of opposite charge attract (net charge = 0):

– > Ionic Bonding

• 90% of all minerals are ~ ionic compounds.

Ionic Bonding

Cation (+) Anion (-)

Ionic Compounds

Fig.3.4

- NaCl- Halite- Table Salt

Important Ions in Minerals

anionsanions chargecharge cationscations chargecharge

O -2Cl -1S -2

Si +4K +1Ca +2Na +1Al +3Mg +2Fe +2 or +3

Covalent Bonding• Electrons are shared between atoms.

• Covalent bonds are much more stable and stronger than ionic bonds.

Atomic Structure of

Diamond

Crystal Structure

• Anions are generally larger than cations.

• The structure of the mineral is determined largely by how the anions are arranged, and how the cations fit between them.

Ionic Radius and Charge

Crystal Structure

Perfect Crystals:Predictable

Interface Angles

Polymorphs• Identical chemical compounds

• Different atomic structure

• Generally stable under different conditions.

“Polymorphs” (although different minerals)

Physical properties of minerals

• We know that minerals are composed of atoms, arranged in a specific order, with a well defined chemical composition.

• We might expect then that the microscopic variations in bond environment will also be manifested in macroscopic physical and chemical properties.

• This is indeed the case.

The Physical Properties of Minerals

• Color

• Streak

• Luster

• Hardness

• External Crystal Form

• Cleavage

The Physical Properties of Minerals (cont.)

• Fracture

• Specific Gravity

• Special Properties

• Other Properties

• Chemical Tests

Important Physical Properties

• Color - Although an obvious feature, it is often unreliable to use to determine the type of mineral.

– Color arises due to electronic transitions, often of trace

constituents, in the visible range of the EM spectrum. For example, quartz is found in a variety of colors.

• Color of a mineral may be quite diagnostic for the trace element and coordination number of its bonding environment.

ColorSome minerals have more than one color for example; purple amethyst and yellow citrine are both varieties of quartz. In contrast, yellow is the only color of sulfur and is therefore a useful tool in identifying this mineral.

Crystal Quartz

Amethyst

Smokey Quartz

Citrine

Agate, Calcedony, Jasper Quartz Gems


• Streak - The color of a mineral in its powdered form; obtained by rubbing the mineral against an unglazed porcelain plate.

– Useful for distinguishing between minerals

with metallic luster.

Streak - The color of a mineral in its powdered form;

Hematite may look black, but it will always produce a RED/BROWN streak on a streak plate. Care must be taken if the mineral being tested is harder than the porcelain, the result will be a powder produced by the porcelain plate being scratched and will always be white.


• Luster - This property describes the appearance of reflected light from the mineral's surface. Nonmetallic minerals are described using the following terms: vitreous, pearly, silky, resinous, and earthy.

Lustera. Metallicb. Non-

metallic• Vitreous• Resinous• Dull• Pearly• Silky• Greasy• Glassy

Hope Diamond: 44.5 carats

http://www.nmnh.si.edu/minsci/hope.htm

Hardness:

We measure the hardness of a mineral by how easy we can scratch it using different tools like finger nails, piece of glass and piece of copper (usually a

penny).

MOH’s hardness scale

External Crystal Form

• Crystal form

• Minerals are grouped into systems according to their crystal symmetry (regularity of form).

• The figure below shows the six main systems.

CleavageCleavage occurs when a mineral has a preferred direction of breakage. Cleavage like most other properties is a function of the crystal structure and the nature of the bonding. When a mineral is broken, it breaks into pieces that resemble one another, then it is said to have perfect cleavage.

Perfect cleavage is due to a higher order of symmetry and is more prevalent in minerals with strong ionic (therefore weak) bonding. Cleavage surfaces usually look almost polished and are very flat. The angles between cleavage surfaces is related to the crystal structure and hence diagnostic of the particular mineral.

Planer Cleavage in Mica

Weak Bonding Yields Planer Cleavage

Fracture• Fracture is the

tendency of a mineral to break along curved surfaces without a definite shape. These minerals do not have planes of weakness and break irregularly.

Mineral Type of Breakage

Halite

Calcite

CLEAVAGE

Cleavage in three directions at right angles (90o). Cubic cleavage.

CLEAVAGE

Cleavage in three directions not at right angles (120o and 60o).

Rhombohedral cleavage.

Mineral Type of Breakage

Quartz

Feldspar

FRACTURE

Mineral does not exhibit cleavage, it breaks or fracture in an irregular manner.

CLEAVAGE

Cleavage in two directions at right angles.

Rhombohedral Cleavage in Calcite

Conchoidal Fracture in Glass

Density and Specific Gravity

• Density - Defined as the mass divided by the volume and normally designated by the Greek letter, rho, mass/volume; SI units: kg/m3 or kg m-3, but

geologists often use g/cm3 as the unit of choice.

• Specific Gravity - Ratio of the mass of a substance to the mass of an equal volume of water. Note that water = 1 g cm-3. S.G. is unitless.

• Examples - quartz (SiO2) has a S.G. of 2.65 while galena (PbS) has a S.G. of 7.5 and gold (Au) has a S.G. of 19.3.

Steps to determine the density of a mineral.

1) Use a balance. In this example the balance to be used is a triple beam balance.

2) Place the specimen in the weighing pan.

3) Record the weight of the specimen, in this case 155.8 grams.

4) Record the level in a graduated cylinder before you put the specimen in. In this case 900ml.

Density = 155.8grams /

20 cc

6) Divide 155.8/20 = 7.79 g/cc. (in this case 20ml = 20cc, because the amount of units displaced are equivalent). So, the density of the minerals is 7.79g/cc.

7) The closest mineral having this density is galena with a density of 7.60 g/cc. (In order to get a precise density sophisticated equipment with a tolerance of five to seven decimals is used).

5) Record the level after the specimen was placed under water. In

this case 920ml.

Special and Other Properties• Striations - Commonly found on plagioclase

feldspar. Straight, parallel lines on one or more of the cleavage planes caused by mineral twinning.

• Magnetism - Property of a substance such that it will spontaneous orient itself within a magnetic field. Magnetite (Fe3O4) has this property and it can be used to distinguish it from other non-magnetite iron oxides, such as hematite (Fe2O3).

• Double Refraction - Seen in calcite crystals. Light is split or refracted into two components giving rise to two distinct images.

Plagioclase striations

Calcite Double Refraction

Important Mineral Groups

Olivine Si, Fe, MgPyroxene Si, Fe, Mg, CaAmphibole Si, Ca, Mg, Fe, Na, KMicas Si, Al, K, Fe, MgFeldspars Si, Al, Ca, Na, KCarbonates C, Ca, MgEvaporites K, Cl, Ca, S

NameName Important constituentsImportant constituents(other than O)

Color and Density• Two broad categories are ferromagnesian and nonferromagnesian silicates,

which simply means iron and magnesian bearing or not. The presence or absence of Fe and Mg strongly affects the external appearance (color) and density of the minerals.

• Ferromagnesian silicates - dark color, density range from 3.2 - 3.6 g/cc

– Olivine - high T, low silica rocks; comprises over 50% of upper mantle

– Pyroxenes - high T, low silica rocks

– Amphiboles - esp. hornblende; moderate T, higher silica rocks

– Mica - esp. biotite; moderate T, higher silica rocks

– Garnet - common metamorphic mineral

• Nonferromagnesian silicates - light color, density close to 2.7 g/cc

– Mica - exp. muscovite; moderate T, higher silica rocks

– Feldspars - plagioclase and orthoclase; most common mineral in crust; form over a wide range of temperatures and melt compositions

– Quartz - low T, high silica rocks; extremely stable at surface, hence it tends to be a major component in sedimentary rocks.

– Clay - esp. kaolinite; different types found in different soils

The Olivine Group is made up of two "end members" one with iron (Fe) and the other with magnesium (Mg). In the real world there is probably never a pure olivine, most are combinations of the two end members. Fe2SiO4

( Fayalite ) and Mg2SiO4 (Foresterite).

A combination formula might look like this: (Mg,Fe)2SiO4

Olivine occurs in the crust, ocean crust, and upper mantle and is not rare, except as clean crystals. It normally grows in small granular forms. As a gem it is known as peridot.

Important Silicates

Where A can be any of the following: Mg+2, Fe+2, Ca+2, and Mn+2. The B elements can be any of the following: Al+3, Fe+3, Cr+3 or a mixture thereof.

Because it has a wide variety of possible chemical formulas, it can be found in a rainbow of colors.

One of the important uses for garnet is the manufacture of wood sandpaper.Grossular (Ca,Al) tsavorite, rhodinite, pyrope Andradite (Ca,Fe) Almandine (Fe,Al)

The Garnet Group is another with the isolated silicate

structure. As with olivine it is made up of different

substitution patterns, but unlike olivine it has many "end members" and thus a variety of

different chemistries.

The general formula is

A3B2(SiO4)3

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