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

no, #3 YES!

no, #2 basalt obsidian mica gold paper chalk coral no, #5 no, #4 YES!

YES!

no, #1,2 no, #2 no, #2

Mineral characteristics

• •

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 Calcite

CaCO 3 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.

Mineral characteristics

•

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 formula

examples:

gold (Au), quartz (SiO 2 ), orthoclase (KAlSi 3 O 8 ) basalt (like many other rocks) contains variable ratios of different minerals; thus, has no consistent formula

Minerals are …

•

Composed of ordered arrangements of atoms….

–

Repeated patterns

–

Uniform spacing

–atoms arranged in an orderly, repeating, 3-D array.

e.g., Atomic Structure of Diamond

How many minerals are there?

•

Nearly 4,000 types of minerals

– Only ~30 occur commonly – Why not more?

• •

Some combinations are chemically impossible Relative abundances of elements don’t allow more

•

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

anions charge

O Cl S -2 -1 -2

cations

Si K Ca Na Al Mg Fe

charge

+4 +1 +2 +1 +3 +2 +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.

Color

Some 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 Citrine Smokey Quartz Agate, Calcedony, Jasper Quartz Gems

Important Physical Properties

•

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.

Important Physical Properties

•

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.

Luster

• • • • • • •

a. Metallic b. 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.

Cleavage

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

Calcite CLEAVAGE Cleavage in three directions not at right angles (120 o and 60 o ). Rhombohedral cleavage.

Mineral Type of Breakage Quartz FRACTURE Mineral does not exhibit cleavage, it breaks or fracture in an irregular manner. Feldspar 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/m 3 geologists often use g/cm 3 or kg m -3 , but 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 (SiO 2 ) 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.

5) Record the level after the specimen was placed under water. In this case 920ml.

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).

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 (Fe 3 O 4 ) has this property and it can be used to distinguish it from other non magnetite iron oxides, such as hematite (Fe 2 O 3 ).

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

Name

Olivine Pyroxene Amphibole Micas Feldspars Carbonates Evaporites

Important Mineral Groups

Important constituents (other than O)

Si, Fe, Mg Si, Fe, Mg, Ca Si, Ca, Mg, Fe, Na, K Si, Al, K, Fe, Mg Si, Al, Ca, Na, K C, Ca, Mg K, Cl, Ca, S

•

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;

Pyroxenes -

high T, low silica rocks comprises over 50% of upper mantle

Amphiboles Mica -

esp. biotite; moderate T, higher silica rocks

Garnet -

esp. hornblende; moderate T, higher silica rocks common metamorphic mineral • – –

Nonferromagnesian silicates Mica

- exp. muscovite; moderate T, higher silica rocks

Feldspars

light color, density close to 2.7 g/cc - 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

Important Silicates

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. Fe 2 SiO 4 ( Fayalite ) and Mg 2 SiO 4 (Foresterite). A combination formula might look like this: (Mg,Fe) 2 SiO 4 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.

The general formula is A 3 B 2 (SiO 4 ) 3 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.

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)