Rocky mountains and earthly ground minerals

Minerals groups and Associations

Some minerals belong to chemical groups or series called solid solutions.

In some circumstances, minerals are found together in groupings known as

associations or assemblages. These patterns of occurrence can provide

clues as to the minerals' origin.

Solid Solutions :

Some minerals do not have specific chemical compositions. Instead , they are homogeneous

mixtures of two minerals. These homogeneous mixtures are known as solid solutions.

For example, the olivine group of silicates includes forsterite and fayalite. Forsterite is a

magnesium silicate while fayalite is an iron silicate. Most olivine specimens are homongenous

mixtures of the two, with the relative content of magnesium and iron varying in specimens.

These minerals are described as part of a solid-solution series, in which forsterite and fayalite

are the end-members.

Primary and Secondary Minerals :

Primary minerals crystalize directly from magma and remain unaltered. They include esstential

minerals used to assign a classification name to a rock and accessory minerals that are present

in lesser abundance and do not affect the classification of a rock. Secondary minerals are

produced by the alteration of a primary mineral after its formation. For example, when copper-

bearing primary minerals come into contact with carbonated water, they alter into secondary

azurite or malachite.

Mineral Associations :

Some minerals are consistently found together over large areas because they are found in

the same rock type. Other associations occur in encrustations, veins, cavities, or thin layers.

The fact that certain minerals are likely to be found together can help in the discovery and

identification of minerals. Lead and zinc ore minerals are often associated with calcite and

barite, while gold is frequently found in association with quartz.

Associated minerals that form almost simultaneously and are usually present in a specific

rock type make up an assemblage. Orthoclase, albite, biotite, and quartz form an assemblage

for granite, and plagioclase, augite, magnetite, and olvine for gabbro. Assemblages are key

indicators of the environments in which minerals form.

Classifying Minerals

Classification of minerals is an ongoing study among mineralogists,

geologists who specifically study minerals. The ability to develop deep

into the structure and chemistry of minerals has increased dramatically

with advances in instruments and techniques.

The term "mineral" is commonly applied to certain organic substances, such as

coal, oil, and natural gas, when referring to a nation's wealth in resources. However,

these materials are more accurately referred to as hydrocarbons. Gases and liquids

are not, in the strict sense, minerals. Although ice, the solid state of water, is a

mineral, liquid water is not; nor is liquid mercury, which can be found in mercury ore

deposits. Synthetic equivalents of minerals, for example emeralds and diamonds

produced in the laboratory, are not minerals because they do not occur naturally.

The "minerals" referred to in foods are also not strictly minerals, they refer to elements,

such as iron, calcium , or zinc.

Chemical Formulae :

A chemical formula identifies the atoms present in a mineral and their proportions. In

some minerals, the atoms and their proportions are fixed. Pyrite, for example, is always

FeS2, denoting iron (Fe) and sulfur (S) in a 1:2 ratio. In solid solutions, the components

may be variable. For olivine, where complete substitution is possible between iron and

magnesium (Mg), the formula is (Fe, Mg)2 SiO4, indicating that iron and magnesium

are found in varying amounts.

Identifying Minerals

There are certain physical properties determined by the crystalline structure and chemical

composition of a mineral. These are commonly help to identify minerals without the use of

expensive equipment. Even a beginner can readily use these pointers.

Color :

Some minerals have characteristic colors, the bright blue of azurite, the yellow of sulfur, and

the green of malachite allow for easy identification. This is not true of all minerals, fluorite

occurs in virtually all colors, so it is best identified by other properties.

In minerals, color is caused by the absorption or refraction of light of particular wavelengths.

This can happen for several reasons. One is the presence of trace elements, "foreign" atoms

that are not part of the basic chemical makeup of the mineral in the crystal structure. As few as

three atoms per million can absorb enough of certain parts of the visible-light spectrum to give

color to some minerals. Color can also result from the absence of an atom or ionic radical from

a place that it would normally occupy in a crystal. The structure of the mineral itself, without any

defect or foreign element, may also cause color : opal is composed of minute spheres of silica

that diffract light; and the thin inter-layering of two feldspars in moonstone gives it color and sheen.

Luster:

A mineral's luster is the appearance of its surface in reflected light.

There are two broad types of luster : metallic and nonmetallic.

1) Metallic luster is that of an untarnished metal surface, such as gold, silver, or copper.

These minerals tend to be opaque.

2) Minerals with nonmetallic luster commonly show transparency or translucency.

Vitreous describes the luster of a piece of broken glass; adamantine, the brilliant luster of diamond;

resinous, the luster of a piece of resin; and pearly, the luster of mother-of-pearl or pearl. Greasy luster

refers to the appearance of being covered with a thin layer of oil, and silky, the appearance of the

surface of silk or satin. Dull luster implies little or no reflection, and earthly luster the non-lustrous

look of raw earth.

Streak :

The color of the powder produced when a specimen is drawn across a surface such as

unglazed porcelain is known as streak. A mineral's streak is consistent and is a more useful

diagnostic indicator than its color, which can vary. Streak can help distinguish between minerals

that are easy to confuse. For example, the iron oxide hematite has a red streak, while magnetite,

another iron oxide gives a black streak.

Cleavage :

The ability of a mineral to break along flat, planar surfaces is called cleavage.

It occurs in the crystal structure where the forces that bond atoms are the weakest.

Cleavage surfaces are generally smooth and reflect light evenly. Cleavage is described

by its direction relative to the orientation of the crystal and by the ease with which it is produced.

If cleavage easily produces smooth, lustrous surfaces, it is called perfect. Distinct, imperfect,

and difficult indicate less easy kinds of cleavage. Minerals may have different quality cleavages

in different directions. Some have no cleavage at all.

Fracture :

Some minerals can break in directions other than along cleavages planes. These breaks, known

as fractures. For example, hackly fractures (with jagged edges), are often found in metals, while

shell-like conchoidal fractures are typical of quartz. Other terms for fractures include even (rough but

more or less flat), uneven (rough and completely irregular), and splintery (with partially separated

fibers).

Tenacity :

The term tenacity describes the physical properties of a mineral based on the cohesive force

between atoms in the structure. Gold, silver, and copper are malleable and can be flattened without

crumbling. Sectile minerals can be cut smoothly with a knife; flexible minerals bend easily and stay

bent after pressure is removed; ductile minerals can be drawn into a wire; brittle minerals are prone

to breakage; and elastic minerals return to the original form after they are bent.

Hardness :

The hardness of a mineral is the relative ease or difficulty with which it can be scratched. A harder

mineral will scratch a softer one, but not vice versa. Minerals are assigned a number between 1 to 10

on the Mohs scale, which measures hardness relative to ten minerals of increasing hardness. Hardness

differs from toughness or strength; very hard minerals can be quite brittle. Most hydrous minerals, those

that contain water molecules are soft, as are phosphates, carbonates, sulfates, halides, and most sulfides.

Anhydrous oxides, those without water molecules, and silicates are relatively hard.

Refractive Index :

Light changes velocity and direction as it passes through a transparent or translucent mineral. The extent

of this change is measured by the refractive index : the ratio of light's velocity in air to its velocity in the

crystal. A high index causes dispersion of light into its component colors. Refractive indices can be found

using specialized liquids or inexpensive equipment.

Fluorescence :

Some minerals exhibit fluorescence, that is, they emit visible light of various colors when subjected to

ultraviolet radiation. Ultraviolet lights for testing fluorescence can be obtained from dealers selling

collectors' equipment. Fluorescence is an imperfect indicator of a mineral's identity because not all

specimens of a mineral show fluorescence, even if they look identical and come from the same location.

What are crystals?

Virtually all minerals are crystalline, solids in which the component atoms are arranged

in a particular, repeating , three-dimensional pattern. All crystals of a mineral are built with

the same pattern. Some are 10 feet ; others can only be seen with a microscope.

Atomic Structure :

A crystal is built up of individual, identical structural units of atoms or molecules called

unit cells. A crystal can consist of only a few unit cells or billions of them. The unit is

repeatedly repeated in 3-D, forming the larger internal structure of the crystal. The shape

of the unit cell and the symmetry of the structure determine the positions and shapes

of the crystal's faces.

Crystals of many different minerals have unit cells that are similar in shape but are made

of different chemical elements. The final development of the faces of a crystal is determined

by the symmetry of the atomic at the time of its formation. Certain faces may be emphasized,

while others disappear altogether. The final form taken by a crystal is known as its habit.

Crystal Symmetry :

All crystals exhibit symmetry because each crystal is built up of repeating geometric patterns.

These patterns of crystal symmetry are divided into six main groups, or crystal system. The

first of these symmetrical patterns is the cubic system, in which all crystals exhibit cubic

symmetry. The characteristics of cubic symmetry may be explained a follows : if opposite

face centers of a cube-shaped cubic crystal, such as halite, are held between the thumb

and forefinger and the crystal rotated through 360 degrees, the pattern of faces will appear

identical four times as the different faces and edges come into view.

All cubic crystals have three axes of fourfold symmetry. They have other axes of symmetry,

but these differ among classes within the cubic system. For example, cube-shaped crystals

 of halite have three axes of fourfold symmetry, in addition to its four axes of threefold symmetry.

Twin Crystals :

When two or more crystals of the same species (a group of minerals that are chemically similar),

such as gypsum or fluorite, form a symmetrical inter-growth, they are referred to as twinned crystals.

Twins can be described as interpenetrating or contact. Penetration twinning may occur with

individual crystals at an angle to one another, for example, forming a cross. It can occur with

individual crystals parallel to one another, as in Carlsbad twinning. If twin involves three or more

individual crystals, it is referred to as a multiple twin or a repeated twin. Albite often forms multiple

twins. Many other minerals form twins, but they are particularly characteristic of some, such as the

fishtail contact twins of gypsum or the penetration twins of fluorite.

Crystal Habits

Habit refers to the external shape of a crystal or an assemblage of inter-grown. It includes

names of crystal's faces, such as prismatic and pyramidal, names of forms , such as cubic

and octahedral, and descriptive terms, such as bladed and dendritic.

Crystal Faces :

The three types of crystal face, prism, pyramid, and pinacoid, are determined by a relationship

to a crystallographic axis. Prism faces are parallel to the axis; pyramid faces cut through the

axis at an angle; and pinacoid faces are at right angle to the axis.

Crystal Forms :

Habits can be named after crystal forms: "cubic" implies crystallizing in the form of cubes;

"dodecahedral", in the form of dodecahedrons; and "rhombohedral", in the form of rhombohedrons.

When crystals of one system crystallize in forms that appear to be the crystals of another system,

the habit name is preceded by the word "pseudo". When terminations take different forms in the

same crystal, the habit is known as hemimorphic.

Aggregates :

Aggregates are groups of intimately associated crystals. In general, aggregates are inter-growths of

imperfectly developed crystals. In some aggregates, the crystals may be microscopic. The type of

aggregation is often typical of a particular mineral species. Terms used to describe aggregates include

granular, fibrous, radiating, botryoidal, stalactitic, geodic, and massive.

Crystal Appearance :

Some habits are descriptions of the general appearance of a crystal. The term "tabular" describes

a crystal with large, flat parallel faces; "bladed" describes elongated crystals that are flattened

like a knife blade, "stalactitic" describes crystal aggregates shaped like stalactites; and "blocky"

or "equant" describes crystals will faces that are roughly the same size in all directions.

Crystal Systems

Crystals are classified into six different systems according to the maximum symmetry

of their faces. Each crystal system is defined by the relative lengths and orientation of its

three crystallographic axes, imaginary lines that pass through the centre of an ideal crystal.

1) Cubic :

Cubic crystals have three crystallographic axes (a, a2, and a3) at right angles and of

equal length, and four threefold axes of symmetry. The main forms within this system

are cube, octahedron, and rhombic dodecahedron. Halite, copper, gold, silver, platinum,

iron, fluorite, and magnetite crystallize in the cubic system, which is also known as the

isometric system.

2) Tetragonal :

Tetragonal crystals have three crystallographic axes at right angles, two equal in length

(a, and a2), and the third (c) longer or shorter. These crystals have one principal, fourfold

axis of symmetry. Crystals look like square or octahedral prisms in shape. Rutile, zircon,

cassiterite, and calomel are minerals that crystallize in the tetragonal system.

3) Hexagonal and Trigonal :

Some crystallographers consider hexagonal and trigonal crystals to comprise a single

system, whereas others regard them as forming separate systems. Both crystalline

forms have three crystallographic axes (a, a2, and a3) of equal length. These are at

120 degrees to one another and to a fourth axis (c), which is perpendicular to the plane

of the other three axes. Trigonal crystals have only threefold symmetry, whereas

hexagonal crystals have sixfold symmetry. Minerals that crystallize in the hexagonal

system include beryl (emerald and aquamarine) and apatite. Some of the minerals that

crystallize in the trigonal system are calcite, quartz, and tourmaline.

4) Monoclinic :

The term "monoclinic" means "one incline." Monoclinic crystals have three crystallographic

axes of unequal length. One (c) is at right angles to the other two (a and b). These two

axes are not perpendicular to each other, although they are in the same plane. The crystals

have twofold axis of symmetry. More minerals crystallize in the monoclinic system than in

any other crystal system. Example are gypsum, orthoclase, malachite, and jadeite.

5) Orthorhombic :

Orthorhombic means "perpendicular parallelogram". Crystals in this system have three

crystallographic axes (a, b, and c) at right angles, all of which are unequal in length.

They have three twofold axes of symmetry. Minerals that crystallize in this system include

olivine, aragonite, topaz, marcasite and barite.

6) Triclinic :

Trilinic crystals have the least symmetry shape, with three crystallographic axes of

unequal length (a, b and c) inclined at angles other than 90 degrees to each other.

The orientation of a triclinic crystal is arbitrary. Minerals that crystallize in this system

include albite, anorthite, kaolin, and kyanite.



Gem Cuts

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Gems

A gems is any mineral that is highly prized for its beauty, durability, and rarity.

It is enhanced in some manner by altering its shape, usually by cutting and

polishing. Most gems begin as crystals of minerals or as aggregates of crystals.

The use of gemstones in human history goes back to the Upper Paleolithic Period

(25,000 --- 12,000 BCE.). People were initially drawn by the bright colors and beautiful

patterns of gems. When the shaping of stones for adornment first began, opaque and

soft specimens were used. As shaping techniques improved, harder stones began to

be cut into gems. Beads of the quartz varieties hard carnelian and rock crystal were

fashioned in Mesopotamia (now Iraq) in the 7th millennium BCE. Records of the time

suggest that people thought that stones had a mystic value, a belief that persists to

the present.

Gem Mining :

Gemstone deposits form in different geological environments. Perhaps the best known

are the "pipes" of kimberlite, from which most diamonds are recovered by the hard-rock

methods of drilling and blasting. Other gems also recovered from the rock in which

they form are quartz varieties, opal, tourmaline, topaz, emerald, aquamarine, some

sapphires and rubies, turquoise, lapis lazuli, and chrysoberyl. Hard and dense gemstones

that are impervious to chemical weathering are carried by water to placer deposits such

as river beds, beaches, and the ocean floor. Placer mining techniques mimic the creation

of the placer by separating denser minerals in running water. The simplest methods are

panning and sieving, or passing gravel through a trough of flowing water with baffles at the

bottom. The lighter material washes away but denser gemstones remain.

Faceting :

Gemstones can be shaped in several ways. Opaque or translucent semiprecious stones,

such as agate and jasper, are tumble-polished, carved, engraved, or cut with a rounded upper

surface and a flat underside. Grinding and polishing of flat faces on the stone is called faceting.

Facets are placed in specific geometric positions at specific angles according to the bending

of light within a particular stone. Transparent stones, such a amethyst, diamond, and sapphire,

are faceted to maximize their brilliance and "fire" or enhance color. Although much material is

ground away while cutting, the final value is much enhanced.

What is a Rock?

A rock is a naturally occurring and coherent aggregate of one or more minerals.

There are three major classes of rock, igneous, sedimentary, and metamorphic.

Each of these three classes is further subdivided into group and types.

Types of Rock :

Igneous rocks form from melted rock called magma. When magmas solidify

underground, intrusive rocks such as granite are created. Intrusive rocks are

also known as plutonic rocks. If the magma flows onto the surface of the land

or ocean bed, extrusive rocks such as basalt, are formed.

Sedimentary rocks are usually made of deposits laid down on Earth's surface

by water, wind, or ice. They almost always occur in layers or strata. Stratification

survives compaction and cementation and is a distinguishing feature of sedimentary

rocks. Some sedimentary rocks are of chemical origin, having been deposited in solid

form from a solution. Others are of biochemical origin and are composed predominantly

of the compound calcium carbonate.

When existing rocks are subjected to extreme temperatures or pressures, or both,

their composition, texture, and internal structure may be altered to form metamorphic

rocks. The original rocks may be igneous, sedimentary or metamorphic.

Meteorites are not considered igneous, sedimentary, or metamorphic but are a group

of their own. Many are remnants of the formation of the Solar System. Some meteorites

are remains of the nickel-iron cores of asteroids; some contain nickel-iron and minerals

such as olivine from the mantles of asteroids; and others are made up principally of

silicate minerals.

The Rock Cycle :

The series of processes by which rocks are created , broken down, and reconstituted

as new rocks is known as the rock cycle. These processes depend on pressure,

temperature, time, and changes in environmental conditions in Earth's crust and surface.

At various stages in the rock cycle, old rocks are broken down, new minerals form, and

new rocks originate from the components of the old. Thus a rock that began at the surface

as an igneous rock may be reworked into a sedimentary rock to continue the cycle again.

Native Copper

Copper :

In its free-occurring metallic state, copper was probably the first metal to be used

by humans. Neolithic people are believed to have used copper as a substitute for

stone by 8,000 BCE. Around 4,000 BCE, Egyptians cast copper in molds. By

3,500 BCE, copper began to be alloyed with tin to produce bronze.

Copper is opaque, bright, and metallic salmon pink on freshly broken surfaces but

soon turns dull brown. Copper crystals are uncommon, but when formed are either

cubic or dodecahedral, often arranged in branching aggregates. Most copper is

found as irregular, flattened, or branching masses. It is one of the few metals that

occur in the "native" form without being bonded to other elements. Native copper

seems to be a secondary mineral, a result of interaction between copper-bearing

solutions and iron-bearing minerals.

(Copper Wire)

Platinum

The first documented discovery of platinum was by the Spaniards in the 1500s, in the alluvial

gold mines of the Rio Pinto, Colombia. They called it platina del Pinto, from platina which

means "little silver", thinking that it was an impure ore of silver. It was not recognized as a

distinct metal until 1735. It is opaque, silvery gray, and markedly dense.

Platinum usually occurs as disseminated grains in iron- and magnesium-rich igneous rocks and

in quartz veins associated with hematite, chlorite, and pyrolusite . When rocks weather, the

heavy platinum accumulates as grains and nuggets in the resulting placer deposits. Crystals

are rare, but when found they are cubic. Most platinum for commercial use is recovered from

primary deposits. Native platinum typically contains iron and metals such as palladium, iridium,

and rhodium.

Iron

Five percent of Earth's crust is made up of iron. Native iron is rare in the crust and

is invariably alloyed with nickel. Low-nickel iron is called kamacite, and high-nickel

iron is called taenite. Both crystallize in the cubic system. A third form of iron-nickel,

mainly found in meteorites and crystallizing in the tetragonal system, is called

tetrataenite. All three forms are generally found either as disseminated grains or as

rounded masses.

Kamacite is the major component of most iron meteorites. It is found in most chondritic

meteorites, and occurs as microscopic grains in some lunar rocks. Taenite and

tetrataenite are mainly inter-grown with kamacite. Iron is also plentiful in the Sun and other

stars.

Native Bismuth

As a native metal, bismuth has been known since the Middle Ages. A German monk

Basil Valentine first described it in 1450. Bismuth is often found uncombined with

other elements, forming indistinct crystals, often in parallel groupings. It is hard, brittle,

and lustrous. It is also found in grains and as foliated masses. Silver-white, it usually

has a reddish tinge that distinguishes it. Specimens may have an iridescent tarnish.

Bismuth is found in hydrothermal veins and in pegmatites and is often associated with

ores of tin, lead, copper, from which it is separated as a by-product. Bismuth expands

slightly when it solidifies, making its alloys useful in the manufacture of metal castings

with sharp detailing. Bismuth salts are often used as soothing agents for digestive

disorders.

Antimony



Although recognized as a metal since the 8th century or earlier, antimony was only

identified as an element in 1748. Crystals are rare but when found are either psuedo-cubic

or thick and tabular. Antimony usually occurs in massive, foliated, or granular form. It is

lustrous, silvery, bluish white in color, and has a flaky texture that makes it brittle. It

almost always contains some arsenic and is found in veins with silver, arsenic, and other

antimony minerals.

Antimony is extremely important in alloys. Even in minor quantities, it imparts strength and

hardness to other metals, particularly lead, whose alloys are used in the plates of automobile

storage batteries, in bullets, and in coverings for cables. Combined with tin and lead, antimony

forms anti-friction alloys called babbitt metals, which are used as components of machine

bearings. Like bismuth, antimony expands slightly on solidifying, making it a useful alloying metal

for detailed castings.