The main goal of this paper is to
introduce and describe five species of minerals which are represented in a
small collection brought out of the copper mines of Butte, Montana during
the 1950's. The species included in the collection are native copper,
azurite, malachite, rhodocrosite, and chrysocolla. Their description is
contained in Section 6, which is the final portion of the paper.
Section 6 states the species and class
of the five minerals represented in the collection. It then describes the
physical characteristics of these species. It discusses their color,
mentions their streak, luster, density, and hardness, and notes
characteristic patterns of cleavage and fracture. It relates their
crystal class, and states their crystal habit and preferred patterns of
crystalline growth. Finally it mentions in what type of environment and
in the company of which other species these minerals may be found.
Sections 1 through 5 attempt to
explain the terminology and ideas which are utilized in Section 6 to
describe the minerals of the collection. In Section 1 the paper first
attempts to define the set of characteristics which cause certain
crystalline natural substances to be defined as minerals. In Section 2 it
continues with a description of the physical attributes of minerals.
These attributes include such characteristics as color, luster, density,
hardness, and preferred patterns of crystal growth. Section 3 then
considers the structure, symmetry and shapes which may be possessed by
crystals. It introduces the six systems and thirty-two classes into which
crystals are categorized on the basis of their symmetry features. The
fourth section of the paper explains the system of classification under
which each mineral species is assigned to a particular mineral class
according to certain chemical criteria. Section 5 explains the different
geologic processes by which rocks and minerals are formed. Section 6
finally utilizes the information presented in the first five sections in
order to describe the five mineral species represented in the small
collection.
A mineral is defined to be a naturally
occurring, homogeneous crystalline solid. A mineral must first of all be
a solid; this requirement excludes all liquids and gases. A crystalline
material is not merely a solid but also possesses an orderly and periodic
molecular structure. This molecular structure is composed of repeated
units, each of which contains a specified set of atoms or ions placed
according to a specified geometric arrangement. The periodic repetition
of such unit cells results in discrete translational invariance within the
crystal lattice. An amorphous solid such as amber, coal, or glass does
not possess this orderly, periodic, translationally invariant crystalline
lattice structure and thus may not be considered a mineral.
Minerals are expected to be
homogeneous: the chemical composition of a mineral may not differ
significantly from sample to sample within a species or from location to
location within a sample. However, small variations in chemical
composition are permissable. In certain species the chemical composition
may vary over a well-defined range. Two samples, each of which possesses
a chemical composition which falls within the required range, are then
considered to be minerals of the same species despite any difference in
their compositions. Trace amounts of impurities, or elements which
are not an integral part of the crystal structure, may also be present
within the crystal lattice. The atoms of an impurity may either occupy a
previously uninhabited position within a few individual unit cells or they
may replace a few atoms which are regularly part of the lattice. Such
impurities may not be present in large quantities and they may not greatly
affect any physical properties of the mineral species except color,
hardness, and conductivity.
The requirement that a mineral be
naturally occurring was conceived before the production of synthetic
materials was prevalent. In recent years, however, a multitude of
synthetic materials which would otherwise satisfy the definition of a
mineral have been produced in the laboratory. These substances are
rapidly gaining in importance in both industry and experiment, and it now
seems logical to extend the definition of 'mineral' in order to include
such synthetic materials.
The physical properties of a mineral
are determined by its chemical composition and its crystalline structure.
Within the limits of the permissible variation in chemical composition,
different samples of a single mineral species are expected to display the
same set of physical properties. These characteristic physical properties
are therefore very useful to the field geologist in identifying and
describing a specimen.
Properties which describe the physical
appearance of a mineral specimen include color, streak, and
luster. Mass-dependent properties include density;
mechanical properties include hardness, cleavage, fracture, and
tenacity. Properties relating to the growth patterns and physical
appearance of crystals, both individually and in aggregate, are described
in terms of crystal habit, crystal form, and crystal system.
Some minerals derive their color from
the presence of an element which is defined to be part of the crystal
lattice. Coloration of this type is termed allochromatic
coloration. Different samples of a mineral species which possesses
allochromatic coloration will all display the same color. Other minerals
are colored by the presence of certain elements in mixture. These
minerals may exhibit a range of colors. Still other mineral species may
usually be colorless, but may display several startling and different
colors when trace amounts of impurities are present. This type of
coloration is termed idiochromatism. Certain elements are strong
pigmenting agents and may lend vivid colors to specimens when they are
present, whether as a part of the crystal lattice, in mixture, or as an
impurity. Such elements are called chromophores. The color which
a mineral displays when it has been ground to a fine powder is termed
streak. Trace amounts of impurities do not tend to affect the
steak of a mineral, so this characteristic is usually more predictable
than color.
Minerals are either opaque or
transparent. A thin section of an opaque mineral will not transmit light,
whereas a thin section of a transparent mineral will. Relative
differences in opacity and transparency are described as luster.
The characteristic of luster refers to the amount and quality of light
which is reflected from a mineral's exterior surfaces, and provides a
description of how much the mineral surface 'sparkles'.
The property of density is
defined as mass per unit volume. Hardness is defined as the amount
of difficulty with which a mineral specimen may be scratched. Hardness
has traditionally been measured according to the Mohs scale, but the
diamond indentation method now provides a more quantitative measurement.
The cleavage of a crystal refers to its propensity for splitting
along a smooth plane. A cleavage plane is a plane of structural weakness.
The property which is termed cleavage refers to both the ease with which
the mineral cleaves and to the character of the exposed surface. Not
every mineral exhibits cleavage. Fracture takes place when a
mineral sample is split in a direction which does not serve as a plane of
cleavage. A mineral fractures when it is broken or crushed. In some
mineral species fractured surfaces may possess a characteristic
appearance.
The term crystal habit
describes the favored growth pattern of the crystals of a mineral species.
The crystals of particular species sometimes form very
distinctive, characteristic shapes. Crystal habit is also greatly
determined by the environmental conditions under which a crystal develops.
The descriptive terminology of the
discipline of crystallography is applied to crystals in order to
describe their structure, symmetry, and shape. This terminology describes
the crystal lattice, which provides a mineral with its ordered internal
structure. It also describes and analyzes various types of symmetry. By
considering what type of symmetry a mineral species possesses, the species
may be categorized as a member of one of six crystal systems and one of
thirty-two crystal classes.
The concept of symmetry
describes the periodic repetition of structural features. Two general
types of symmetry exist. These include translational symmetry and
point symmetry. Translational symmetry describes the periodic
repetition of a motif across a length or through an area or volume. Point
symmetry, on the other hand, describes the periodic repetition of a motif
about a single point. Reflection, rotation, inversion, and rotoinversion
are all point symmetry operations.
A specified motif which is translated
linearly and repeated many times will produce a lattice. A lattice
is an array of points which define a repeated spatial entity called a
unit cell. The unit cell of a lattice is the smallest unit which
can be repeated in three dimensions in order to construct the lattice.
The number of possible lattices is
limited. In the plane only five different lattices may be produced by
translation. The French crystallographer Auguste Bravais (1811-1863)
established that in three-dimensional space only fourteen different
lattices may be constructed. These fourteen different lattices are thus
termed the Bravais lattices.
The reflection, rotation, inversion,
and rotoinversion symmetry operations may be combined in a variety of
different ways. There are thirty-two possible unique combinations of
symmetry operations. Minerals possessing the different combinations are
therefore categorized as members of thirty-two crystal classes.
In this classificatory scheme each crystal class corresponds to a unique
set of symmetry operations. Each of the crystal classes is named
according to the variant of a crystal form which it displays.
Each crystal class is grouped as one of the six different crystal
systems according to which characteristic symmetry operation it
possesses.
A crystal form is a set of
planar faces which are geometrically equivalent and whose spatial
positions are related to one another by a specified set of symmetry
operations. If one face of a crystal form is defined, the specified set
of point symmetry operations will determine all of the other faces of the
crystal form. A simple crystal may consist of only a single crystal form.
A more complicated crystal may be a combination of several different
forms. Example crystal forms are the parallelohedron, prism, pyramid,
trapezohedron, rhombohedron and tetrahedron.
Each crystal class is a member of one
of six crystal systems. These include the isometric, hexagonal,
tetragonal, orthorhombic, monoclinic, and triclinic crystal systems.
Every crystal of a certain crystal system shares a characteristic symmetry
element - for example, a certain axis of rotational symmetry - with the
other members of its system. The crystal system of a mineral species may
sometimes be determined by examining a particularly well-formed crystal of
the species.
The Berzelian mineral classification
system was named in honor of the Swedish chemist and mineralogist Jons
Jakob Berzelius (1779-1848). The Berzelian system categorizes mineral
species according to the main anion group present in their chemical
composition. Mineral classes may then be further subdivided according to
physical features, the presence of certain cations in the lattice, the
presence or absence of water or the hydroxyl anion, or structural
considerations. The main classes which are recognized under Berzelius'
scheme include the native elements; sulfides and sulfosalts; oxides and
hydroxides; halides; carbonates, nitrates and borates; sulfates;
phosphates; and silicates. The antimonides, arsenides, selenides, and
tellurides closely resemble the sulfides, while the chromates, molybdates
and tungstates resemble the sulfates. The arsenates and vanadates are
closely akin to the phosphates.
The native elements include all
mineral species which are composed entirely of atoms in an uncombined
state. Such minerals either contain the atoms of only one element or else
are metal alloys. The native elements are divided into metallic,
semimetallic, and nonmetallic subgroups. Metals tend to be dense and
malleable substances which possess a characteristic metallic luster and
conduct electricity well, while semimetals and nonmetals tend to be
brittle and to conduct poorly.
Minerals of the sulfide class
are compounds which contain the nonmetallic element sulfur in combination
with atoms of a metal or a semimetal. Compounds in which anions of
antimony (Sb), arsenic (As), selenium
(Se), or tellurium (Te) replace the sulfur
anion and bond with metallic or semimetallic cations are classed
respectively as antimonides, arsenides, selenides,
and tellurides. If the sulfur anion, a metallic element, and a
semimetal are all present then the mineral is categorized as one of the
rare sulfosalts. Most sulfides and sulfosalts are soft, dark,
heavy, and brittle, possessing a metallic luster and high conductivity.
The minerals of the oxide class
contain oxygen bonded to one or more metallic elements.
Hydroxides are compounds of a metallic element and water or the
hydroxyl anion (OH-). The oxide minerals tend
to be relatively hard. Many provide important metal ores, and some of
them may be used as gemstones. Minerals of the hydroxide class tend to be
softer and less dense than oxides.
In members of the halide class
an element of the halogen group such as fluorine (F),
chlorine (Cl), bromine (Br), or iodine
(I) bonds to a metal or semimetal cation such as sodium
(Na), potassium (K), magnesium
(Mg), calcium (Ca), aluminum
(Al), copper (Cu), or silver
(Ag). Halides are constructed entirely of ionic bonds.
The halides tend to be soft, brittle, easily soluble in water, and possess
medium to high melting points. In solid state they are poor conductors.
Mineral species which are members of
the carbonate class are compounds of a metal or semimetal with the
carbonate anion
(CO3)2-. The
nitrates are structurally very closely akin to the carbonates with
the nitrate anion
(NO3)- replacing the
carbonate anion. The nitrates tend to be softer and to possess lower
melting points than do the carbonates. Minerals of the borate
class contain the borate radical
(BO3)3- rather than
the carbonate or nitrate radicals. However, borate radicals may unlike
the carbonate or nitrate radicals be linked into polymerized chains,
sheets, or groups. This trait grants the minerals of the borate class
unique structural characteristics.
This sulfate radical
(SO4)2- forms the
basic structural unit of the minerals of the sulfate class.
Minerals of the chromate class are compounds of metallic cations
with the chromate anion group
(CrO4)2-. Just as
sulfur and chromium form the anion groups
(SO4)2- and
(CrO4)2-, the ions of
molybdenum (Mo) and tungsten (W) bond with
oxygen atoms to create the anion groups
(MoO4)2- and
(WO4)2-. These anion
groups then bond with metal cations to form the minerals of the
molybdate and tungstate classes.
Like sulfur, the elements phosphorous
(P), arsenic (As), and vanadium
(V) form anion groups in combination with oxygen. The
resulting phosphate radical,
(PO4)3-, provides the
basic structural unit of the minerals of the phosphate class; the
arsenate and vanadate radicals
(AsO4)3- and
(VaO4)3- form the
basic structural units of the arsenate and vanadate classes.
The mineral species of these three classes are thus composed of the
respective phosphate, arsenate, and vanadate radicals linked by various
metal and semimetal cations.
The basic constituent of the minerals
of the silicate class is the silicate radical
(SiO4)4-. Silicate
radicals may remain structurally isolate, join together in pairs, or
polymerize into frameworks, sheets, chains, or rings. The various species
of the silicate class are grouped according to their structural type.
Silicate minerals are usually of relatively great hardness, and single
crystals are often translucent.
The term petrogenesis refers to
the means by which a rock or mineral deposit is formed. Three different
modes of formation are common; these consist of igneous activity,
metamorphism, and sedimentary processes. Mineral deposits therefore
occur in these three types of environments and also in vein environments.
Igneous rocks are formed when
magma or molten rock cools and solidifies. Some igneous rocks form
from magma which cooled at great depths within the earth's crust, while
others formed when magma erupted from a volcano or vent at the earth's
surface. Metamorphism describes the set of solid state processes
which transform one type of rock into another. Any type of rock, whether
igneous, metamorphic, or sedimentary, may be metamorphised. Metamorphism
typically occurs at the high temperatures and pressures found deep within
earth's crust. Veins are mineral deposits which form when a
fracture or fissure within a larger body of rock is filled with new
crystalline material. Veins are believed to form when aqueous solutions
migrate through fissures in rock and deposit minerals onto the fissure
walls. Sedimentary rock is formed when loose sediments are
collected together, compacted, and cemented into rock through the action
of heat, pressure, and chemical agents. These sediments may be derived
from the erosion of preexisting rocks, by precipitation from aqueous
solution, or from the skeletal remains of organisms. They are transported
and collected via meteorological processes such as flowing water, wind,
and migrating glaciers.
The small collection of minerals with
which this paper is concerned contains five species. All samples in the
collection were removed from the copper mines of Butte, Montanna during
the 1950's. Native copper, the copper carbonates azurite and malachite,
and the amorphous copper silicate chrysocolla are copper-bearing
substances. Rhodocrosite (MnCO2) is, like
azurite and malachite, a carbonate mineral, but is an ore of manganese
rather than a copper ore.
Copper (Cu) is a
native metal which has long been utilized as a resource by humans. It is
composed of only elemental copper and its physical features are derived
from the prevalent metallic bonds of its structure. Like all metals,
native copper is dense, soft, malleable, and ductile; it conducts well.
It is opaque and possesses the shiny, highly reflective luster typical of
metals. In color it is the familiar, rosy copper red; it possesses a red
streak. Native copper demonstrates a face-centered cubic lattice, which
is the most dense and symmetric of all possible crystalline structures.
It is of the isometric or cubic crystal system. Individual copper
crystals may may be of cubic, dodecahedral, tetrahedral, and very rarely
of octahedral shape. Aggregates of crystals may be wiry in habit or form
a dendritic, arborescent branching structure.
The two minerals azurite
(Cu3(CO3)2(OH)2)
and malachite
(Cu2Co3(OH)2) are closely
related. Both are hydrous copper carbonates. Azurite is royal blue or
brilliant blue in color while malachite may be bright to dark green. The
coloration in both species is idiochromatic and is due to the presence of
the chromophore copper, which has a strong pigmenting effect. Aggregates
of crystals of both species are frequently botryoidal or mammillary in
habit. Large individual crystals of malachite are rare. More commonly
the mineral occurs in rings, bands, and concentric layers of varying
shades of green. Both azurite and malachite occur as earthy, incrusted
masses.
Rhodocrosite
(MnCO3) is member of the carbonate class, a
member of calcite group, and an ore of manganese. Its characteristic rose
pink or red color is idiochromatic and is due to the presence of the
chromophore manganese (Mn). Rhodocrosite is translucent
and will transmit light diffusely; it possesses a vitreous or pearly
luster.
Rhodocrosite is like all
calcite minerals a member of the rhombohedral crystal system.
Rhodocrosite's cleavage is perfect in three directions and is
rhombohedral. Crystals are rare but may be rhombohedral in shape; more
typically the mineral demonstrates botryoidal, encrusting habit.
The hydrous silicate
chrysocolla
(Cu2H2Si2O5(OH)4)
is an ore of copper. Chrysocolla is not a true mineral and does not
possess a crystalline lattice. Instead, it is an amorphous 'silica gel'
or gelatinous precipitate. It is, however, a copper-bearing solid
substance and is commonly found in association with minerals such as
azurite, malachite, and native copper. Chrysocolla is green to sky-blue
in color. This coloration is idiochromatic and is due to the presence of
the chromophore copper (Cu). Chrysocolla is translucent;
samples may be of vitreous or glassy luster or appear greasy, dull, or
earthy. It has no cleavage and demonstrates uneven or conchoidal
fracture. Specimens of chrysocolla are typically of massive or earthy
habit. The substance may also display reniform or bulbous, botryoidal
habit.
Azurite, malachite and chrysocolla are
commonly found in the oxidized zones of copper veins and deposits.
Rhodocrosite occurs in veins and hydrothermal replacement deposits in the
company of manganese minerals as well as other metal ores such as copper.