My small collection of minerals contains five species, all of which were removed from the copper mines of Butte, Montanna during the 1950's. Of these five species, 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. However, rhodocrosite is an ore of manganese rather than copper.
Copper (Cu) is a native
metal which has long been utilized by humans as a natural resource. The
crystalline lattice of this mineral contains only elemental copper and its
structure is derived from metallic bonds. Like all metals, native copper
is dense, soft, malleable, and ductile. It conducts well. It is opaque
and demonstrates the shiny, highly reflective luster typical of metals.
In color it is the familiar, rosy copper red; it possesses a red streak.
Exposed surfaces may tarnish to black oxide or green carbonate.
Native copper possesses 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. Mineral
specimens demonstrating whole individual crystals and well-formed crystal
faces are relatively rare.
The two minerals azurite
(Cu3(CO3)2(OH)2)
and malachite
(Cu2Co3(OH)2) are closely
related. Both are hydrous copper carbonates, which contain the carbonate
anion group, (CO3)2-, the hydroxyl
anion (OH-), and the copper cation
(Cu2+). 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 copper, which
is a chromophore and possesses a strong pigmenting effect. Malachite's
green results from the fact that the copper which it contains is more
highly oxidized than that of azurite.
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 composed of varying shades of green.
Both azurite and malachite occur as earthy, incrusted masses.
Rhodocrosite
(MnCO3) is member of the carbonate class 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.
Like azurite and malachite,
rhodocrosite is a member of the carbonate class. However, rhodocrosite
contains neither water (H2O) nor the hydroxyl
anion (OH-) and is therefore not a hydrous
carbonate. Instead it is a member of the calcite group. Rhodocrosite is
like all calcite minerals a member of the rhombohedral crystal system.
Rhodocrosite's cleavage is perfect in three directions and provides
external indication of its rhombohedral internal structure. 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 a hydrous or basic copper silicate
because it contains the hydroxyl anion (OH-).
This species 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 which is
commonly found in association with minerals such as azurite, malachite,
and native copper.
In color chrysocolla is green to
sky-blue. 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 is very brittle, sometimes fragile; 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. It lacks macroscopic crystals.
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.
Native copper has been utilized by
humans as a natural resource since perhaps 3,400 B.C. The word 'copper'
is
derived from the Latin cuprum, which was in turn a modification of
the Greek name Kyprios. The word Kyprios indicated the
island of Cyprus, where the metal was mined in ancient times.
Copper is a member of the class of
native elements. The minerals of this class either contain only the atoms
of a single element or else are metal alloys. A metal alloy may contain
two or more metallic elements in solid solution, but its constituent atoms
must be bonded only by metallic bonds. Native copper is a metal and its
lattice structure contains only elemental copper.
Native copper gains its structure and
many of its properties from metallic bonds. When metal atoms bond
together into a crystal the valence electrons become delocalized and are
free to roam throughout the lattice. The attraction of the positively
charged atoms for the surrounding electrons bonds the atoms into an
ordered structure. The positive ions surrounded by the negatively charged
electron resevoir retain an almost perfectly spherical shape. In a pure
metal, these atoms are all of one element and are therefore all of the
same atomic radius. Because the atoms are spherical, equally sized,
equally charged, and surrounded by negative, delocalized electrons they
pack closely together to form a dense and very ordered crystalline
structure. Metals tend to be dense and to have a high degree of symmetry.
Copper possesses a typically high density of 8.9
g/cm3. Parallel planes of equally sized and
charged atoms may glide across one another, resulting in the softness,
malleability, ductility, and sectility which are characteristic of metals
and are demonstrated by native copper. As expected, copper is soft,
possessing a hardness of 21/2 to 3 on
the Mohs scale.
The many delocalized valence
electrons present within a metallic lattice serve as conducting electrons
and account for the high conductivity of metals. Copper, like all native
metals, conducts well.
Electromagnetic waves cannot propagate
through a metal. Instead, they are absorbed and reflected. The
absorption index of a metal is very high, and nearly all incident light is
reflected. This renders the metal completely opaque and makes it appear
shiny and brilliant. Copper is therefore opaque and will not transmit
light in thin section; the mineral possesses a typical shiny, reflective
metallic luster.
In color, native copper consistently
demonstrates the familiar and distinctive copper red; its streak is also
red in hue. The red of native copper is an example of idiochromatic
coloration, or color which is derived directly from the presence of one of
the main constituent elements of the mineral. Idiochromatic color is a
property which is directly related to the chemical composition of a
mineral species and is therefore possessed by all specimens of that
species. This characteristic red of native copper is attributable to the
presence of copper, which has strong pigmenting capabilities and is the
only constituent of the crystal lattice. (For an introduction to
idiochromatism please refer to Section 2.) Exposed surfaces of native copper crystals
may tarnish to black oxide or green carbonate according to the following
reaction:
2Cu + O2 -----------> 2CuO
2Cu + 2CO2 + O2
----------->
2CuCO3
Native copper has no cleavage and does
not tend to break along regular planes. Its fracture is 'hackly', or
jagged, containing many small irregularities.
The lattice structure occupied by
copper is that of cubic closest packing or face-centered cubic. This type
of structure is based around a cube on which an atom occupies each of the
eight corners. Atoms are emplaced at the center of each of the six
exterior faces as well. This is the most dense of all possible lattice
structures. (Please refer to the discussion of crystalline lattices in Section 3.) Native copper
is of the isometric crystal system. Isometric crystals possess four
three-fold axes of symmetry and are measured against three axes of equal
length which occur at right angles to one another. (Please refer to the
discussion of crystal system in Section 3.)
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. (Please refer to the discussion of
crystal habit in Section 2.) Fine mineral specimens with whole
individual crystals and well-formed crystal faces are quite rare.
The mineral azurite
(Cu3(CO3)2(OH)2)
is an ore of copper, and provided human civilization with one of its
earliest sources of this metal. Both azurite and its close relative
malachite have been mined for thousands of years. Because of its amazing
blue color, azurite has long been used as a pigment in dyes and as a
decorative gemstone. The species was named 'azurite' in honor of this
distinctive azure blue. The mineral is also called 'chessylite', a name
reminiscent of Chessy, France, where large quantities have been mined.
Single crystals of azurite are a dark,
royal blue; the mineral exhibits a brilliant azure blue color when it is
of dull or earthy luster. Its streak is blue. This coloration is
idiochromatic and is due to the presence of the chromophore copper, which
grants a strong pigmenting effect, within the crystal lattice. (Please
refer to Section
2 for a discussion of idiochromatism and the chromophores.)
Azurite's characteristic blue color is useful in identification.
Azurite produces some specimens of
adamantine luster which are very sparkly and highly translucent with a
high refractive index. It forms other samples of
vitreous or glassy
lustre and medium refractive index; its surface may also be rough, giving
a dull, earthy and almost lustreless appearance.
Azurite possesses a density of 3.77 to
3.83 g/cm3 and a Mohs scale hardness of
31/2 to 4. Its cleavage is good in
two directions; its fracture is conchoidal, or shell-like, resulting in a
series of concentric rings about the stressed point. The mineral is of
brittle tenacity and is not malleable. Like other minerals of the
carbonate class, azurite effervesces slightly in hydrochloric acid. This
provides a useful means of identification in the field.
In shape azurite crystals may be
equidimensional; prismatic, or elongated in one dimension; or tabular,
appearing in flat plates. Aggregates of crystals are frequently
botryoidal in habit, having a globular form said to resemble a bunch of
grapes. Azurite may also form radiating crusts or earthy masses. (Please
refer to the description of crystal habit in Section 2.)
The mineral malachite is a
semi-precious stone and a common ore of copper. Its name is derived from
the greek term malache, 'mallow', which refers to its leaf green color.
Like azurite, malachite has been used for centuries as a pigment in dyes
and as a semi-precious stone.
Malachite may vary from bright to dark
green in color; its streak is consistently light green. The coloration of
malachite is idiochromatic and due to the presence of copper within the
chemical formula
(Cu2Co3(OH)2). Specimens
may be of adamantine, silky, or dull lustre.
Malachite possesses a density between
3.6 and 4.0 g/cm3 and a hardness of
31/2 to 4 on the Mohs scale. It
demonstrates perfect cleavage in one direction; however, this cleavage
occurs rarely. More frequently observed is the conchoidal or splintery
fracture.
Large individual crystals of malachite
are very rare, but may occur in prismatic form. Aggregates of crystals
are frequently mammillary, forming smooth, bulbous masses. They may also
be botryoidal and globular. They may occur in radial assemblages, forming
layers and concentric rings in varying colors of green. Malachite is
likewise observed to produce fibrous threads and tufts, stalactitic
formations, and earthy, incrusted masses. Distinctive concretionary
banding patterns occur in many samples, particularly in botryoidal masses.
These are caused by varying oxidation levels in the aqueous solution from
which dissolved minerals exsolute. The mechanism by which this occurs is
not fully understood. (For a discussion of terms such as 'mammillary',
'botryoidal' and 'concretionary' please refer to the description of
crystal habit contained in Section 2.)
Classification and Association of Azurite and Malachite
Both azurite and malachite are
hydroxide-containing members of the copper carbonate class. Carbonate
minerals compose a class in which
(CO3)2- anions are
linked by various cations within the unit cell. Copper carbonates in
particular are hydrous carbonate minerals which contain both the copper
cation (Cu2+) and hydroxyl anions
(OH-). Azurite possesses the chemical formula
Cu3(CO3)2(OH)2;
the formula of malachite is
Cu2(CO3)(OH)2.
Azurite and malachite are commonly
found together and in conjunction with native copper and other copper
ores.
Crystal System and Molecular Structure
Both azurite and malachite belong to the monoclinic crystal system. Members of this crystal system possess three axes of unequal length, two of which are perpendicular to each other. The end faces are inclined relative to the side faces rather than being orthogonal to them. Such crystals are usually short, having the appearance of a distorted rectangle. (Please refer to the discussion of crystal structure found in Section 3.)
The molecular structure of azurite
consists of square groups composed of two (O2-)
and two (OH-) anions attached to a single,
central
Cu2+ cation. The anions may be paired
along one
side of the square or across its diagonal. These square groups are then
linked in chains by plane triangular
(CO3)2- anions.
In the molecular scheme of malachite a
single Cu2+ cation is surrounded by an
octahedron composed of O2- and
(OH-) anions. This octahedron is composed of
four of either anion (O2- or
(OH-)) and two of the other. Many such
octahedra in turn form chains linked by
(CO3)2- groups.
Color
Azurite and malachite have very similar chemical formulae:
| Azurite:
Malachite: |
Cu3(CO3)2(OH)2
Cu2Co3(OH)2 |
or
or |
Cu(OH)2 • 2(CuCO3)
Cu(OH)2 • (CuCO3) |
The vibrant greens and blues exhibited
by these two minerals are due to the inclusion of copper, which is a very
effective pigmenting agent, among their chemical constituents. This type
of coloration is termed idiochromatic.
Malachite possesses a ratio of 1:1
Cu(OH)2 to CuCO3 while
azurite possesses a ratio of 1:2 for the same substances. Since
Cu(OH)2 is more highly oxidized than
CuCO3 and malachite possesses the higher ratio
of this cation, malachite thus occupies a later stage in the oxidation
process than does azurite. The more prevalent oxidation of malachite is
responsible for its bright green color as compared to the deep blue of
azurite.
Pseudomorphism in Azurite and Malachite
Malachite is more stable than azurite;
over time azurite will invert to malachite. Sometimes the external form
of the original azurite is preserved during this process. Malachite can
thus form a pseudomorph of azurite as azurite unit cells are replaced over
time by those of malachite.
When a pseudomorph of a crystal forms,
the unit cells of the original mineral are gradually replaced cell for
cell by those of a new mineral. The original macroscopic crystal shape
remains intact, but the chemical substance of which it is composed is now
that of a different mineral. Although the external, macroscopic form has
been preserved, the internal microscopic structure has changed. A
pseudomorph thus possesses the chemical composition of the new mineral
species while its external crystal form preserves the shape of the
original, replaced species.
A pseudomorph of malachite after
azurite retains the same shape as the original azurite crystal but is
composed of malachite rather than azurite. The pseudomorph is therefore
malachite green in color rather than azurite blue.
The chemical formula describing the
inversion of azurite to malachite is:
|
2
[Cu(OH)2 • 2(CuCO3)]
+ H2O
2 azurite + water |
----------->
-----------> |
3
[Cu(OH)2 • (CuCO3)]
+ CO2
3 malachite + carbon dioxide |
Mineral specimens containing only
azurite, only malachite, and varying portions of each substance exist.
The contrast between azurite's intense blue and malachite's bright green
is very pleasing to the eye. Samples in which the transformation process
has begun but remains incomplete can therefore be quite beautiful.
Rhodocrosite
(MnCO3) is member of the carbonate class and an
ore of manganese. Its name is derived from the Greek words rhodon,
meaning 'rose', and chros, color. The name was given in reference
to the mineral's characteristic rose pink or red color.
Rhodocrosite's distinctive pink color
is an example of idiochromatism which is due to the presence of the
chromophore manganese (Mn) in the chemical formula.
(Please refer to Section 2 for a discussion of idiochromatism and the
chromophores.) This characteristic pink color may darken upon exposure to
the atmosphere. Rhodocrosite is known to alter to black manganese oxides
or hydroxides. The mineral displays a white streak. It is translucent
and will transmit light diffusely; it possesses a vitreous or pearly
luster.
Rhodocrosite has a density of 3.4 to
3.6 g/cm3 and a hardness of
31/2 to 4 on the Mohs scale. It is
brittle and displays uneven fracture.
Like the hydrous copper carbonates
azurite and malachite, rhodocrosite is a member of the carbonate class.
The carbonate minerals are compounds of a metal or semimetal with the
carbonate anion (CO3)2-.
Rhodocrosite is not, however, a hydrous carbonate because it contains
neither water (H2O) nor the hydroxyl anion
(OH-). Instead it is a member of calcite group.
The minerals of the calcite group possess a simple geometry in which
layers of (CO3)2- radicals alternate
with layers of (2+) metallic cations. Ionic bonds join
the carbonate anions to the metallic cations.
Rhodocrosite and all calcite group
minerals are of rhombohedral crystal system. Crystals of this system are
measured against three vertical axes which intersect one another at
120° angles and one horizontal axis which is perpendicular to the other
three. A rhombohedron may be envisioned as an equilateral parallelogram
which has been turned up onto one corner. (For more information on
crystal systems, please refer to Section 3.)
The cleavage of rhodocrosite is
perfect in three directions. Like the cleavage of all minerals of the
calcite group, it is rhombohedral. Crystals are rare but may be
rhombohedral; more typically the mineral demonstrates botryoidal,
encrusting habit.
Rhodocrosite is found in veins and
hydrothermal replacement deposits in the company of manganese minerals as
well as other metal ores. It may alter to manganese oxides or hydroxides.
The hydrous silicate chrysocolla
(Cu2H2Si2O5(OH)4)
is an ore of copper. The name of the species is derived from the Greek
words chrysos, meaning gold, and kolla, 'glue'. It was
given this name because it strongly resembled a similar material which was
once used to solder gold.
Chrysocolla is a hydrous or basic
copper silicate because it contains the hydroxyl anion
(OH-). Unlike true minerals it does not possess
a crystalline lattice. Instead it is an amorphous 'silica gel' or
gelatinous precipitate. Because it is amorphous and lacks a crystalline
structure, chrysocolla is not a mineral in the strictest sense. However,
it is a copper-bearing solid substance which is found in the oxidized
zones of copper veins and is associated with minerals such as azurite,
malachite, cuprite, and native copper.
In color chrysocolla is green to
sky-blue. This coloration is idiochromatic and is due to the presence of
the chromophore copper (Cu) within the material. The
streak is very pale blue or green. Chrysocolla is translucent and is able
to transmit light when sliced into thin sheets. Samples may be of
vitreous or glassy luster; they may also appear greasy, dull, or earthy.
Chrysocolla is quite soft, possessing
a hardness of 2 - 4 on Mohs scale; it has a density of 2.0 - 2.4
g/cm3. It is
very brittle, sometimes fragile. It has no cleavage and demonstrates the
uneven or conchoidal fracture which is typical of glassy, amorphous
materials. This type of fracture displays a series of rounded, concentric
rings radiating away from the point of impact. Conchoidal fracture may
sometimes be observed on a piece of thick broken glass such as the base of
a glass bottle.
Specimens of chrysocolla are typically
of massive or earthy habit. The substance may also display reniform or
bulbous, lumpy botryoidal habit. It lacks macroscopic crystals.
An amusing field test may aid in the
identification of chrysocolla. When touched to the tongue a specimen of
this substance will usually adhere lightly. It sometimes tastes bitter or
basic.