Section 6:

Description of Specimens

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  1. Introduction
  2. Native Copper
  3. Azurite and Malachite
  4. Rhodocrosite
  5. Chrysocolla


1. Introduction

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      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.


2. Native Copper

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Photos

      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.


3. Azurite and Malachite

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Azurite
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      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.)

Malachite
Photos

      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)22(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)22(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.


4. Rhodocrosite

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      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.

5. Chrysocolla

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      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.


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