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14.2: Native Elements

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    18652
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    Metals
    gold Au
    silver Ag
    platinum Pt
    copper Cu
    Semimetals
    arsenic As
    bismuth Bi
    antimony Sb
    Nonmetals
    diamond C
    graphite C
    sulfur S

    Gold, silver, platinum, and copper are the most common of the native metals. Additionally, iron, zinc, nickel, lead, and indium have occasionally been reported from meteorites or altered igneous rocks. All native metals have similar properties: metallic luster (if not tarnished), high thermal and electrical conductivity, malleability, and opaqueness to visible light. Complex solid solutions are possible, and many natural alloys have been given their own names. Kamacite and taenite, for example, are Fe-Ni alloys.

    The native semimetals (arsenic, bismuth, and antimony), all rare, are found in hydrothermal deposits but rarely have economic importance. The native nonmetals are diverse in occurrence and properties. Graphite is common as an accessory mineral in many metamorphic rocks, sulfur exists in massive beds or as encrustations associated with fumaroles, and diamond is primarily restricted to kimberlite pipes, alluvium derived from kimberlites, or mantle nodules

    For more general information about native elements, see the Section 9.2.1 in Chapter 9.

    Gold Au

    Origin of Name
    The name of this mineral refers to its color.

    14.230.jpg
    Figure 14.230: Native gold on quartz from the Sierra Nevada foothills; the sample is 4 cm tall
    14.231.jpg
    Figure 14.231: Hydrothermal gold from the Mother Lode of the Sierra Nevada Mountains
    14.232.jpg
    Figure 14.232: Gold nugget on quartz

    Hand Specimen Identification
    Gold is metallic and yellow. High specific gravity, sectile nature, and slightly different (more buttery yellow) color and luster distinguish gold from the yellow sulfides pyrite and chalcopyrite. Chalcopyrite, also, commonly tarnishes to obtain a slightly greenish hue, making it distinct from gold.

    Physical Properties

    hardness 2.5 to 3
    specific gravity 15.6 to 19.3
    cleavage/fracture none/hackly
    luster/transparency metallic/opaque
    color golden yellow
    streak gold-yellow

    Crystallography
    Gold is cubic, a = 4.0783, Z = 4; space group \(P\dfrac{4}{m}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{m}\overline{3}\dfrac{2}{m}\).

    Habit
    Gold crystals, when they exist, are octahedra, rarely showing other forms. More typically, gold is arborescent, fills fractures, or is found as nuggets, grains, or wire scales. Most gold crystals are exceptionally small.

    Structure and Composition
    Gold’s face-centered cubic structure is the same as the atomic arrangement in platinum and copper. Its composition is sometimes close to pure Au, but substantial Ag may be present in solid solution. Small amounts of other elements, such as Cu and Fe, may be present.

    Occurrence and Associations
    Gold is most often found in quartz veins associated with altered silicic igneous rocks. Associated minerals include quartz, pyrite, chalcopyrite, galena, stibnite, sphalerite, arsenopyrite, tourmaline, and molybdenite. It is also concentrated in placer deposits.

    Varieties
    Most natural gold contains up to 10% alloyed metals, thus giving rise to a number of slightly different colors and properties.

    Related Minerals
    Electrum is a name for intermediate Ag-Au solutions. Other gold-bearing minerals include calaverite (AuTe2), petzite (Ag3AuTe2), maldonite, (Au2Bi), and uytenbogaardtite (Ag3AuS3).

    Silver Ag

    Origin of Name
    From the Old English word for this metal, seolfor.

    14.233.jpg
    Figure 14.233: Blocky cubic silver crystals from Kongsbberg, Norway; FOV is 10 cm across
    14.234.jpg
    Figure 14.234: Silver wire aggregates from the Czech Republic; FOV is 2 cm across
    14.235.jpg
    Figure 14.235: Native silver wires with calcite, Valenciana Mine, Mexico

    Hand Specimen Identification
    Silver may occur as cubic crystals, but more commonly has a wire-like, dendritic, or arborescent habit. It may have a metallic silver color, but only when fresh. Most of the time it tarnishes like the specimens seen in the three photos here. Silver has high specific gravity and is quite malleable. It is occasionally confused with the platinum group minerals. If cubic and tarnished it may be confused with galena.

    Physical Properties

    hardness 2.5 to 3
    specific gravity 10.1 to 10.5
    cleavage/fracture none/hackly
    luster/transparency metallic/opaque
    color silver-white but typically tarnished
    streak silver-white

    Crystallography
    Silver is cubic, a = 4.0856, Z = 4; space group \(F\dfrac{4}{m}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{m}\overline{3}\dfrac{2}{m}\).

    Habit
    Distorted cubes, octahedra, or dodecahedra are known, but silver is typically acicular. Flakes, plates, scales, and filiform or arborescent masses are common.

    Structure and Composition
    Silver has a face-centered cubic structure that is isostructural with copper. It may contain substantial amount of Au, Hg, Cu, As, Sb, Bi, Pt, or Fe in solid solution.

    Occurrence and Associations
    Silver is found with sulfides and arsenide in oxidized zones of ore deposits, or in hydrothermal deposits. The many associated minerals include, most significantly, species containing Co, Ni, and As.

    Varieties
    Amalgam is a solid solution of Ag and Hg. Electrum is a solid solution of Ag and Au.

    Related Minerals
    Silver is isostructural with copper. Other Ag minerals include dyscrasite (Ag3Sb), argentite (Ag2S), proustite (Ag3AsS3), and pyrargyrite (Ag3Sb3).

    Platinum Pt

    Origin of Name
    From the Spanish platina, meaning “silver.”

    14.236.jpg
    Figure 14.236: Platinum nuggets from California and Sierra Leone; the largest nugget is about 2 cm tall

    Hand Specimen Identification
    Platinum is most easily identified by its malleability, silvery-gray color and streak, and very high specific gravity. Although a cubic mineral, euhedral crystals (generally distorted cubes) are rare; it most commonly occurs as nuggets, often with rounded corners, like the nuggets seen in top row of Figure 14.236.

    Physical Properties

    hardness 4 to 4.5
    specific gravity 21.47
    cleavage/fracture none/hackly
    luster/transparency metallic/opaque
    color gray-silver, steel-gray
    streak gray-silver, steel-gray

    Crystallography
    Platinum is cubic, a = 3.9237, Z = 4; space group \(F\dfrac{4}{m}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{m}\overline{3}\dfrac{2}{m}\).

    Habit
    Euhedral or subhedral platinum crystals, generally poorly formed, are exceptional. Masses, nuggets, or small grains are typical.

    Structure and Composition
    Platinum has a cubic closest packed structure similar to gold’s structure. It forms alloys with other elements, notably Fe, Cu, Pd, Rh, and Ir.

    Occurrence and Associations
    Primary platinum is found with chromite, spinel, and olivine in ultramafic rocks. It is also found in some placer deposits.

    Related Minerals
    Platinum is isotypical with copper.

    Copper Cu

    Origin of Name
    From the Greek word kyprios, referring to Cyprus, one of the earliest places where copper was mined.

    14.237.jpg
    Figure 14.237: Copper from near Copper Harbor, Michigan; 11.1 cm across
    14.238.jpg
    Figure 14.238: Native copper with malachite from near White Pine, Michigan; 2 cm across
    14.239.jpg
    Figure 14.239: Arborescent native copper from Pima County, Arizona

    Hand Specimen Identification
    Native copper has a copper-red or pale rose-red color, and most commonly forms as scales of flakes (Figures 14.237 and 14.238), or branching arborescent crystals (Figure 14.239). It commonly tarnishes, has a hackly fracture, is malleability, and has high specific gravity. The photos in Figures 14.237 and 14.239 show copper that is altering to green malachite (Cu-carbonate).

    Physical Properties

    hardness 2.5 to 3
    specific gravity 8.7 to 8.9
    cleavage/fracture none/hackly
    luster/transparency metallic/opaque
    color copper color, copper-red or rose-red; sometimes tarnished
    streak copper-red

    Crystallography
    Copper is cubic, a = 3.6153, Z = 4; space group \(F\dfrac{4}{m}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{m}\overline{3}\dfrac{2}{m}\).

    Habit
    Copper may form cubes, octahedra, or dodecahedra. When euhedral, contact or penetration twins are common. Most copper is in the form of malformed crystals, or dendritic, arborescent, or irregular plates, scales, or masses.

    Structure and Composition
    Copper has a cubic closest packed structure similar to gold and platinum. It often contains solid solutions of Ag, Fe, As, or other elements.

    Occurrence and Associations
    Copper is found in the oxidized zones of many copper deposits, and as primary mineralization from hydrothermal fluids passing through mafic lavas. Copper is often deposited in voids or cracks. Associated minerals include silver, sulfides, calcite, chlorite, zeolites, cuprite, malachite, and azurite.

    Related Minerals
    Gold, silver, platinum, and lead are isotypical with copper.

    Diamond C

    Origin of Name
    From the Greek word adamas, meaning “invincible.”

    14.240.jpg
    Figure 14.240: Placer diamonds; the largest shown is about 1.1 cm in long dimension
    14.241.jpg
    Figure 14.241: A large diamond crystal in kimberlite; the large crystal is about 7 mm across

    Hand Specimen Identification
    Diamond is distinguished by its occurrences, hardness, octahedral cleavage and sometimes octahedral shape, and luster. Diamonds are mined from alluvial (placer) deposits and from kimberlite pipes. Figure 14.240 shows alluvial diamonds, and Figure 14.241 shows a large diamond in kimberlite.

    Physical Properties

    hardness 10
    specific gravity 3.5
    cleavage/fracture perfect octahedral {111}/conchoidal
    luster/transparency adamantine/transparent
    color typically colorless but rare, colored varieties may be valuable
    streak white

    Properties in Thin Section
    Diamond is isotropic, n = 2.419.

    Crystallography
    Diamond is cubic, a = 3.5668, Z = 8; space group \(F\dfrac{4}{d}\overline{3}\dfrac{2}{m}\); point group \(\dfrac{4}{m}\overline{3}\dfrac{2}{m}\).

    Habit
    Diamond crystals are usually octahedral and often distorted or twinned. More rarely, diamond forms cubes or dodecahedra. Curved faces are common.

    Structure and Composition
    Diamond is essentially pure carbon but may contain inclusions of other material.

    Occurrence and Associations
    Diamond is found in altered ultramafic rock of mantle origin or in placer deposits. Associated minerals include pyrope, olivine, kyanite, and zircon.

    Related Minerals
    Graphite is a polymorph of diamond.

    Graphite C

    Origin of Name
    The name comes from the Greek word graphein, meaning “to write,” because of its use in pencils.

    14.242.jpg
    Figure 14.242: Massive graphite
    14.243.jpg
    Figure 14.243: Hexagonal graphite crystal, 1 mm across, in calcite

    Hand Specimen Identification
    Graphite is easily recognized by its greasy feel, softness, shiny luster, dark color and streak, and foliated nature. Figure 14.242 shows a typical massive example. Occasionally graphite is in the form of hexagonal crystals but they are generally quite small. Figure 14.243 shows two examples.

    Physical Properties

    hardness 1 to 2
    specific gravity 2.1 to 2.2
    cleavage/fracture perfect basal (001)/elastic, flexible
    luster/transparency submetallic/opaque
    color lead-gray, black
    streak black

    Crystallography
    Graphite is hexagonal, a = 2.46, c = 10.06, Z = 6; space group \(R\overline{3}\dfrac{2}{m}\); point group \(\overline{3}\dfrac{2}{m}\).

    Habit
    Well-formed graphite crystals are hexagonal tablets. Foliated and scaly masses are common; radiating or granular aggregates are less common.

    Structure and Composition
    Graphite‘s structure contains stacked planes of covalently bonded C atoms arranged in a hexagonal pattern. Graphite is essentially pure carbon.

    Occurrence and Associations
    Graphite is common in a wide variety of metamorphic rocks including schists, marbles, and gneisses. It is a rare mineral in some igneous rocks. Graphite is usually disseminated as fine flakes, but may form large books.

    Related Minerals
    Graphite is a polymorph of graphite.

    Sulfur S

    Origin of Name
    From the Middle English word sulphur, meaning “brimstone.”

    14.244.jpg
    Figure 14.244: Sulfur with gray/white gypsum that makes up a salt dome cap rock in Germany; FOV is 11 cm across
    14.245.jpg
    Figure 14.245: Native sulfur from Mt. Etna, Sicily; FOV is 4.8 cm across

    Hand Specimen Identification
    Sulfur can be easily identified by its yellow color, hardness, density, and sometimes eggy odor. It is occasionally confused with orpiment, the only other relatively common yellow mineral, or yellow sphalerite.

    Physical Properties

    hardness 2
    specific gravity 2.1
    cleavage/fracture poor {101} and {110}/ conchoidal
    luster/transparency resinous or dull/transparent to translucent
    color bright yellow
    streak white

    Properties in Thin Section
    Sulfur is characterized by extreme relief and birefringence. It is pale yellow in thin section and commonly pleochroic. Biaxial, α = 1.958, β = 2.038, γ = 2.245, δ = 0.29, 2V = 69o.

    Crystallography
    Sulfur crystals are orthorhombic, a = 10.44, b = 12.84, c = 24.37, Z = 128; space group \(F\dfrac{2}{d}\dfrac{2}{d}\dfrac{2}{d}\); point group \(\dfrac{2}{m}\dfrac{2}{m}\dfrac{2}{m}\).

    Habit
    Typically, sulfur is massive, colloform, or stalactitic, but tabular crystals may display combinations of orthorhombic prisms, dipyramids and pinacoids.

    Structure and Composition
    The sulfur structure consists of covalently bonded groups, stacked parallel to the c-axis, and weakly connected to each other. Sulfur is essentially pure S but may contain small amounts of Se in solid solution.

    14.246.jpg
    Figure 14.246: Sulfur deposit in crater of White Island Volcano, New Zealand.

    Occurrence and Associations
    Sulfur is found as a deposit associated with volcanic fumaroles (Figure 14.246). It also occurs in veins where it forms from sulfides, or in sediments where it forms by the reduction of sulfates by bacterial action. The most substantial occurrences are thick evaporite beds in sedimentary sequences. Associated minerals include other sulfur-containing minerals (celestite, gypsum, anhydrite), and carbonates.

    Related Minerals
    Sulfur has several different polymorphs.


    This page titled 14.2: Native Elements is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Dexter Perkins via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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