14.2: Native Elements

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.

Figure 14.230: Native gold on quartz from the Sierra Nevada foothills; the sample is 4 cm tall

Figure 14.231: Hydrothermal gold from the Mother Lode of the Sierra Nevada Mountains

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.

Figure 14.233: Blocky cubic silver crystals from Kongsbberg, Norway; FOV is 10 cm across

Figure 14.234: Silver wire aggregates from the Czech Republic; FOV is 2 cm across

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.”

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.

Figure 14.237: Copper from near Copper Harbor, Michigan; 11.1 cm across

Figure 14.238: Native copper with malachite from near White Pine, Michigan; 2 cm across

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.”

Figure 14.240: Placer diamonds; the largest shown is about 1.1 cm in long dimension

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.

Figure 14.242: Massive graphite

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.”

Figure 14.244: Sulfur with gray/white gypsum that makes up a salt dome cap rock in Germany; FOV is 11 cm across

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 = 69^o.

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.

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.