GEOL 333 #10 - Native Elements, Sulfides, Oxides/Hydroxides

These mineral groups (ore minerals) contain elements that can be mined for profit and used typically for variety of purposes.

I) Native Elements - (single element) 20 examples; metals and non-metals (and rare semi-metals - As, Bi, Sb - Antimony). Highly valued because they require little processing for economic exploitation (and some are rare, precious, and useful metals).

Native Metals - Most important (abundant) examples = native Au (link #2, photo), native Ag (link #2, photo), native Cu* (link #2, photo), and native Pt (link #2, photo) (Fe is rare except in Fe-meteorites). High specific gravity (G = 5 ­ 21) due to heavy elements, metallic luster (fresh surfaces, see photos of native Au, Ag, Cu and Pt), soft, and malleable; weak metallic bonds = good conductors of heat and electricity; (atoms of same size in 12-fold coordination = high symmetry, typically isometric), among first metals used by early humans. All (except Fe) used as precious metals in jewelry, Cu used in electrical wiring, and Pt used as catalyst (catalytic converter in cars) and in other items (high technology, defense). Most of world's supply of Ag and Cu is mined in form of Ag-sulfides and Cu-sulfides, not as native elements. Occurrences = hydrothermal alteration (Ag, Cu, Au), ultramafic igneous rocks (Pt), and sedimentary placer ore deposits (Au), involve weathering of hydrothermal Au ore deposit and deposition in river deposit (Au settles to bottom due to high G). Alloys of Cu = brass (Cu and Zn) and bronze (Cu and Sn). Pure (24 ct) gold = deep (orange) yellow; white gold involves alloys with Pd and Ni, which commonly causes skin irritation; Au alloys with Ag and Zn cause only moderate whitening; “colors’ of Au (compositional triangle) = green, pale yellow, pink and red due to adding Ag and Cu (photo, function of metals added to gold, properties of typical gold alloys).

Non-metals (S and C) - Native/elemental sulfur* (link #2, photo) (S) yellow color, low melting point (~120°C), and low hardness (weak Van der Waals bonds). Major occurrence = salt domes, forms by reduction of gypsum (S6+ --> S0) during burial. Mined by Frasch method (figure), involves pumping superheated steam down into sulfur horizon, melting sulfur and then pumping up to surface with compressed air. Sulfur also occurs around steam vents of active volcanoes (fumaroles). Most important use = making sulfuric acid, which is used to make phosphate fertilizer (also used in insecticides, explosives, rubber, plastics, pharmaceuticals, and many other goods).

Graphite* (link #2, photo) (C) 6-membered rings, strongly bonded within ring but weak Van der Waals bonds between rings, very low hardness; black; occurs in metamorphic rocks from metamorphism of coal; major use = pencils and crucibles in steel industry (containers for carrying molten steel) due to very high melting point (~3,000°C).

Diamond* (link #2, photo) (C) opposite extreme from graphite: hardest mineral, clear to light colors from impurities. Diamonds most commonly occur in kimberlite, ultramafic rock from deep crust or upper mantle (rapidly, sometimes explosively, moved up to surface). Uses = gemstone and abrasive (most important industrial abrasive; small diamond pieces in drilling bits for oil exploration, hard diamond cuts through much softer rock, even quartz).

II) Sulfides (link #2) - 1 or more metal cation bonded to sulfide anion, usually S2- but occasionally S1- Pyrite (FeS2), chalcopyrite (CuFeS2), sphalerite (ZnS), galena (PbS) and cinnabar (HgS). Many sulfide minerals are important ore minerals (economic source usually for metals). Occur mainly in hydrothermal ore deposits, produced as elements are carried and concentrated in flowing hot (T = ~50 - 600°C) waters created as magma cools or by water flowing through hot, fractured rock. Often, several sulfide minerals occur together (with gangue/host minerals, typically quartz and various carbonates, not economically exploited). Most sulfide minerals have high G (heavy elements) and metallic luster (except cinnabar and sphalerite). Variable crystal structures.

Pyrite* (link #2, photo) (FeS2) - pale brass color, hard (~6), commonly occurs as cubic crystals with no cleavage; pyrite = most common and widespread sulfide mineral occurs as very small grains in sedimentary rocks (black, organic-rich shale) and in many hydrothermally altered rocks (e.g., Au deposits). Source of S for sulfuric acid (Fe ore only if no Fe ore deposits with Fe-oxides). Less stable polymorph of pyrite = marcasite (radiating needle-like crystals), much less common.

Chalcopyrite* (link #2, photo) (CuFeS2) - usually massive (poor crystals) bright yellow color (tarnishes easily), green-black streak, softness (~3.5) distinguishes it from pyrite. Most important Cu ore mineral, common in sulfide hydrothermal ore deposits.

Sphalerite* (link #2, photo) (ZnS) - usually massive, characteristic resinous luster and yellow/brown streak (and 6 perfect cleavage directions and isometric symmetry), variable color (yellow, brown, black - solid solutions with Fe, Mn, and Cd); typically occurs with galena (Pb ore) in Pb/Zn deposits of sedimentary and igneous origin; major source of Zn, used to galvanize iron (i.e., plate Zn metal onto iron in battery cell), and in brass (with Cu).

Galena* (link #2, photo) (PbS) - isometric; lead-gray cubic crystals with cubic cleavage and high G (~7.5); occurs with sphalerite (Zn ore) in Pb/Zn deposits of sed./ig. Origin; major source of lead, used in storage batteries, pipes, shot, and sheets for radioactive shielding. Pb used to be used as filler in paint (as PbO) and as additive in gasoline (improve engine performance) before serious human health effects were understood (Pb concentrates in body and causes brain damage).

Cinnabar (link #2, photo) (HgS) - brilliant red color and streak, high G (~8); crusts on fracture surfaces; most important source of mercury, hydrothermal ore deposits of volcanic and sedimentary origin.

Minerals related to sulfides include sulfosalts (with semi-metals As and Sb, Antimony, instead of metal cations), selenides (Se as anion), and tellurides (tellurium as anion).)

III) Oxides and Hydroxides - 1 or more metal cation bonded to oxygen (O2- = oxides) or hydroxyl (OH- = hydroxides). Oxides = relatively hard, moderate to high G, and minor constituent in igneous and metamorphic rocks (a few exceptions). Hydroxides = much softer, lower G than oxides, and form at lower temperatures in sedimentary settings (chemical weathering where water for hydroxyl is abundant). Both groups dominated by ionic and covalent bonds and are rather insoluble (little dissolves in water). Many are mined as ore minerals with considerable economic value. Oxides with other metal cations include Mn, Cr (Chromite, link #2), Sn, and U (Uraninite, link #2 - principal ore for uranium used in generating nuclear energy).

Corundum* (link #2) (Al2O3) second hardest mineral (H = 9), relatively high G (4), colored varieties = gemstones ruby (red due to trace amounts of Cr, photo) and sapphire (blue from Fe or Ti, photo). Occurs in metamorphic Al-rich rocks; used as gemstone and abrasive (emery paper/board = corundum and magnetite).

Hematite* (link #2, photo #1, #2) (Fe2O3, Fe3+) occurs as soft, earthy variety - brown or red color and hard (H = 6) specular variety - black with metallic luster; brick red streak = diagnostic. Hematite (and magnetite) are most important sources of iron ore and steel manufacturing. Occurs in all rock types, economic deposits from sedimentary banded iron formation (photo #1, #2) - alternating layers of Fe-oxides (hematite and magnetite) and chert (SiO2). Banded iron formation = common in Precambrian (2.5 - 2 billion years) rock, represents large increase in O2 levels in atmosphere (and ocean water), which caused massive chemical precipitation of Fe-oxides from oceans (O2-poor water carries abundant dissolved Fe and O2-rich water carries little dissolved Fe).

Magnetite* (link #2) (Fe3O4 or Fe2+1 Fe3+2 O4) strongly magnetic, photo, very common as minor phase in igneous (and metamorphic and sedimentary) rocks, useful for paleomagnetic studies (determination of location and polarity of Earth's magnetic pole during geologic past and paleolatitude). Sedimentary banded iron formation, other major source of iron ore.

Rutile (link #2, photo) (TiO2) and Ilmenite (link #2, photo) (FeTiO3) major sources of Ti metal, used for alloys. Ti metal = high strength, low weight, and resists corrosion; used in aircraft and as strategic metal (replacement hips). Rutile - used in paint and ceramics as pigment and in facial products; prismatic crystals, pale brown streak, and sub-metallic to brilliant luster; occurs in granite and metamorphic rocks. Ilmenite = metallic black, weakly magnetic; minor phase in mafic igneous rocks and heavy mineral concentrate in "black sands."

Goethite* (link #2, photo) (FeOOH) forms radiating fibrous masses, botryoidal (grape-like); color and streak = yellow-brown (limonite = mix of Fe-oxides and hydroxides); very common alteration product of Fe-bearing minerals in soil; minor source of iron ore.

Bauxite (link #2, photo) (not mineral, rock = mix of Al-hydroxides) yellow to red-brown (from Fe impurities), earthy luster, common pisolitic texture (round ­ oval grains); occurs in soils called laterites which form in tropical climates due to intense chemical weathering (heavy rainfall leaches easily dissolved elements from soil, leaving insoluble Al-hydroxides (and Fe-hydroxides)); major source of Al ore; very energy intensive process to convert bauxite to Al metal.