native element carbonate (orthorhombic, aragonite structure) sulfur, S cerussite PbCO3

Diagnostic features: yellow colour, characteristic smell. Diagnostic features: high specific gravity (G = 6.55) and crystal form (often dendritic or in aggregates of criss- crossing crystals). Association with Pb- and Zn-bearing sulfides and carbonates. Habit: usually found as incrustations or irregular masses that are imperfectly crystallized. When well formed the crystals are orthorhombic and dipyramidal. Habit: single crystals are generally acicular, often dendritic.

Colour: yellow (but can be orange or greenish when impurities substitute for S). Colour: white. Streak: white (colourless).

Hardness: 1.5-2.5 Cerussite is often formed by the action of carbonated hydrothermal waters on galena. The alteration of galena Note 1: most of the specimens in your drawers are not examples of natural sulfur. They (PbS) and sphalerite (ZnS), commonly found together and with pyrite (FeS2), gives rise to associations of are massive forms of microcrystalline sulfur that must often be extracted from oil or secondary minerals which may include cerussite, smithsonite and limonite (a microcrystalline or hydrated form of natural gas before they are sold as fossil fuels. Sulfur reacts readily with oxygen to form goethite). a serious atmospheric pollutant.

Note 2: The text mentions that small single crystals can be heard to crack when held in hand, close to the ear. (We have not been able to test this with single crystals in the past.) This can happen because sulfur, a non-metal, is a poor conductor of heat: for this reason, the surface layers may expand because of your body heat but the interior of the crystal does not, and the single crystal could crack.

carbonate (orthorhombic) sulfide aragonite CaCO3 sphalerite ZnS

Diagnostic features: moderate hardness (H = 3.5-4), stronger effervescence in room-temperature HCl than other Diagnostic features: moderate hardness (H = 3.5-4), many planes of cleavage, resinous luster. common carbonate minerals (including calcite) and the lack of rhombohedral cleavage. Habit: well-formed tetrahedral crystals (left, below) are rare. They cleave into dodecahedra (6 cleavage planes, Habit: single crystals are orthorhombic, often acicular or in radiating aggregates. The polysynthetic twins formed below right). Even in aggregates, the many cleavage planes give a “sparkling” appearance to the specimen. by three intergrown crystals have a pseudo-hexagonal appearance (righ, below), but their re-entrant angles are visible at the contact between the individual crystals. In varved deposits (thinly layered) and stalactitic aggregates Colour: colourless when pure but generally yellow to brownish, sometimes black. The streak is colourless in pure sphalerite and its colour is yellow to brown with increasing Fe content. individual crystals are not visible to the naked eye.

The structure of sphalerite is a derivative of that of diamond where half of the atoms are now S, and the other half Colour: generally colorless or white, but can be variously coloured by impurities. 2+ 2+ are cations (Zn or Fe ), and all ions are in tetrahedral coordination. Calcite and aragonite are polymorphs of calcium carbonate. Calcite, less dense, is the more stable form at the Earth’s surface temperature and pressure. Some marine invertebrate (oysters, corals) secrete aragonite which, after the death of the animal, are replaced by the less soluble calcite during fossilization.

sulfide sulfide cinnabar HgS covellite CuS

Diagnostic features: red colour is somewhat variable but its scarlet streak is characteristic. Diagnostic features: indig-blue colour (purple iridescence is common), micaceous habit. Habit: well formed crystals are rare. Occurs as vein fillings or scattered among other suflides. Habit: tabular hexagonal crystals. Colour: red (vermilion to brownish red) Colour: blue, with a metallic luster. Often tarnishes to shades of purple. Hardness: 2.5 (very easy to streak, even when present as small grains) Hardness: 1.5-2. This mineral is the only important ore of mercury. Often associated with other copper minerals: chalcocite, chalcopyrite, bornite, and enargite. It is derived from them by alteration and usually occurs as a coating (it is often present in the tarnish of bornite).

halide halide halite NaCl sylvite KCl

Diagnostic features: low hardness (H = 2.5, scratched by a tough fingernail), perfect cubic cleavage, salty taste. Diagnostic features: low hardness (H = 2), perfect cubic cleavage, bitter taste (really awful).

Habit: crystals are usually cubic. “Hopper-shaped” skeletal crystals grow from rapidly evaporating solutions. Habit: crystals are usually cubic, often modified by octahedral faces.

Colour: colourless when pure. Often white because of inclusions (small bubbles filled with fluid that were trapped Colour: colourless when pure, but natural crystals are usually coloured by microscopic inclusions of other during crystal growth) or coloured by impurities or inclusions of other minerals. minerals or the reddish remains of halophilic (i.e. salt-tolerant) algae which thrive in the evaporating saline water bodies where this mineral precipitates. The structure of halite, NaCl, and sylvite, KCl, are identical. Both minerals are less dense (G = 2-2.1) than most non-metallic minerals. They are part of a sequence of minerals commonly formed by evaporation of seawater: Specific gravity: G = 1.99, lower than that of most minerals with non-metallic luster. aragonite or calcite - gypsum - halite - sylvite. Identical in structure to halite (NaCl), sylvite forms at a later stage of seawater evaporation.

halide carbonate (rhombohedral, calcite-like structure) fluorite CaF2 magnesite MgCO3

Diagnostic features: octahedral cleavage, moderate hardness (H = 4, easily scratched by the knife), cubic habit. Diagnostic features: no noticeable reaction with room-temperature HCl solution.

Habit: usually cubes, sometimes modified by octahedra. Often found in aggregates of interpenetrating crystals. Habit: crystals are rhombohedral, but good forms are rarely seen. Usually found in compact masses. The shape of small fragments should reflect the rhombohedral cleavage. Colour: very variable, the mineral is coloured by a wide range of impurities substituting for the Ca2+ ions. Colour: usually white or gray. The phenomenon of fluorescence received its name because it was observed early in some varieties of fluorite. Not all fluorite, however, is fluorescent. The property depends on the substitution of small amounts of rare-earth Hardness: 3.5-5 (varies with crystal size). elements () for the Ca2+ ions. Magnesite is quite rare and has two main origins. The first one is hydrothermal alteration of Mg-rich igneous and Avoid pouring dilute HCl on fluorite samples. Dissolving even small amounts of fluorite in an acidic solution can metamorphic rocks, in which case it is often associated with opaline silica, talc, chlorite or serpentine. The second be quite corrosive to the skin. possibility is by alteration of limestone (CaCO3) in which case it is associated with dolomite.

carbonate (rhombohedral, calcite structure) carbonate (rhombohedral, calcite structure) dolomite CaMg(CO3) 2 siderite FeCO3

Diagnostic features: rhombohedral cleavage, little reaction with HCl at room temperature (more soluble if the Diagnostic features: rhombohedral cleavage, brown colour, vitreous luster, moderate hardness (H = 3.5-4). Should mineral is powdered), often slightly rusty on weathered surface. barely react to HCl at room temperature.

Habit: unit rhombohedra are common, sometimes with curved faces (“saddle” dolomite, shown to the right, below). Habit: crystals are usually simple unit rhombohedra, often with curved faces. Also occurs in compact, granular Perfect rhombohedral cleavage like calcite and other rhombohedral carbonates. Polysynthetic twinning is common, masses. Perfect rhombohedral cleavage. giving rise to striations along the long diagonal of the rhombic faces. Colour: light to dark brown. Vitreous luster. Colour: white or in shades of flesh or pink. Hardness: 3.5-4. Hardness: 3.5-4. Siderite is associated with some coal beds and metallic ore deposits of hydrothermal origin. It has been mined as an Ankerite, CaFe(CO3)2, is the name given to Fe-rich varieties of dolomite. The mineral is typically yellowish to ore of iron in Europe. yellowish-brown, with a weak reaction to HCl similar to that of dolomite.

carbonate (rhombohedral, calcite structure) carbonate (rhombohedral) rhodochrosite MnCO3 calcite CaCO3

Diagnostic features: pink colour, rhombohedral cleavage, moderate hardness (H = 4). Weak reaction to room Diagnostic features: noticeable reaction (effervescence) with HCl at room temperature, rhombohedral cleavage, temperature HCl solution. moderate hardness (H = 3-3.5).

Habit: rhombohedral and scalenohedral crystals are rare. Usually in cleavable compact masses, often banded. Habit: highly variable but a three-fold or six-fold symmetry is often clearly visible. Crystals can be nearly acicular or flattened plates. Most are combinations of rhombohedra and prism(s) or scalenohedra.. Colour: usually pink (characteristic), though the shade may vary from light pink to red or dark brown.. Colour: colourless when pure, but highly variable because of the presence of fluid inclusions, organic matter, Rhodochrosite is a relatively rare hydrothermal vein mineral. inclusions of other minerals or substitution of Ca2+ ions by various impurities.

Cleavage fragments of limpid, colourless calcite crystals displays pronounced double refraction. There is no double refraction if one looks through the crystal down its c axis.

Calcite and aragonite are polymorphs of calcium carbonate.

carbonate (rhombohedral, calcite structure) smithsonite ZnCO3

Diagnostic features: botryoidal habit, slightly harder (H = 4-4.5) than some other carbonates, weak or no reaction to HCl at room temperature.

Habit: single crystals (rhombohedral or scalenohedral, below and to the right), are rare. Aggregates are usually reniform (below, to the left), botryoidal or stalactitic.

Colour: light green to dirty brown (but we have lilac-coloured specimens in the teaching collection).

Smithsonite is a zinc ore formed by the alteration of sphalerite (ZnS) by carbonated hydrothermal waters. It is commonly associated with sphalerite, galena, cerussite, calcite and limonite (the amorphous form of goethite).