Small water-worn crystals of spinel with different colouring. Some, where broken, show the vitreous lustre. These examples are from a placer deposit in India.
OXIDES AND HYDROXIDES
Minerals in the oxide group are compounds of various elements combined with oxygen. Hydroxide minerals are compounds of metallic elements and the hydroxyl radical, (OH).
SPINEL
Small water-worn crystals of spinel with different colouring. Some, where broken, show the vitreous lustre. These examples are from a placer deposit in India.
| Group | Oxides |
| Composition | MgAl2O4 |
| Crystal System | Cubic |
| Habit | Crystals cubic, dodecahedral; granular, massive |
| Cleavage | Indistinct |
| Fracture | Uneven or conchoidal |
| Specific Gravity | 3.6 |
| Hardness | 7½ to 8 |
| Colour | Brown, red, black, green, blue |
| Streak | White |
| Lustre | Vitreous |
Spinel occurs in a number of metamorphic rocks including the contact metamorphic rock marble and the regionally metamorphosed gneiss. It has also been found in serpentinite and peridotite. Because of its hardness, spinel accumulates in alluvial sediments. There are two varieties of spinel with different chemistry: pleonaste contains iron and is dark in colour, while picotite contains chromium. Good quality spinel is used as a gemstone.
ZINCITE
Typical red zincite with dark franklinite from New Jersey, USA.
| Group | Oxides |
| Composition | (Zn,Mn)O |
| Crystal System | Trigonal/hexagonal |
| Habit | Crystals pyramidal; massive, foliated, granular |
| Cleavage | Perfect |
| Fracture | Conchoidal |
| Specific Gravity | 5.7 |
| Hardness | 4 |
| Colour | Red, orange |
| Streak | Orange-yellow |
| Lustre | Adamantine |
Zincite, a relatively rare mineral, is found mainly in metamorphic rocks formed by heating due to the proximity of igneous rocks. Willemite and calcite are associated minerals. Zincite can be manufactured synthetically and both natural and man-made zincite have been used as semiconductors.
CUPRITE
Massive and crystalline cuprite with typical red colouring. Two crystals at the bottom of the specimen show an adamantine lustre. Specimen from Chile.
| Group | Oxides |
| Composition | Cu2O |
| Crystal System | Cubic |
| Habit | Crystals cubic, octahedral; granular, massive, earthy |
| Cleavage | Poor |
| Fracture | Uneven or conchoidal |
| Specific Gravity | 6.1 |
| Hardness | 3½ to 4 |
| Colour | Bright red |
| Streak | Brown |
| Lustre | Metallic, adamantine or earthy |
Cuprite forms where copper minerals and veins have been altered by oxidation. Native copper, iron oxides, azurite, malachite and chrysocolla are found with it. Because of its colour and lustre, cuprite has been used as a gemstone. That it has not become very valuable in this respect is due to its low hardness, as it is easily scratched in everyday use.
PYROLUSITE
Prismatic crystals of pyrolusite with characteristic metallic lustre.
| Group | Oxides |
| Composition | MnO2 |
| Crystal System | Tetragonal |
| Habit | Crystals prismatic; fibrous, dendritic, massive, columnar |
| Cleavage | Perfect |
| Fracture | Uneven |
| Specific Gravity | 5.1 |
| Hardness | 2 to 6½ |
| Colour | Black or dark grey |
| Streak | Black or bluish-black |
| Lustre | Metallic or dull |
Pyrolusite occurs where manganese veins, containing manganite, have been altered, and in bogs and lakes. The manganese nodules found on the deep ocean bed contain pyrolusite. Some specimens are relatively hard, but the dendritic material is soft and leaves a sooty impression on the fingers. Dendritic pyrolusite is often mistaken for plant fossils, especially by the producers of flooring slabs, where their products are called ‘fossil’. It has industrial use, including the green and violet colouring of paints, glass and pottery.
MAGNETITE
Tiny black crystals of magnetite, with typical metallic lustre, set in white calcite. Specimen from Norway.
| Group | Oxides |
| Composition | Fe3O4 |
| Crystal System | Cubic |
| Habit | Crystals octahedral, dodecahedral; granular, massive |
| Cleavage | None |
| Fracture | Uneven or subconchoidal |
| Specific Gravity | 5.2 |
| Hardness | 5½ to 6½ |
| Colour | Black |
| Streak | Black |
| Lustre | Metallic or dull |
Magnetite occurs in mineral veins and as a replacement deposit. It also forms in basic igneous rocks and is found in beach and alluvial sands, where it accumulates because of its hardness and resistance to weathering. This mineral is an important ore of iron. Major sources include China, Peru, Sweden, Western Australia, the USA, South Africa and India. As its name suggests, magnetite is magnetic; a specimen will attract iron filings and move a compass needle.
ILMENITE
A massive specimen of dark ilmenite showing a bright metallic lustre. Specimen from South Africa.
| Group | Oxides |
| Composition | FeTiO3 |
| Crystal System | Trigonal/hexagonal |
| Habit | Crystals rhombohedral, tabular; massive, granular, compact |
| Cleavage | None |
| Fracture | Uneven or conchoidal |
| Specific Gravity | 4.7 |
| Hardness | 5 to 6 |
| Colour | Brownish, black, silvery |
| Streak | Black |
| Lustre | Metallic or dull |
Ilmenite occurs in small amounts in many igneous rocks, such as coarse-grained pegmatites, and also in hydrothermal mineral veins. It accumulates in alluvial sands as a placer mineral. Ilmenite is the main ore of titanium and the mineral is found in huge amounts in Western Australia in sand deposits. Economically useful quantities also exist in Canada, Norway and the Ukraine. Titanium is used in making aircraft engines and framework, because it has the strength of steel but is 45% lighter and does not suffer metal fatigue.
HAEMATITE
This specimen shows reddish, reniform kidney ore and black crystalline specularite. Specimen from Cumbria, England.
| Group | Oxides |
| Composition | Fe2O3 |
| Crystal System | Trigonal/hexagonal |
| Habit | Crystals prismatic, rhombohedral, tabular; reniform, massive, fibrous |
| Cleavage | None |
| Fracture | Uneven or subconchoidal |
| Specific Gravity | 5.3 |
| Hardness | 5 to 6 |
| Colour | Brown, reddish, black |
| Streak | Red-brown |
| Lustre | Metallic or dull |
Haematite forms as a replacement mineral in sedimentary rock, especially limestone, and in hydrothermal veins. It commonly occurs in a reniform habit with a reddish colour, giving rise to its name of kidney ore. Haematite is an important metal ore, having a high iron content. Large deposits are worked in Canada, Brazil and Venezuela. The black crystalline variety of haematite is known as specularite. A form called iron rose is composed of small tabular crystals in a rosette.
CASSITERITE
Black crystals of cassiterite with adamantine lustre from Bolivia.
| Group | Oxides |
| Composition | SnO2 |
| Crystal System | Tetragonal |
| Habit | Crystals prismatic, pyramidal; reniform, massive, granular, botryoidal |
| Cleavage | Imperfect |
| Fracture | Uneven or subconchoidal |
| Specific Gravity | 7.0 |
| Hardness | 6 to 7 |
| Colour | Black, brown, yellow |
| Streak | Brown, grey |
| Lustre | Greasy or adamantine |
Cassiterite occurs in hydrothermal mineral veins, with many other minerals including pyrite, chalcopyrite, quartz, sphalerite and galena. This mineral is the main ore of tin. It is mined in Malaysia, Burma, China and Thailand, and in Brazil and Bolivia. The presence of cassiterite in Cornwall, England, was exploited for hundreds of years until the world market changed and the industry there faded. The main uses of tin are in alloys and for tin plating.
CHRYSOBERYL
Pale greenish crystals and grains of chrysoberyl set in white quartz.
| Group | Oxides |
| Composition | BeAl2O4 |
| Crystal System | Orthorhombic |
| Habit | Crystals prismatic, tabular; massive, granular |
| Cleavage | Distinct |
| Fracture | Uneven or conchoidal |
| Specific Gravity | 3.7 |
| Hardness | 8½ |
| Colour | Yellow, greenish |
| Streak | White |
| Lustre | Vitreous |
Chrysoberyl occurs in coarse-grained igneous pegmatites, as well as metamorphic marbles, schists and gneisses. Because of its great hardness and resistance to physical weathering, chrysoberyl is found in alluvial sands. With a considerable hardness, vitreous lustre and transparency, chrysoberyl has a gem variety, alexandrite. This has the remarkable property of being red in artificial light and green in daylight.
CORUNDUM
Red corundum (ruby) crystals set in calcite. Specimen from India.
| Group | Oxides |
| Composition | Al2O3 |
| Crystal System | Trigonal/hexagonal |
| Habit | Crystals rhombohedral, tabular, prismatic, bipyramidal; massive, granular |
| Cleavage | None |
| Fracture | Uneven or conchoidal |
| Specific Gravity | 4.0 |
| Hardness | 9 |
| Colour | Brownish, grey, blue, red, yellowish, white |
| Streak | White |
| Lustre | Adamantine or vitreous |
Corundum occurs in basic igneous rocks and in some metamorphic rocks. It is one of the very hardest minerals, being the defining mineral at point nine on the hardness scale. Corundum is used as an abrasive, but the coloured forms are greatly valued as gemstones. Sapphire (blue, illustrated on p.75) and ruby (red) are colour varieties of corundum. Both can be found in alluvial placer deposits, where they accumulate because of their great hardness.
Brownish, prismatic corundum crystal from Zimbabwe.
RUTILE
Slender, needle-like crystals of rutile, with acicular habit, embedded in transparent, colourless quartz.
| Group | Oxides |
| Composition | TiO2 |
| Crystal System | Tetragonal |
| Habit | Crystals acicular, prismatic; massive |
| Cleavage | Distinct |
| Fracture | Uneven or conchoidal |
| Specific Gravity | 4.2 |
| Hardness | 6 to 6½ |
| Colour | Black, brown, red, yellowish |
| Streak | Yellow, brown |
| Lustre | Adamantine or metallic |
Rutile commonly occurs as acicular (needle-shaped) crystals within quartz, and as granular or compact masses. It is also found as an accessory mineral in many igneous rocks and in metamorphic rocks such as gneiss, metaquartzite and schist. Along with anatase and brookite, which have the same chemical formula as rutile and form a series of minerals with it, rutile is an important ore of titanium.
URANINITE
Typical dark uraninite, with uneven fracture and submetallic lustre.
| Group | Oxides |
| Composition | UO2 |
| Crystal System | Cubic |
| Habit | Crystals cubic, octahedral, dodecahedral; massive, granular, botryoidal |
| Cleavage | Indistinct |
| Fracture | Uneven or conchoidal |
| Specific Gravity | 6.5 to 10 |
| Hardness | 5 to 6 |
| Colour | Dark brown, black |
| Streak | Black, brown |
| Lustre | Greasy, dull, pitch-like or submetallic |
Uraninite occurs in hydrothermal mineral veins and in acid igneous rocks, including granite and pegmatite. It can also be found in some sandstones. Uraninite is a radioactive mineral and great care and sensible precautions have to be taken when examining a specimen. An alternative name, pitchblende, arises from the pitch-like colour of some forms of the mineral. It is the main ore of uranium, and is mined in the USA, Australia, Canada, China, Russia and South Africa.
QUARTZ
A mass of small, transparent, hexagonal quartz prisms, with grey, metallic galena.
| Group | Oxides |
| Composition | SiO2 |
| Crystal System | Trigonal/hexagonal |
| Habit | Crystals prismatic, pyramidal, rhombohedral; massive, granular |
| Cleavage | None |
| Fracture | Uneven or conchoidal |
| Specific Gravity | 2.65 |
| Hardness | 7 |
| Colour | White, colourless, grey, pink, purple, black, brown, yellow, green |
| Streak | White |
| Lustre | Vitreous |
Quartz is one of the most common minerals. It is often classified with the silicate group of minerals, but strictly it is an oxide. Quartz forms in hydrothermal mineral veins and in many rocks, especially granite, gneiss, schist and metaquartzite. In mineral veins it occurs with metallic ore minerals and is referred to as a ‘gangue’ mineral. Many of the colour varieties have their own names and are used as gemstones. These include purple amethyst, pink rose quartz and yellow citrine. Quartz has many economic uses and when found in great bulk, as in sand and sandstone, is used in the construction industry. Much of the silicon used in the electronics industry is made artificially. Quartz defines point seven on the hardness scale.
Purple amethyst from a geode. Specimen from Brazil.
Dark Cairngorm (smoky quartz) specimens. One shows hexagonal prisms, another is polished and two are cut and polished.
CHALCEDONY
A mass of botryoidal chalcedony coated with minute sparkling quartz crystals.
| Group | Oxides |
| Composition | SiO2 |
| Crystal System | Trigonal/hexagonal |
| Habit | Microcrystalline; massive, botryoidal or mammilated |
| Cleavage | None |
| Fracture | Conchoidal or uneven |
| Specific Gravity | 2.65 |
| Hardness | 7 |
| Colour | White, brown, green, blue, pink, red, black |
| Streak | White |
| Lustre | Vitreous or waxy |
Chalcedony is found in veins and cavities in a variety of rocks, especially in cavities in igneous rocks. It has a similar chemical composition to quartz, but is microcrystalline. The colour varieties have different names and many are used as ornamental stones and gemstones. The red variety is carnelian, chrysoprase is green and sard is light to dark brown. Agate is chalcedony with a concentric banded structure, while jasper is red and opaque.
Agate showing alternating bands of red, white and blue chalcedony. Specimen from the Cheviot Hills, Scottish Borders.
OPAL
Shimmering opal from Australia, showing vitreous lustre and varied colouring.
| Group | Oxides |
| Composition | SiO2.nH2O |
| Crystal System | Amorphous |
| Habit | Non-crystalline; massive, concretionary, botryoidal, globular, reniform |
| Cleavage | None |
| Fracture | Conchoidal |
| Specific Gravity | 2.0 to 2.2 |
| Hardness | 5½ to 6½ |
| Colour | White, blue, yellow, red, green, black |
| Streak | White |
| Lustre | Waxy, resinous or vitreous |
Opal occurs as the result of precipitation from silica-rich solutions, especially around hot springs. It is used as a gemstone and has a very rich play of colours which can change when the mineral is warmed. The red to orange variety is known as fire opal. Wood opal is the form which replaces plant material and can have concentric tree-rings preserved in its structure. Hyalite is colourless, with a bubbly habit. If heated, opal loses its water molecules and may become quartz.
Brownish wood opal with some concentric rings. Specimen from Arizona, USA.
GOETHITE
A mass of small, dark, prismatic goethite crystals. Specimen from Germany.
| Group | Hydroxides |
| Composition | FeO(OH) |
| Crystal System | Orthorhombic |
| Habit | Crystals prismatic; massive, stalactitic, botryoidal |
| Cleavage | Perfect |
| Fracture | Uneven |
| Specific Gravity | 3.3 to 4.3 |
| Hardness | 5 to 5½ |
| Colour | Yellow-brown, brown, black |
| Streak | Yellow-brown |
| Lustre | Dull or adamantine |
Goethite occurs in regions where iron minerals have been altered by oxidation. This can be the result of weathering or the action of subsurface fluids. Though not as rich in iron as some ores, goethite is used as an ore of this metal. Limonite, usually yellowish-brown in colour, is a similar mineral, but with an added molecule of water.
MANGANITE
A massive specimen of manganite from Sweden showing submetallic lustre and dark colouring.
| Group | Hydroxides |
| Composition | MnO(OH) |
| Crystal System | Monoclinic |
| Habit | Crystals prismatic; massive, columnar, granular, fibrous, concretionary |
| Cleavage | Perfect |
| Fracture | Uneven |
| Specific Gravity | 4.3 |
| Hardness | 4 |
| Colour | Black, dark grey |
| Streak | Black or red-brown |
| Lustre | Submetallic |
Manganite occurs in hydrothermal mineral veins and in bogs and lakes. It is often found in clay and laterite deposits with other oxides of manganese. Crystalline manganite frequently forms as bundles of striated prismatic crystals. It can be altered to pyrolusite by fluids circulating through the Earth’s crust. Manganite has been used as an ore of manganese.