Oxides & Hydroxides - The oxide group includes the silicon oxide quartz, also called silica.
One of the most common minerals, quartz occurs in many areas in a variety of forms.
Semiprecious gem stones of quartz include amethyst, tigereye, agate, and onyx. Siliceous sinter, or geyserite, is an impure quartz deposited by hot springs and is a form of opal. The fire opal has an internal iridescence of intense orange to red. Diatomaceous earth, or diatomite, was formed from the siliceous shells of diatoms, the microscopic plants that exist in both fresh water and seawater. It is also called kieselguhr and tripolite. The powdery substance is used for insulating and filtering material and in the manufacture of polishing and scouring powders. Another abundant oxide of silicon is tridymite.
Among the oxides of metals that exist as minerals are cuprite, or copper oxide; zincite, or zinc oxide; cassiterite, or tin oxide; and rutile, or titanium oxide. Pyrolusite, or manganese oxide, is the chief ore of manganese. Among the ores of iron are the oxides hematite and magnetite. Lodestone, a form of magnetite, is a natural magnet. Ilmenite, which exists in large deposits, is a mixed oxide of iron and titanium. It is a chief source of the titanium used as a paint pigment and as a purifier in alloys. Aluminum oxide, known in mineralogy as corundum (and in an impure form as
bauxite), exists in two transparent and colored gem forms--sapphire and ruby. Emery, a black granular corundum mixed with iron minerals, is used in a powdered form for grinding and polishing. Spinel is a mixed oxide of magnesium and aluminum, and chromite is an iron and chromium oxide that makes up the chief ore of chromium. Chromium is one of the major components of stainless steels.
The leading radioactive minerals, sources of such elements as radium, thorium, and uranium, include uraninite, carnotite, and autunite. They are all complex oxides of the radioactive and other elements and usually contain lead. Pitchblende, in which mineral radioactivity was first discovered, is an impure form of uraninite. Hydroxides are low-temperature minerals typically formed from products of aqueous alteration or from hydrothermal vents. Among the hydroxide minerals are the aluminum orebauxite and limonite, an iron ore containing hydrated iron oxides.
Oxides and Hydroxides: Brookite
Brookite Native aluminium oxide occurring in cleavable masses or in pyramidal crystals. It includes the ruby and sapphire, and among naturally materials is 9 on the Mohs scale of hardness, next to diamond. It occurs as crystals distributed through granite, cyanite and schist rocks in many parts of the world. Brookite is a polymorph with two other minerals. The minerals rutile and anatase as well as brookite all have the same chemistry, TiO2, but they have different structures. At higher temperatures, about 750 degrees Celsius, brookite will automatically revert to the rutile structure. Rutile is the more common and the more well known mineral of the three. Brookite shares many of the same properties as rutile such as color and luster and some properties are nearly the same such as hardness and density. However due to structural differences brookite and rutile differ in crystal habit and
cleavage.
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Oxides and Hydroxides: Cassiterite
Cassiterite or tinstone is the only abundant tin ore. A concentrically banded form with fibrous structure is called wood tin, or toad's eye when formed on a small scale. Cassiterite has an unusually high specific gravity for a non-metallic mineral. Cassiterite is a mineral that has ornately faceted specimens with high luster. It is generally opaque, but its luster and multiple crystal faces cause a nice sparkle. Cassiterite has been an important ore of tin for eons and is still the greatest source of tin today. Most sources of cassiterite today are not primary deposts but alluvial deposits containing weathered grains. The best source of original-formation cassiterite is at the tin mines of Bolivia, where it is found in hydrothermal veins. Although found throughout the world in many igneous rocks, cassiterite is usually only a minor constituent. The Bolivia veins and those worked and nearly
exhausted in Cornwall, England, somehow concentrated the tin in a way not fully understood by geologists.
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Oxides and Hydroxides: Corundum
Corundum The best known and prized varieties of this hard and durable mineral are rubies (red) and sapphires (blue). Yellow, green, and brown forms occur. Corundum is the second hardest natural mineral known to science. The hardest mineral, diamond is still four times harder than corundum. The hardness of corundum can be partially attributed to the strong and short oxygen-aluminum bonds. These bonds pull the oxygen and aluminum atoms close together, making the crystal not only hard but also quite dense for a mineral made up of two relatively light elements.
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Oxides and Hydroxides: Cuprite
Cuprite A red oxide of copper in crystalline form. Cuprite has been a major ore of copper and is still mined in many places around the world. Of all the copper ores except for native copper, cuprite gives the greatest yield of copper per molecule since there is only one oxygen atom to every two copper atoms. As a mineral specimen, cuprite shows fine examples of well-developed cubic crystal forms. Cuprite's dark crystals show internal reflections of the true deep red inside the almost black crystal. Other varieties, such as chalcotrichite, show tufts of needle-like crystals that have a beautiful red color and a special sparkle that make them popular display cabinet specimens.
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Oxides and Hydroxides: Goethite
Goethite forms attractive but rare crystals. Common forms are massive, with radially fibrous structure.It is found in large amounts as the amorphous mineral assemblage called limonite. Limonite is a blanket term for all forms of hydrated ferric oxide, which has been formed by the oxidation and weathering of iron minerals. Goethite has had a reputation for making rather uninteresting, dull and "dirty" mineral specimens. This is definitely not true of many of the fine specimens available to collectors today. Some good pieces show splendid radiating clusters of deep black crystals. Massive stalactitic forms show rainbows of irridescent colors on surreal landscapes. Many times Goethite forms the backdrop or matrix for other minerals such as red Vanadinite, making a visually appealing mineral specimen.
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Oxides and Hydroxides: Hematite
Hematite is found as crystals and as mammillary, botryoidal, and ocherous, massive material. The best crystals from Franklin were described by Palache and Berman (1927) and Palache (1935). Hematite has several varieties, each with their own unique names. Hematite Rose is a circular arrangment of bladed crystals giving the appearance of the flower of a rose. Tiger Iron is a sedimentary deposit of approximately 2.2 billion years old that consists of alternating layers of silver gray hematite and red jasper, chert or even tiger eye quartz. Kidney Ore is the massive botryoidal form and gives the appearance of lumpy kidney-like masses. Oolitic Hematite is a sedimentary formation that has a reddish brown color and an earthy luster and is composed of small rounded grains. Specularite is a micaceous or flaky stone that is sparkling silver gray and sometimes used as an ornamental
stone.
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Oxides and Hydroxides: Ilmenite
Ilmenite, an ore of titanium, is a black mineral with a metallic lustre when pure. It can contain varying amounts of the related magnesian mineral giekielite, which affects its physical properties. The variety menaccanite, found at Menaccan Sands, Cornwall, England, was one of the first reported descriptions of ilmenite. The occurrence of ilmenite in sand reflects its high resistance to weathering. In magmas, it is prone to segregate with magnetite or hematite, because of its relatively high specific gravity. It is distinguished from hematite by its streak, and from magnetite by its non-magnetic nature. Ilmenite is an economically important and interesting mineral. It is named for its place of discovery, such places are called type localities, at Ilmen Lake in the Ilmen Mountains, Miask in the southern portion of the Ural Mountains of Russia. Ilmenite forms
as a primary mineral in mafic igneous rocks and is concentrated into layers by a process called"magmatic segregation". It crystallizes out of a magma relatively early before most of the other minerals. As a result, the heavier crystals of ilmenite fall to the bottom of the magma chamber and collect in layers. It is these layers that constitute a rich ore body for titanium miners. Ilmenite also occurs in pegmatites and some metamorphic rocks as well as in the sedimentary rocks that are formed from the weathering and erosion of them.
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Oxides and Hydroxides: Magnetite
Magnetite A black iron ore, magnetic iron oxide found in igneous rocks. It may possess polarity ( a lodestone) and was used as a compass as early as the first millennium BC. Magnetite is a natural magnet, hence the name, giving it a very nice distinguishing characteristic. Explaining the magnetism is not easy but here is a go at it. Remember, electricity produces magnetic fields just as magnetism produces electic fields. Magnetite is a member of the spinel group which has the standard formula A(B)2O4. The A and B represent usually different metal ions that occupy specific sites in the crystal structure. In the case of magnetite, Fe3O4, the A metal is Fe +2 and the B metal is Fe +3; two different metal ions in two specific sites. This arrangement causes a transfer of electrons between the different irons in a structured path or vector. This electric vector generates the magnetic
field.
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Oxides and Hydroxides: Opal
Opal A non-crystalline form of silica, occurring in stalactite in volcanic rocks. The common opal is opaque, milk-white, yellow, red, blue or green and lustrous. The precious opal is colorless, having innumerable cracks from which emanate brilliant colours produced by minute crystals of cristobalite. Opal has been a popular gem for many centuries and has a very interesting structure. Opal is considered a mineraloid because this structure is not truly crystalline. The chemistry of Opal is primarily SiO2 and varying amounts of water. The amount of water varies from 5 -10% and greater. This water can help geologists determine the temperature of the host rock at the time the opal formed.
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Oxides and Hydroxides: Pyrolusite
Pyrolusite is an important ore of the metal manganese, which is used in steel making, both to remove oxygen and sulphur, and as an alloying metal. Rare, well-formed crystals are called polianite. The massive form is by far the most abundant, the color and the softness being the main diagnostic feature. The name comes from the Greek words meaning 'fire' and 'wash'., referring to the fact that pyrolusite was once used to remove iron oxide discolouration in glass making. Pyrolusite occurs as a constituent of the mixture of black hydrous manganese-bearing minerals known as wad, together with psilomelane and manganite. Pyrolusite is the most common manganese mineral and is an important ore. Manganese is a strategically valuable metal since it is an essential ingredient in steel and other alloys. The mining term "wad" is used to indicate ores that are a mixture of several manganese oxides
such as pyrolusite, psilomelane and others that are difficult to distinguish.
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Oxides and Hydroxides: Quartz
Quartz One of the commonest minerals minerals of the Earth's crust, it is a crystalline form of silica. When impurities, such as oxides, occur in the crystals, they produce such gem varieties as cairngorm, amethyst, and tourmaline. Quartz is the most common mineral on the face of the Earth. It is found in nearly every geological environment and is at least a component of almost every rock type. It frequently is the primary mineral, >98%. It is also the most varied in terms of varieties, colors and forms. This variety comes about because of the abundance and widespread distribution of quartz. A collector could easily have hundreds of quartz specimens and not have two that are the same due to the many broad catagories. The specimens could be separated by answers to the following questions: color?, shade?, pyramidal?, prismatic?, druzy?, twinned?, sceptered?, tapered?, phantomed?,
inclusions?, coated?, microcrystalline?, stalactitic?, concretionary?, geoidal?, tappered?, banded?, etc. Multiple combinations of these could produce hundreds of unique possibilities.
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Oxides and Hydroxides: Rutile
Rutile Naturally occurring crystalline form of titanium dioxide from which titanium is extracted. It is usually reddish brown to black, with bright (adamantine) surface lustre. Rutile is common in a wide range of igneous and metamorphic rocks and also occurs concentrated in sands. It is also used as a pigment which gives a brilliant white to paint, paper and plastics. Rutile is an interesting, varied and important mineral. Rutile is a major ore of titanium, a metal used for high tech alloys because of its light weight, high strength and resistance to corrosion. Rutile is also unwittingly of major importance to the gemstone markets. It also forms its own interesting and beautiful mineral specimens.
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Oxides and Hydroxides: Tantalite
Tantalite is the most widespread tantalum mineral and makes for an important ore of the industrially useful metal. Tantalum is used in alloys for strength and higher melting points, in glass to increase the index of refraction, and in surgical steel, as it is non-reactive and non-irritating to body tissues. Tantalite forms a series with the mineral columbite. In fact the two are often grouped together as a semi-singular mineral called columbite-tantalite in many mineral guides. A series is where two or more elements can occupy the same places within a crystal structure and their respective percentages can then vary. The two minerals of this series have similar properties since they have the same structure and similar chemistries (tantalum and niobium are very similar elements). Tantalite is the more tantalum rich end member and columbite is the more niobium rich end member. Niobium had been
called columbium hence the name columbite.
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Oxides and Hydroxides: Background Information
Hydroxides are a common mineral group that is formed by the weathering and hydration of other minerals. They area combination of a metallic element and water (H2O) or hydroxyl (OH). They are often found as aggregates of fine-grained material that includes other minerals and clays. Two hydroxide minerals, in particular, are important because their structure serves as a starting point for describing silicates. Both gibbsite and brucite have two sheets of OH- anions, in between which is a layer of cations (Mg or Al). The result is a series of neutrally charged octahedral sheets that are held together by relatively weak electrostatic bonds.
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Oxides and Hydroxides: Oxides & Hydroxides Information
The oxide class of minerals is a rather diverse class. It includes minerals that are quite hard (corundum) and some that are quite soft such as psilomelane. It has metallic minerals such as hematite and gemstones such as corundum, chrysoberyl and spinel. Many oxides are black but others can be very colorful. The large diversity of oxides can be partially attributed to the extreme abundance of oxygen in the Earth's crust. Oxygen comprises over 45% of the Earth's crust by weight. Most of this is locked up in more complex minerals based on chemical complex anions such as CO3, BO3, SO4, NO3, SiO4, PO4 and others.
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