# Rocks and Minerals (O-Z)

Oligoclase
Oligoclase has the formulae (Na,Ca)AlSi3O8 and a relative hardness of 7. It is a plagioclase feldspar. Of interest to petrologists and collectors. Varieties: labradorite, anorthite.

Olivine
Olivine has the formulae (Mg,Fe)2SiO4 and a relative hardness of 7. It's name is derived from the characteristic olive green colour. A common rock forming mineral. A clear green variety is called peridot and has some uses as a gem.

Onyx
Onyx is a mineral formed of silica.

Opal
Opal has the formulae SiO2ùnH2O and a relative hardness of 6. It is found lining and filling cavities in igneous and sedimentary rocks where it's been deposited by hot waters. The ordinary varieties are common but the precious varieties are quite valuable.

Orpiment
Orpiment has the formulae As2S3 and a relative hardness of 2. It is a rare mineral usually associated with realgar. Used in dyeing but is poisonous. Distinguished from sulphur by its perfect cleavage.

Orthoclase
Orthoclase has the formulae KAlSi3O8 and a relative hardness of 6. It is used in the manufacture of porcelin and for other industrial purposes.

Orthorhombic
Orthorhombic refers to a rectangular crystal with three axes of different lengths and all at right angles to each other. A closed book is a basic example.

Oxide
Oxide refers to a group of minerals where oxygen joined with a metal is a major constituent.

Oxidized zone
Oxidized zone refers to the portion of an ore body that has been altered by downward percolating groundwater and which contains dissolved oxygen and carbon dioxide.

Pagodite
see "Agalmatolite"

Pearly
Pearly refers to a luster with the iridescent look of a pearl. Most commonly seen on surfaces parallel to cleavage planes.

Pectolite
Pectolite has the formulae NaCa2Si3O8(OH) and a relative hardness of 5. It is formed from hydrothermal solutions filling cavities in basalts. Associated with zeolites, prehnite, calcite. Of interest to collectors.

Pegmatite
Pegmatite refers to an igneous rock of very coarse grain size. Usually found as dikes within a larger rock mass. They are often excellent sources of fine crystals.

Penninite
Penninite has the formulae Mg3(Si4O10)(OH)2ùMg3(OH)6 and a relative hardness of 3. It is a member of the chlorite group of minerals.

Pentlandite
Pentlandite has the formulae (Fe,Ni)9S8 and a relative hardness of 4. It is the principal ore of nickel. The major use of nickel is in the manufacture of steel.

Perlite
A perlite is an obsidian, or other vitreous rock with a concentric structure and which is expansible by heating.

Petalite
Petalite has the formulae LiAlSi4O10 and a relative hardness of 7. It is an ore of lithium. Associated with spodumene, lepidolite, tourmaline.

Petroleum
Petroleum is a mineral oil from which petrol is derived.

Phenakite
Phenakite has the formulae Be2SiO4 and a relative hardness of 8. It is a rare mineral found in pegmatite dikes associated with topaz, beryl, and apatite. From the Greek word for 'a deceiver' in that it can be mistaken for quartz.

Phillipsite
Phillipsite has the formulae (K2,Na2Ca)(Al2Si4)O12ù4-5H2O and a relative hardness of 5.
It is a hydrothermal mineral found lining cavities in basalt rocks associated with chabazite. Formed as an alteration product of feldspars and volcanic ashes.

Phlogopite
Phlogopite has the formulae K(Mg,Fe)3(AlSi3)O10(F,OH)2 and a relative hardness of 3.
It occurs as a result of the metamorphism of crystalline magnesium limestones or dolomitic marbles. Also found in serpentine. Rarely found in igneous rocks.

Phosphates
Phosphates refers to a group of minerals where phosphate (PO4) is an important constituent.

Pipe
Pipe refers to a cylindrical, vertical mass of igneous rock.

Placer
Placer refers to a concentrated deposit of mineral particles that have weathered out of rock. Usually deposited by stream action.

Plagioclase
Plagioclase has the formulae Na(AlSi3O8) and a relative hardness of 6.
It is the plagioclase feldspar group includes: albite, oligoclase, andesine, labradorite, bytownite, anorthite.

Playa
Playa refers to a shallow basin or plain in a desert where water collects after a rain and then evaporates.

Plutonic rock
Plutonic rock refers to a granular igneous rock that has solidified at great depth and shows a distinct grain structure. Ex: granite

Polybasite
Polybasite has the formulae (Ag,Cu)16SbS11 and a relative hardness of 3.
It is an ore of silver and resembles hematite but is much softer.

Polyhalite
Polyhalite has the formulae KaCa2Mg(SO4)4ù2H2O and a relative hardness of 4.
It is a source of potassium and occurs in bedded deposits associated with sylvite, carnallite, halite, and has a characteristic red colour.

Potash
Potash is the name given to any mineral containing potassium.

Potassium argon dating
Potassium argon dating is a technique used in geology for estimating the age of a mineral or rock, based upon the rate of decay of radioactive potassium into argon.

Potstone
Potstone (Lapis ollaris) is a species of talc containing an admixture of chlorite. It is green in colour and varys in shade, greasy and soft but hardens on exposure to the air. It's English name comes from its capability to be turned into vases and pots by turning.

Prase
Prase is a dark green variety of quartz, the colour being due to an admixture of hornblende.

Precipitation
Precipitation refers to the process by which disolved or suspended solids are separated from a liquid.

Prehnite
Prehnite has the formulae Ca2Al2Si3O10(OH)2 and a relative hardness of 7.
It occurs as a crusty lining in cavities in basalt and related rocks. Associated with zeolites, datolite, pectolite, and calcite. Has a characteristic green colour. Resembles hemimorphite but is of lower specific gravity and fuses easily.

Proustite
Proustite has the formulae Ag3AsS3 and a relative hardness of 3.
It is an ore of silver. Has a characteristic ruby-red colour, vermilion streak and a brilliant luster.

Pseudomorph
Pseudomorph refers to a mineral that has taken the outward crystal form of a different mineral.

Psilomelane
Psilomelane has the formulae BaMnO16(OH)4 and a relative hardness of 6.
It is an ore of manganese. Usually occurs with pyrolusite. Different from other manganese oxides in that it is of greater hardness and has an apparent lack of crystal structure.

Pumice
Pumice is a light volcanic rock.

Purpurite
Purpurite has the formulae (Mn,Fe)PO4 .
It has a relative hardness of 5.
It is an alteration product of lithiophilite that occurs in pegmatites. Of interest to collectors.

Pyragyrite
Pyragyrite has the formulae Ag3SbS3.
It has a relative hardness of 3.
It is an ore of silver. Similar to proustite but has a deeper red colour and is less translucent. Forms in silver veins formed at low temperatures and is one of the last minerals to crystallize in the process of deposition.

Pyrite
Pyrite has the formulae FeS2 and a relative hardness of 7.
It is iron pyrite. Formed from cooling magma. It is found as an igneous segregation and also in metamorphic rocks and as vein deposits. Often found in sedimentary rocks being both primary and secondary in origin. Often mined for the gold or silver associated with it.

Pyrolusite
Pyrolusite has the formulae MnO2 and a relative hardness of 3.
It is the major ore of manganese which is used in the manufacture of steel. Pyrolusite is formed when manganese is dissolved out of crystalline rocks and then redeposited in dentrites etc.

Pyromorphite
Pyromorphite has the formulae Pb5(PO4)3Cl and a relative hardness of 4.
It is a minor ore of lead which is found in the oxidized portions of lead veins.

Pyrope
Pyrope (fire-garnet, Bohemian garnet) is a dark-red variety of garnet found embedded in trap tufa in the mountains of Bohemia, and in serpentine in Germany.

Pyrophyllite
Pyrophyllite has the formulae A12Si4O10(OH)2 and a relative hardness of 2.
It is appears very similar to talc. A comparatively rare mineral found in metamorphic rocks and often with kyanite.

Pyroxenes
Pyroxenes refers to a group of closely related and dark coloured rock forming minerals. Ex: augite, diopside.

Pyrrhotite
Pyrrhotite has the formulae FeS and a relative hardness of 5.
It is a magnetic common mineral often found in igneous rocks. It is mined for the nickel minerals associated with it.

Quartz
Quartz has the formulae SiO2 and a relative hardness of 7.
It is a very widespread mineral which occurs as an important constituent of igneous rocks which have an excess of silica. Very resistant to mechanical and chemical attack. Massive, fine grained types are called jasper, chert, flint, agate. Coarse crystalline types called amethyst, rose quartz etc.

Realgar
Realgar has the formulae AsS and a relative hardness of 2.
It is found in veins of lead, silver, and gold ores associated with arsenic minerals and stibnite. Also occurs as a deposit from hot springs. When mixed with saltpeter and burned it gives a bright white light and was used in fireworks.

Red Ochre
Red Ochre is a soft, earthy variety of haematite mineral.

Replacement
Replacement refers to the process by which one mineral is replaced by another and the original physical form is often retained.

Resinous
Resinous refers to a luster with the appearance of resin.

Rhodochrosite
Rhodochrosite is an important manganese ore. In South America it is used as an ornamental stone. It has a relative hardness of 4.

Rhodocrosite
Rhodocrosite has the formulae MnCO3.
It has a relative hardness of 5.
It is a minor ore of manganese. Occurs in veins with ores of silver, lead, copper, and other manganese minerals.

Rhodonite
Rhodonite has the formulae (Mn,Fe,Mg)SiO3.
It has a relative hardness of 7.
It often has a pink-red colour. Sometimes polished for use as an ornamental stone. Of interest to collectors.

Rhyolite
Rhyolite is a fine grained igneous rock.

Ruby
Ruby is the red transparent form of corundum. It is a precious stone.

Rutile
Rutile has the formulae TiO2.
It has a relative hardness of 7.
It exhibits a strong luster and often twinned crystals. Found in granite, granite pegmatites, gneiss, mica schist, metamorphic limestone and dolomite. Present as an accessory mineral in the rocks or in quartz veins. Name is derived from the latin word "rutilus"; red.

Sanidine
Sanidine is a clear, glassy, often cracked variety of orthoclase felspar, which is confined to modern volcanic rocks, such as trachyte, rhyolite and phonolite.

Sapphire
Sapphire is the blue transparent form of corundum.

Scapolite
Scapolite has the formulae (Na,Ca,K)4A13(Al,Si)3Si6O24(Cl,SO4,CO3) and a relative hardness of 6.
It shows fluorescence. Occurs in the crystalline schists, gneisses, and often is probably derived from the alteration of plagioclase feldspars. Also occurs in crystalline limestones formed by metamorphic contact or igneous intrusion. Associated with diopside, amphibole, garnet, apatite, and zircon.

Scheelite
Scheelite is an ore of tungsten. Found in granite pegmatites, contact metamorphic deposits and high-temperature ore veins associated with granitic rocks. Associated with cassiterite, topaz, flourite, apatite, molybdenite, and wolframite. Sometimes found with gold.
It has the formulae CaWO4 and a relative hardness of 5.

Schist
Schist refers to a metamorphic rock which exhibits fine lamination or layers along which the rock may be easily broken. Mica is a good example.

Schorl
Schorl is a rock compinded of quartz and black tourmaline. It is of igneous origin and occurs associated with granite and crystalline schists. It has a granular texture and is usually a grey colour.

Schwazite
Schwazite is a variety of Tetrahedrite containing upto 17 percent mercury.

Scolecite
Scolecite has the formulae Ca(Al2Si3)O10ù3H2O.
It has a relative hardness of 6.
It is of interest to collectors. Found in lavas and contact metamorphic depostis.

Scorodite
Scorodite has the formulae FeAsO4ù2H2O.
It has a relative hardness of 4.
It is of interest to collectors. Occurs in the oxidized portions of metallic veins as an alteration of arsenic containing minerals.

Secondary minerals
Secondary minerals refers to minerals formed by the alteration of preexisting minerals.

Sepiolite
Sepiolite has the formulae Mg4Si6O15(OH)2ù6H2O.
It has a relative hardness of 3.
It is an alteration product of magnesite and serpentine. Becomes plastic when mixed with water. Used in the manufacture of meerschaum pipes.

Serpentine
Serpentine has the formulae Mg6(Si4O10)(OH)8.
It has a relative hardness of 4.
It is a common mineral group and usually an alteration product of some magnesium silicate, especially olivine, pyroxene, and amphibole. Frequently associated with magnesite, chromite, and magnetite. Two varieties are antigorite(massive) and chrysotile(fibrous; also called asbestos).

Shale
Shale is a fine black sedimentary rock.

Siderite
Siderite has the formulae FeCO3.
It has a relative hardness of 5.
It is becomes magnetic when heated. An ore of iron. Frequently found as an impure admixture with clay materials. Also in concretions with concentric layers.

Silica
Silica refers to silicon dioxide (SiO2). A very common mineral that is found in many forms including quartz, opal, chert.

Silicates
Silicates refers to a group of minerals composed chiefly of SiO4. Ex: quartz, orthoclase.

Silky
Silky refers to a silk-like luster on a mineral. Results from a fine, fibrous and parallel surface.

Sillimanite
Sillimanite is a fibrous silicate with the formulae Al2SiO5.
It has a relative hardness of 7.
It is a somewhat rare mineral found as a constituent of gneiss and schist in metamorhpic rocks. Often occurs with corundum.

Silver
Silver is a white, ductile metallic element that is sonorous, very malleable, capable of a high degree of polish, and chiefly univalent with it's compounds and has the highest thermal and electrical conductivity of any substance. It has the symbol Ag.

Skutterudite
Skutterudite has the formulae CoAs2-3.
It has a relative hardness of 6.
It is an important ore of cobalt and nickel. Associated with native silver, bismuth, calcite, arsenopyrite.

Slate
Slate is a metamorphic rock.

Smithsonite
Smithsonite has the formulae ZnCO3.
It has a relative hardness of 6.
It is an ore of zinc. Usually found with zinc deposits in limestone beds.

Soapstone
see "Steatite"

Sodalite
Sodalite has the formulae Na4Al3(SiO4)3Cl.
It has a relative hardness of 6.
It is usually has an attractive blue colour. Used as polished slabs and for carved ornaments.

Sodium Tetraborate
see "Borax"

Specific gravity
Specific gravity refers to the relative density of a mineral. It is the ratio of:; Weight in Air/(Weigth in Air - Weight in Water). ;This measurement is an easily accomplished procedure using a simple balance or spring scale.

Sperrylite
Sperrylite is a rare natural compound of platinum and arsenic found in the nickel mines of Sudbury in Canada. It has the formulae PtAs2 and a relative hardness of 7.

Sphalerite
Sphalerite is the main ore of zinc. Associated with galena, pyrite, marcasite, chalcopyrite, calcite. Formed as a replacement deposit in limestones and in veins in igneous rocks. It has the formulae (Zn,Fe)S and a relative hardness of 4.

Sphene
Sphene (titanite, calcium silicate or titanate) is a source of titanium. A rather common accessory mineral in igneous rocks. It is often found as crystals and is commonly associated with chlorite. It has the formulae CaTiO(SiO4) and a relative hardness of 6.

Spinel
Spinel is a common metamorphic mineral occuring imbedded in crystalline limestone, gneisses, and serpentine. Occurs as a common accessory mineral in many dark igneous rocks. When transparent and finely coloured it is used as a gem. It has the formulae MgAl2O4 and a relative hardness of 8.

Spodumene
Spodumene is a source of lithium. Found occasionaly as very large crystals in pegmatic dikes. It has the formulae LiAlSi2O6 and a relative hardness of 7.

Staurolite
Staurolite is an accessory mineral in crystalline schists, slates, and sometimes gneisses. Often associated with garnet, kyanite, and tourmaline. May form cross twins. It has the formulae (Fe,Mg,Zn)2Al9Si4O23(OH) and a relative hardness of 8.

Steatite
Steatite (soapstone) is a variety of talc, hydrated magnesic silicate, usually devoid of a distinct crystalline structure.

Stephanite
Stephanite is a minor ore of silver. One of the last minerals to form in silver veins. It has the formulae Ag5SbS4 and a relative hardness of 3.

Stibnite
Stibnite is the major ore of antimony. It is Deposited by alkaline waters, usually in association with quartz. Found in quartz veins or beds in granite and gneiss. May occur as a replacement in limestones and shales.

Stilbite
Stilbite is a mineral of seconadary origin found in cavities in basalts and related rocks. It has a relative hardness of 4.

Stone coal
see "Anthracite"

Strata
see "Stratum"

Streak
Streak refers to the colour of the powder produced when a mineral is rubbed over the surface of a piece of unglazed, white porcelain.

Striations
Striations refers to very small parallel grooves or narrow channels of the faces of a crystal.

Strontianite
Strontianite has the formulae SrCO3.
It has a relative hardness of 4.
It is source of strontium. Physically simialar to cerussite and witherite. Associated with barite, celestite, and calcite in veins in limestone. Occasionaly found in igneous rocks and as a gangue mineral in sulphide veins.

Sulphates
Sulphates refers to a group of minerals in which sulphate SO4 is an important part.

Sulphides
Sulphides refers to a mineral group where sulphur is combined with one or more metals.

Syenite
Syenite is a plutonic igneous rock consisting essentially of alkali-felspar and one or more ferro-magnesian minerals, such as augite, hornblende, or mica.

Sylvanite
Sylvanite is a rare ore of gold, silver, and tellurium formed in low temperature hydrothermal veins. It is associated with calaverite and other tellurides - usually in quartz gangue.

Sylvite
Sylvite is an industrial mineral with a relative hardness of 2 used as a fertilizer. Found in sedimentary evaporite deposits associated with halite.

Tachylite
Tachylite (Tachylyte) is a natural glass, formed by the rapid cooling of molten basalt. It is a black or dark-brown and greasy looking substance. It is very brittle and occurs in basaltic obsidians in dikes, veins and intrusive masses.

Tachylyte
see "Tachylite"

Talc
Talc has the formulae Mg3Si4O10(OH)2 and a relative hardness of 1.
It is a mineral of secondary origin formed by the alteration of magnesium silicates. Usually found in metamorphic rocks where, in a non-crystalline form, it occurs as 'soapstone' - and can make up large rock masses. Used for laboratory table tops and for many industrial uses. It has a greasy feel.

Tenacity
Tenacity refers to the ability of a substance to resist being separated.

Tetragonal
Tetragonal refers to a crystal with four rectangular(not square) sides and two square bases. A butter package is an example.

Tetrahedrite
Tetrahedrite has the formulae (Cu,Fe)12Sb4Si3 - (Cu,Fe)12As4S13.
It has a relative hardness of 5.
It is an ore of copper and silver. Commonly found in hydrothermal veins formed at low to moderate temperatures. Usually associated with other silver, lead, and copper minerals.

Thenardite
Thenardite has the formulae Na2SO4.
It has a relative hardness of 3.
It dissolves easily in water and has a weak salty taste. Forms in sedimentary evaporite deposits in lakes and playas of desert climates. Used in the glass and paper-making industries.

Till
A till is a stiff unstratified clay mixed with sand, gravel and boulders.

Tillite
A tillite is a rock composed of consolidated till.

Titanate
see "Sphene"

Titanite
see "Sphene"

Topaz
Topaz has the formulae Al2SiO4(F,OH)2.
It has a relative hardness of 8.
It is formed by flourine-bearing vapors given off during the last stages of the solidification of igneous rocks. It is used as gem stone.

Torbernite
Torbenite is a uranium ore. It contains up to 61 percent uranium. It has a relative hardness of 2.5.

Tourmaline
Tourmaline has the formulae (Na,Ca)(Al,Fe,Li,Mg)3A16(BO3)3(Si6O18) and a relative hardness of 8.
It is most commonly found in granite pegmatites. It is usually black in colour, but lighter coloured gem varieties are also found.

Trachyte
Trachyte is grey, yellow, brown, green and red volcanic rock consisting chiefly of alkali felspar, and often containing crystals of glassy felspar, mica, hornblende, or augite. Trachyte occurs in lava, intrusive sheets, and dykes from the early Tertiary period.

Travertine
Travertine is a white or light-coloured crystalline concretionary limestone deposited from springs and used for building.

Tremolite
Tremolite has the formulae Ca2Mg5Si8O22(OH)2 and a relative hardness of 6.
The fibrous variety has been used for asbestos. The compact variety is called nephrite and is used for ornamental purposes in the orient. It is most often found in impure limestones where the rock has recrystallized during metamorphism.

Triclinic
Triclinic refers to a crystal with six faces as parallelograms and three axes of unequal length all inclined to each other. An uncommon form of crystal.

Trydimite
Trydimite has the formulae SiO2 and a relative hardness of 7.
It is of interest to scientists and collectors due to its rarity.

Tufa
Tufa is a soft white porous rock of calcium carbonate deposited from solution in spring water or percolating ground water.

Tuff
Tuff is rock debris consisting of volcanic ashes and igneous rocks of fine-grained material.

Turquoise
Turquoise has the formulae CuAl6(PO4)4(OH)8ù5H2O and a relative hardness of 6.
It is a mineral of secondary origin usually found in small veins and stringers. Used as a gem stone.

Twin
Twin refers to a mineral specimen comprised of two or more single crystals intergrown in a systematic arrangement.

Ulexite
Ulexite has the formulae NaCaB5O9ù8H2O and a relative hardness of 2.
It is an ore of boron and is formed during the evaporation of lake basins.

Umber
Umber is a naturally occurring mineral used as a pigment.

Uraninite
Uraninite has the formulae UO2 and a relative hardness of 6.
It is radioactive. An important source of uranium. Occurs in pegmatites and veins in granite; also as sedimentary deposits.

Vanadinite
Vanadinite has the formulae Pb5(VO4)3Cl and a relative hardness of 3.
It is source of vanadium and a minor ore of lead. Found in the oxidized portion of lead veins associated with other lead minerals. Vanadium is used as a steel-hardening metal.

Vein
Vein refers to a sheetlike extension of mineral matter cutting through preexisting rock.

Vermilion
see "cinnaba"

Vesicle
Vesicle refers to a small cavity in a volcanic rock.

Vesuvianite
Vesuvianite has the formulae Ca10(Mg,Fe)2Al4(SiO4)5(SiO7)2(OH)4 and a relative hardness of 7.
It is usually found in crystalline limestones and is formed as a result of contact metamorphism.

Vitreous
Vitreous refers to a luster like that of glass. Quartz is an example.

Vivianite
Vivianite has the formulae Fe3(PO4)2ù8H2O and a relative hardness of 3.
It is a rare mineral of secondary origin, associated with pyrite in copper and tin veins. Forms as a weathering product from primary iron-manganese phosphates in pegmatites. Also found in beds of clay.

Wavellite
Wavellite has the formulae Al3(PO4)2(OH)3ù5H2O and a relative hardness of 4.
It is a rare mineral of secondary origin. Found frequently in small amounts in crevices in aluminous, low-grade metamorphic rocks and in limonite and phosphorite deposits.

Willemite
Willemite has the formulae Zn2SiO4 and a relative hardness of 6.
It is an ore of zinc. Found in crystallized limestone. Associated with calcite, zincite, franklentie.

Witherite
Witherite has the formulae BaCO3 and a relative hardness of 4.
It is a somewhat rare mineral found most often in veins associated with galena. It's a minor source of barium.

Woolastonite
Woolastonite has the formulae CaSiO3 and a relative hardness of 5.
It is used in the manufacture of tile. Occurs mainly as a contact metamorphic mineral in crystalline limestones.

Wulfenite
Wulfenite has the formulae PbMoO4 and a relative hardness of 3.
It often displays brilliant colours and a tabular habit. It is a minor source of molybdenum and is found in the oxidized portion of lead veins with other secondary lead minerals.

Ytterbite
see "Gadolinite"

Zincite
Zincite has the formulae (Zn,Mn)O and a relative hardness of 5.
It is an ore of zinc and used in the production of zinc oxide.

Zircon
Zircon (zirconium silicate) has the formulae ZrSiO4 and a relative hardness of 8.
It is a common accessory mineral found in all types of igneous rocks. When transparent it's used as a gemstone.

Zoisite
Zoisite is a mineral formed in high temperature metamorphic rocks. It has the formulae Ca2Al3(Si3O12)(OH) and a relative hardness of 7.

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