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The Palaeocene was the thirteenth geological period, 80,000,000 years ago.
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Palagonite is a fine dark red rock, usually found in thin layers between ancient flows of basaltic lava. It is really a weathered and oxidised basic tuff or ash-bed, composed of shreds of black volcanic glass, pumice and rock fragments.
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Pearly refers to a lustre with the iridescent look of a pearl. Most commonly seen on surfaces parallel to cleavage planes.
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A pebble is a class of rock between 2 and 64 mm in diameter.
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Pectolite is a brittle mineral, a hydrous silicate of sodium and calcium with the formulae NaCa2Si3O8(OH) and a relative hardness of 5. It is formed from hydrothermal solutions filling cavities in basalts. Pectolite was confirmed as a distinct species in 1828 and is associated with zeolites, prehnite and calcite.
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In physical geography, a pediment is a broad, gently sloping rock surface that extends outwards from the abrupt foot of a mountain. Pediments are found in arid and semi-arid regions and are usually covered with a layer of alluvium.
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In geology, the term pediocratic refers to a period during which erosion predominates over crustal upheaval, leading to an overall lessening of relief.
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In crystallography, a pedion is a crystal form consisting of a single face, without any symmetrically equivalent face.
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Pegmatite is a variety of extremely coarse-grained igneous rock chemically similar to and closely associated with granite. The mineral constituents of
pegmatite are largely those typical of acidic intrusive rocks: orthoclase feldspar, quartz, and mica. Individual mineral crystals may be up to several meters in length. The growth of crystals this large indicates that the parent magma, from which the rock solidified, was able to cool very slowly, after injection into fissures extending outward from a central magma chamber.
Pegmatite is widely distributed in the crust of the earth but is found especially in older mountain chains, where it is restricted essentially to those surface areas in which igneous and metamorphic rocks are prevalent. Minerals containing the elements boron, beryllium, and lithium are abundant in some pegmatites. Other pegmatites contain commercial quantities of feldspar, mica, and gem-grade tourmaline.
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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.
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Pentlandite is the most important industrial source of nickel and has the formulae (Fe,Ni)9S8 and a relative hardness of 4. Pentlandite was confirmed as a distinct species in 1856 and named after Joseph Pentland, the discoverer of it.
Pentlandite forms in basic igneous rocks such as diabase and gabbro through magmatic segregation.
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Peridote is a yellow variety of tourmaline found in Sri Lanka.
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Peridotite is an intrusive igneous rock thought to be one of the major components of the Earth's mantle. Peridotite is generally dark coloured and formed from dense, heavy minerals, of which more than 40 percent are olivine.
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A perlite is an obsidian, or other vitreous rock with a concentric structure and which is expansible by heating.
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A perlitic structure in volcanic rocks consists of small concentric cracks, along which the rock readily breaks down, yielding rounded pearl-like fragments. The phenomena is found in some pitchstones and obsidians.
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The Permian was the eighth geological period, 205,000,000 years ago. It marked the evolution of the reptiles.
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Perthite is a variety of orthoclase feldspar or microcline in which small veinlets of plagioclase feldspar (albite) are present. It is a common mineral in rocks such as granite and syenite.
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Petalite has the formulae LiAlSi4O10 and a relative hardness of 7. It is an ore of lithium. Associated with spodumene, lepidolite, tourmaline.
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Petroleum is a mineral oil from which petrol is derived.
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Pharmacosiderite is a secondary mineral formed through the oxidation of pre-existent arsenic-bearing minerals such as arsenopyrite. It is also commonly found associated with goethite, pyrite, realgar and scorodite. Pharmacosiderite was confirmed as a distinct species in 1813.
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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.
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A phenocryst is one of the prominent embedded crystals of a porphyry.
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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.
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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.
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Phonolite or clinkstone is a volcanic rock, consisting essentially of nepheline and sanidine feldspar. It usually also contains some form of augite, hornblende or biotite.
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Phosphates refers to a group of minerals where phosphate (PO4) is an important constituent.
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Phosphorite is an impure massive form of apatite, mined for the calcium phosphate it contains, and forming one of the chief sources of phosphorus and its compounds, and of artificial manures.
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Phyllite is a clay rock, resembling slate, but containing a large percentage of fine, silky, pale- coloured mica. It splits readily, and has a shimmering lustre on the surface.
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Picrite is a crystalline igneous rock which consists essentially of olivine and augite, but usually also contains iron oxides, a little feldspar, biotite, hornblende and apatite.
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Piemontite (piedmontite) is a rare reddish-brown coloured mineral first discovered in New Zealand and confirmed as a distinct species in 1853. It is a hydrous silicate of calcium, aluminium and ferric iron and usually contains between five and twenty percent manganese oxide. Piemontite is found in metasomatic manganese ore deposits in andesite, rhyolite and low-grade schist. Rarely it also occurs in pegmatite. Piemontite has a relative hardness of 6.
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Pipe refers to a cylindrical, vertical mass of igneous rock.
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Pitchstone is a dark, glassy igneous rock produced by the rapid cooling of lava. It is composed essentially of feldspar and quartz and is a natural glass, the glass forming because the rock cools too quickly for the minerals to form into crystals. Pitchstone is closely related to obsidian, but where pitchstone has cooled more slowly than obsidian, it is less shiny and shows signs of devitrification.
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Placer refers to a concentrated deposit of mineral particles that have weathered out of rock. Usually deposited by stream action.
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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.
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Platinum is a rare, silvery coloured, heavy metallic element with the symbol Pt. It is relatively inert and has a high melting point. Platinum is used in jewellery and as a catalyst.
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Plattnerite is a black and heavy secondary mineral of the rutile group formed through the alteration of primary lead-bearing minerals and the oxidation of lead itself. Plattnerite was confirmed as a distinct species in 1845 and named after K F Plattner, professor of metallurgy and assaying at the University of Freiburg,Germany. Plattnerite is a lead oxide and has a brilliant metallic lustre and tetragonal crystals. Plattnerite is closely related and is dimorphous to scrutinyite.
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Playa refers to a shallow basin or plain in a desert where water collects after a rain and then evaporates.
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The Pleistocene (ice age) was the seventeenth geological period, 500,000 years ago. It marked the evolution of tool-making man.
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The Pliocene was the sixteenth geological period, 8,000,000 years ago. It marked the modern formation of the continents.
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Plumbogummite is a brightly coloured and widely distributed secondary mineral formed mainly in the oxidation zones of lead ores. Plumbogummite was confirmed as a distinct species of mineral in 1819, and is a hydrous hydrated phosphate of lead and aluminium. It is typically white but can be grey, yellow, greenish blue or reddish brown in colour, and is often brightly coloured. Plumbogummite has a relative hardness of 4.5 to 5.
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Plutonic rock is igneous rock that has solidified at depth in the Earth's crust. Having cooled slowly, plutonic rocks are coarse-grained: granites and gabbros are typical plutonic rocks. A body of plutonic rock can be called a pluton, but this word has also been used in a narrower sense for a granite mass occurring at a relatively high level in the crust and more or less circular in plan.
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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.
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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.
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Porter (black and gold) is a type of marble found in the Apennines. Porter has a black background clouded with grey and is strongly veined with chain-like formations in yellow - whence it's alternative name of 'black and gold'.
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Potash is the name given to any mineral containing potassium.
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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.
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Potstone (Lapis ollaris) is a species of talc containing an admixture of chlorite. It is green in colour and varies 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.
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Prase is a dark green variety of quartz, the colour being due to an admixture of hornblende.
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The Pre-Cambrian period was the second geological period. It was the time when the first animal life appeared, about 1,125,000,000 years ago.
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Precipitation refers to the process by which dissolved or suspended solids are separated from a liquid.
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Prehnite is a mineral named after Colonel Hendrick Von Prehn who first discovered it at the Cape of Good Hope in South Africa. It was confirmed as a distinct species of mineral in 1788. Prehnite is a hydrous silicate of calcium and aluminium with the formulae Ca2Al2Si3O10(OH)2 and a relative hardness of 6 to 7. Prehnite occurs as a crusty lining in cavities in basalt and related rocks and is associated with zeolites, datolite, pectolite, and calcite. Prehnite has a characteristic green colour. Resembles hemimorphite but is of a lower specific gravity and fuses easily.
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In geology, a propylite is an altered andesite or similar rock containing calcite, chlorite, etc., produced by the action of hot water.
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Proustite is an ore of silver with a characteristic ruby-red colour, vermilion streak and a brilliant lustre. It has the formulae Ag3AsS3 and a relative hardness of 3.
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Pseudomalachite is a rare green coloured mineral named for its superficial resemblance to malachite. Both minerals contain copper, but pseudomalachite is a hydrous hydrated phosphate rather than a hydrous carbonate. Pseudomalachite was confirmed as a distinct species of mineral in 1813 after being discovered at Virneberg in Germany. Pseudomalachite has a relative hardness of 4.5 to 5.
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Pseudomorph refers to a mineral that has taken the outward crystal form of a different mineral.
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Psilomelane is an ore of manganese. It usually occurs with pyrolusite but is different from other manganese oxides in that it is of greater hardness and has an apparent lack of crystal structure. It has the formulae BaMnO16(OH)4 and a relative hardness of 6.
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Pudding-stone is a popular name for a conglomerate.
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Pumice is a light igneous volcanic rock used as a scourer, abrasive, polisher and in the manufacture of soundproofing tiles. Pumice can occur in any type of lava, but is more common in rhyolite and trachyte than in basalt. Pumice is formed from molten lava that cools so quickly that it doesn't have time to form either crystals or glass. Pumice may be heated until it melts, and will then form obsidian. Pumice is full of tiny vesicles which are the solidified remains of bubbles that were in the molten rock. These vesicles may pumice very light and porous - pumice will float and absorb water until saturated.
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Purpurite is an alteration product of lithiophilite that occurs in pegmatites. It has the formulae (Mn,Fe)PO4 and a relative hardness of 5.
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Pyragyrite is an ore of silver. It is similar to proustite but has a deeper red colour and is less translucent. Pyragyrite forms in silver veins at low temperatures and is one of the last minerals to crystallize in the process of deposition. It has the formulae Ag3SbS3 and a relative hardness of 3.
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Pyrite is one of the most common minerals. It is a shiny yellow mineral resembling gold, whence it's name 'fool's gold' formed from cooling magma and is found as an igneous segregation and also in metamorphic rocks and as vein deposits. Often it is found in sedimentary rocks being both primary and secondary in origin. Pyrite is often mined for the gold or silver associated with it. Pyrite has the formulae FeS2 and a relative hardness of 7. During the Second World War pyrite was mined in the USA for its sulphur which was used to produce sulphuric acid.
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In mineralogy, a pyroclast is a fragment of volcanic rock scattered by an explosive eruption.
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Pyrolusite is an oxide of manganese of the rutile group with 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. Pyrolusite was confirmed as a distinct species of mineral in 1827.
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Pyromorphite is a chlorophosphate of lead with 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. Pyromorphite forms through the alteration of the primary minerals anglesite and galena in the oxidation zones of lead veins where it is associated with cerussite and limonite.
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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.
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Pyrophyllite is a comparatively rare mineral found in metamorphic rocks and often with kyanite. It is a hydrous silicate of aluminium with the formulae A12Si4O10(OH)2 and a relative hardness of 2. It appears very similar to talc. Pyrophyllite is used industrially as a refractory mineral, as a filler for rubber, in paints and in insecticides.
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Pyroxenes refers to a group of closely related and dark coloured rock forming minerals. Examples are aegirine, diopside, enstatite, hedenbergite, jadeite and spodumene. Pyroxenes are common rock-forming minerals which are found in most igneous and many metamorphic rocks. Their presence in a rock indicates that it crystallised at a high temperature.
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Pyroxenite is a medium to coarse-grained intrusive igneous plutonic rock made up almost completely of pyroxenes. Containing less than 45 percent silica,
pyroxenite contains large quantities of iron- and magnesium-bearing minerals.
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Pyrrhotite is a magnetic common mineral often found in igneous rocks, and confirmed as a distinct species of mineral in 1835. It is an iron sulphide with the formulae FeS and a relative hardness of 5. It is mined for the nickel minerals associated with it.
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Abbreviations
Rocks & Minerals
