▪ phenocryst ◊ piezoelectric ◊ porphyroblast ◊ porphyroclast ◊ pyroelectric ◊ pyroxenes ◊ |
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 Pegmatites are very coarse-grained igneous rocks in which crystal grains are larger than 20 mm, usually larger than 50 mm. Most pegmatites are composed of quartz, feldspars, and micas (granitic composition). More rarely, intermediate and mafic pegmatites contain amphiboles, Ca-plagioclase feldspar, pyroxenes and other minerals in recrystallized zones and apophyses associated with large layered intrusions. Feldspars within pegmatites may display exaggerated and perfect twinning, exsolution lamellae. When affected by hydrous crystallisation, macroscale graphic texture is known, with feldspar and quartz intergrown. Perthite feldspar within pegmatite often shows gigantic perthitic texture visible to the naked eye. Three theoretical explanations for pegmatite formation: ◙ metamorphic – pegmatite fluids created by devolatilisation (dewatering) of metamorphic rocks, particularly felsic gneiss. ◙ magmatic – pegmatites are often found in the aureoles of granites, often closely matching the compositions of the nearby granites – such pegmatites could represent exsolved granitic material that has crystallised in the country rocks. ◙ metasomatic – some pegmatites could be explained by the hydrothermal action of hot fluids upon a rock mass (producing bulk chemical and textural alteration). Pegmatitic growth mechanisms probably involve a combination of: ◙ Low rates of crystal nucleation coupled with high diffusivity, forcing growth of a few large crystals instead of many smaller crystals. ◙ High vapor and water pressure, assisting the enhancement of conditions of diffusivity. ◙ High concentrations of fluxing elements such as B and Li that lower the temperature of magmatic solidification. ◙ Low thermal gradients coupled with a high wall rock temperature, explaining the preponderance for pegmatite to occur only within greenschist metamorphic terranes |
| links: rocks: pegmatite with large mica crystals, pegmatite, 2, granitic pegmatite with quartz, feldspar, and large black crystals of tourmaline; formations: replacement pegmatites (r,l) amongst dipping colliform layers, Ivnarmiut Island, Skaergaard intrusion, Greenland, and pre-detachment dikes (crosscutting autolith block) with irregular layers near the margins becoming pegmatitic toward its interior, and mafic pegmatite replacement bodies, 2, and unusual small mafic pegmatite on Uttental Plateau that grades from mafic to felsic, base to top in the Homestead area, and close-up and close-up of 2 of these pegmatite replacement bodies that are frequently associated with the anorthositic replacement features probably related to the availability of water and its control on the residual liquid phase relations (white and gray, plagioclase; black, pyroxene and oxides; rusty patches, olivine), mafic pegmatite with interior that has abundant fine-grained gray granophyre and its contact with gabbro, and close-up of small mafic pegmatite on the Uttental Plateau ; pegmatite in granite boulder, pegmatite with irregular contacts with a fine-grained granite, webpages: Japanese pegmatite localities; locations: map |
 Peridotite is an ultramafic, ultrabasic (less than 45% silica), dense, plutonic igneous rock comprising mostly olivine and pyroxene. Weathering alters the appearance of peridotite from its fresh, green tinge to a brownish appearance due to the formation of iron oxides.
Most of the Earth's upper mantle (asthenosphere) is composed of peridotite that originated during the accretion and differentiation of the Earth, or that has differentiated, by precipitation of olivine ± pyroxenes, from basaltic or ultramafic magmas in turn derived from partial melting of the upper mantle peridotites. Deeper in the crust, olivine is replaced by a high pressure polymorphs, so peridotites do not occur at depths greater than 400 km. Peridotite emplaced in the continental crust is typically found in obducted ophiolite complexes, as xenoliths in basalts and kimberlite pipes, and as orogenic peridotite massifs and alpine peridotites. Peridote outcrops include layered complexes, sills, irregular masses, dikes, and volcanic pipes, and peridotite is the primary reservoir of all naturally occurring diamonds and chromium ore, and peridotites are also a frequent host of platinum metals and talc deposits. In the Kimberley district of South Africa, the peridotite is typically referred to as kimberlite, and contains the only diamonds known to occur in the matrix of their original formation. Olivine is unstable at shallow depths and reacts rapidly with water, so that much surface peridotite has been altered to serpentinite by a process in which the pyroxenes and olivines are converted to green serpentine. |
| links: images: hand-specimens: pale peridotite; weathered peridotite from Ronda in southern Spain; more typical color of peridotite, 2; dark; irregular grains of olivine and pyroxenes in peridotite; peridotite in basalt; greenish brown peridotite; green colored peridotite; peridotite from Pyrénées of France; close-up: slickensides on serpetinized peridotite; peridotite, closer; peridote surface; compare olivine in dunite; purple pyrope porphyroblasts in a peridotite with yellow-brown groundmass of weathered olivine, Otrøy, Western Gneiss Region, Caledonides, Norway; spinel harzburgite xenolith in peridotite nodule; peridotite (olivine + pyroxene) nodule in vesicular basalt; Minéraux Péridotite; Matsoko Coarse Peridotite, and Diamondiferous Peridotite; purple garnet peridotite from Greenland; thin-sections: peridotite fsu, 2; peroditite, Italy; peridotite, 2; kinked pyroxene in peridotite xenoliths in basaltic lava, Montferrier, France; peridotite comprising irregular interlocking crystals of olivine (bright colours) and magnesium-rich pyroxene (large grey crystals); xp ts; jp; mantle xenolith; serpentinized peridotite with a vein of chrysotile; Lame mince Péridotite; formations: exposed peridotite in the world's oldest ophiolite complex at Isua in Greenland (along with oldest sedimentary rocks, oldest banded iron formation, and oldest pillow lavas (wp) – the oldest rocks are the Acasta Gneisses in Canada, and the oldest mineral is Zircon in Australia); Red Hills, NZ, and Little Red Hill, West Otago, 2 miles South of Red Mountain, is a possible infaulted mantle peridotite, as is Mt Richards, and Red Mountain ultramafics; dunite vein in the serpentine (altered peridotite) for which the Lizard in Cornwall is famous; Péridotite incluse dans basalte, 2; peridotite in Norway; Serpentinized peridotite of the Kluane Ranges Mafic-Ultramafic Suite (TrK), Quill Creek Mafic-Ultramafic Complex; serpentinized mud volcano of altered peridotite in roadcut near Lake Berryessa, in northern CA; peridotite block, Shell Beach; the ophiolite sequence at the Smith River area is the site of one of the world's largest exposures of serpentinized peridotite; wispy pyroxene schlieren in pyroxene-dunite separating massive metasomatic dunite pipe (hosting Hall orebody), from surrounding layered websterite-peridotite; peridotite, Tahtalidag (NE continuation of Ala Dag range), Turkey; peridotite Tablelands, and Trout River Pond Trail, Gros Morne – the barren Tablelands (between Trout River and Woody Point in Gros Morne National Park) look more like Arizona than forested Newfoundland due to the ultramafic rock - peridotite - which makes up the Tablelands. Peridotite lacks the usual nutrients required to sustain most plant life, hence its barren appearance. The rock is very low in calcium, very high in magnesium, and has toxic amounts of heavy metals. Peridotite is also high in iron, which accounts for its brownish colour; Humboldt Massif with peridotite mantle rock exposures, and peridotite outcrop, southern New Caledonia, and pyroxenites in mantle peridotites; garnet peroditite in outcrop Gfohl Unit, southern part of the Bohemian Massif, Czech Republic, which mainly consists of acidic granulite and migmatitic gneiss; ASTER, Trodos Ophiolite, Cyprus; webpages: JosephineOphiolite; The Composition and Homogeneity of the Earth's Mantle; Coulée du volcan des Baumes, Cône volcanique des Baumes, Brèche volcanique, Quaternaire (1,5 MA), Volcanisme intra-continental |
Pelites are metamorphic rocks derived from protoliths rich in aluminum (shales and turbidite muds). Metamorphic rocks with sandstone parents are termed quartzo-feldspathic. Whereas pelites are derived from muds or clays, psammites are derivatives of micaceous sandstones. 'In unmetamorphosed turbidite sequences, psammite layers consist predominantly of quartz and feldspar, and pelite layers consist predominantly of phyllosilicates, quartz and feldspar. The predominance of relatively weak phyllosilicates in unmetamorphosed pelite units makes them weaker than interlayered psammite units during layer-parallel shearing at low to intermediate temperatures. However, during prograde metamorphism of pelitic rocks, the relative abundance of phyllosilicates can decrease due to the increased abundance of effectively rigid porphyroblasts, which leads to changes in the relative strengths of pelitic and psammitic rocks.'[pdf] Mineral assemblages in pelites can be employed as indicators of P, T, or PT ratios: andalusite low P metamorphism / kyanite high P / sillimanite high T (first grows as fine-grained acicular fibrolite) / chlorites low T / biotite moderate T / garnets moderate T / staurolite moderate-high P / chloritoid moderate-high P / cordierite low P / paragonite high P / see chart of stability fields of pelitic minerals. |
links: images: hand-specimens: pelite, 2; psammite: psammite interveined with calcite and galena; psammite, laminated, Grampian Group; close-ups: deformed psammite; formations: pelites: contact between pelite and marble; alternating layers of pelite (mud) and psammite (sand); metapelite from Namaqualand with compositional layering that represents former graded bedding, the neosomes developed around large pre-existing garnets in the more aluminous layers; metapelitic rock (Damara belt, Namibia), pink garnet-bearing neosomes are arranged as if they occupy or nucleate around tensional fractures; Wechsellagerung of distal turbidite and dark grey pelite, 2, Portugal, German (Engl); Folded limestone and pelite, Otter Lake, Quebec; Ailnack gorge - black schists (graphitic pelite to semi-pelite); Burrator Quarry exposing the irregular granite-pelite contact; folded pelite; Folded limestone and pelite, Otter Lake, Quebec; migmatitic semipelite in Ruthven Semipelite Formation, Grampian Group, Scotland; Psammite/pelite turbidites of the Longstaff Bluff Formation, 2; psammites: climbing ripple lamination (mica) in a psammite (quartzose bed); psammite with thin pelite (Claudy Formation), Kildoag Quarry, Co. Londonderry; amphibolite grade psammite with well preserved cross bedding, Gaick Psammite Formation, Grampian Group, and recumbent folds in Gaick Psammite Formation; Deeply weathered psammite, Gaick plateau; sheared psammites at Portvasgo; shear fold developed in Moine psammites; fold pair in highly sheared Moine psammite; undeformed; highly flattened conglomerate within the Moine metasedimentary sequence; webpages: Pelitic Rocks; related: pelite axehead |
Piezoelectric crystals, such as hemimorph tourmalines, quartz, and topaz develop an electrical potential (opposite electrical charges, dipole) at each end of the crystal when compressed or vibrated. Similarly, pyroelectric crystals generate electrical charge when heated.
Porphyroblasts are large, usually euhedral megacrysts that have formed within the finer matrix of those metamorphic rocks that display porphyroblastic texture due to static recrystallization (subsequent high-temperature metamorphism). Euhedral crystals are distinct, well-formed crystals with sharp, easily-recognized faces (image of euhedral almandine garnet in quartzitic gneiss at left). Euhdral crystals are distinguished from the interlocked grains of anhedral textured rocks that have cooled in the crowded environment of magma chambers. Subhedral crystals are intermediate in character between distinct euhedral crystals and enmeshed anhedral textures. Garnets and staurolite are the commonest porphyroblasts in the platy mica matrix of well-foliated, metamorphosed mudstones and siltstones (together known as metapelites). | |||||
Porphyroblasts are often confused with porphyroclasts, paleoblasts which, as fragments of the original rock before dynamic recrystallization produced the groundmass, are older than the matrix of the rock. (images - click to enlarge - above left, this mylonite shows a clear red garnets (at left) and smaller white feldspathic porphyroclasts; below right, purple pyrope porphyroblasts in a peridotite with a groundmass of weathered olivine.) Porphyroclasts remain as remnants of less easily deformed fragments of the original rock that were little or not affected by recrystallisation. Porphyroclasts may have been phenocrysts or porphyroblasts in the original rock. A porphyroclast is a relict grain larger than the other grains surviving in a deformed rock. Whereas porphyroblasts are larger than surrounding grains because they grew within the rock, porphyroclasts are larger because they have survived the size reduction suffered by surrounding grains. Deformed porphyroclasts act as shear sense indicators – grain tail complexes align with shear, sigma clasts have wedge shaped tails that do not cross the reference plane of shear, and delta clasts have curved tails that cross the reference plane of shear. Mica fish are large isolated mica crystals within a fine-grained recrystallized mylonitic matrix; they are named for their appearance of a lozenge shape and monoclinic shape symmetry with one curved and one planar side, and they can also show stair-stepping. Mica fish lie with their long axis in the extensional quadrant of the deformation, showing steeper inclination to the fabric attractor than mylonitic foliation. This, together with their asymmetry and stair-stepping of trails can be used as a shear sense indicator. The evolution of mica fish probably result from a combination of slip on the basal plane, rigid body rotation, boudinage, and recrystallization at the edges.  
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| Dynamic recrystallization involves nucleation and growth of new grains, which occurs during metamorphic deformation, rather than resulting from later high-temperature, contact metamorphism as in static recrystallization. | |||||
| In igneous rocks, phenocrysts are large crystals that result from two-stage cooling of the intruding magma. | |||||
| links: images: porphyroclasts: tiny feldspar porphyroclasts in ultramylonite; garnet schist with quartz porphyroblast, Wisconsin; distorted porphyroclasts: assymetric K-spar porphyroclast, 2; sigma clast showing sinistral (left-lateral) shear; sigma clast; pyrite sigma clast; delta clast; contact mylonite-ultramylonite in quartzofeldspathic rock, the ultramylonite matrix is very fine grained and composed of mixed quartz, mica, and K-feldspar with a large albitic content; porphyroclasts in garnet gneiss; thin-sections: micas that form foliations and cleavages in foliated schist; crenulation cleavage; crenulation cleavage that has formed as a result of shortening at a low angle to a pre-existing cleavage; dextral shear in mylonitic quartzofeldspathic rock, blue line indicates discrete C-zone and the green line shows orientation of S-surface in recystallized quartz; distorted porphyroclast; peridotite ultramylonite with extreme grain size reduction associated with localization into 5 mm to 30 m wide shear zones after relatively large and uniform strain, North Pyrenees; mica fish, 2; asymmetric muscovite porphyroclast (mica fish), dextral shear, Goat Rock Fault Zone mylonite in the Georgia Piedmont; sheared amphibolite with assymetric feldspar porphyroclast (blue) and an adjacent NSC surface that has possibly separated the two feldspar grains - brown flaky material is biotite, and green material is hornblende; webpages: Ductile Rocks (porphyroclasts, mica fish, porphyroblasts); | |||||
Mineral assemblages depends upon protolith composition: ◙ mafic rocks yield chlorites, prehnite (above left), albite, pumpellyite (above right), and epidote ◙ ultramafic yield serpentine, talc, forsterite, tremolite and chlorites ◙ argillaceous sedimentary rocks yield quartz, illite, albite, and stilpnomelane chlorite ◙ carbonate sediments yield calcite, dolomite, quartz, clays, talc, and muscovite. |
| links: images: crystals: prehnite, 1, 2, 3, 4, prehnite with calcite, 2, prehnite on epidote, 2, 3; prehnite nodules and greenish tan chlorite; prehnite nodules in vugs lined with chalcopyrite crystals coated with greenish tan chlorite (specimen extracted from a large quartz lens in coarse grained amphibolite); Andradite var. Melanite (Garnet Group), Prehnite, Epidote; pumpellyite, 2, 3: hand-specimens: rock containing prehnite and pumpillyite, Hancockite (brick-red) andradite (brownish-yellow) and prehnite (white); Prehnite crystals (white) in hancockite (brown), with minor andradite (dark brown) and franklinite (black); pumpellyite; close-up: pumpellyite; formations: 1; block of pumpellyite-jadeitite (with George Harlow) at Quebrada La Peña, La Ensenada, Guatemala; amygdaloidal ore with some vesicles filled with native copper and others with dark green pumpellyite. thin-sections: prehnite, XP, 2, 3, 4, 5, 6; Prehnite, Prehnite (2), Pumpellyite, Pumpellyite (2) pumpellyite; webpages: Metamorphic Facies; Prehnite; Pumpellyite |
Pseudotachylites are associated with faulting in impact zones and brittle shear zones, and have an appearance that resembles the basaltic glass, tachylyte. Pseudotachylites are typically found along fault surfaces or in impact craters (often as matrix within breccias), or as veins injected into shear zone walls of the fault. Evidence usually reveals that the pseudotachylyte formed by frictional melting of the wall rocks during rapid fault movement associated with a seismic event – leading pseudotachylites to be dubbed "fossil earthquakes". |
| link: images: hand-specimens: pseudotachylite (pt), Vredefort, 2; pt, Siljan; glass melted onto sandstone; pt; pseudotachylite veins in granite gneiss (12x8 cm), Vredefort Dome impact structure, South Africa; formations: impact pt; outcrop of brecciated pseudotachlite (melt is black, pink is gneiss) at Vrederfort Dome Impact Crater, south of Johannesburg where the high pressure of the impact melted the crustal rocks, creating a ~ 300 km impact crater 3.2 Ga, and Vredefort shatter cones, exposed by erosion, boulder, pseudotachylite in quarry in the impact crater, and 2, 3, 4, quarry, and pseudotachylite of Vredefort ring structure, 2, 3, 4, and Vredefort dome from space; Voldeford structure; shatter cones and breccias, Sudbury impact crater, pseudotachylite breccia, Matachewan dike; pseudotachylite vein crossing granite boulder, and pseudotachylite outcrop, 2; pseudotachylite injection vein (red arrow), a pseudotachylite slip surface with secondary associated, high angle oblique microfaults (violet arrow), and a chloritized fault breccia (yellow arrow) - slip is parallel to the gneissic layering seen in the adjacent rocks, basement rocks, South Carolina; outcrop; black pseudotachylitic breccia with clasts of granitic country rock, clasts are found mostly on one side of the pseudotachylite vein (exposure ~ 6 metres long), and close-up, 2; close-ups: mantle xenolith (lherzolite) within brecciated kimberlite matrix, South Africa; thin-sections: pseudotachylite; webpages: Impact melts from the Rubielos de la Cérida impact basin; Impactites; Impakt-Schmelzgesteine; Rochechouart Impaktstruktur; |
mineral / chemical formula |
properties / significance / occurrence |
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pyroxenes XY(Si,Al)2O6 XYSi2O6, where X and Y are two divalent cations or a monovalent and a trivalent cation |
image - click to enlarge - host basalt with xenolith dominated by green peridot olivine, minor black orthopyroxene and spinel crystals, and rare grass-green diopside grains.
Family of common, monoclinic and orthorhombic, rock-forming inosilicates important in the mantle (with olivine). Crystallize from magmas before lavas erupt, so initially viewed as impurities in volcanic glasses. Pyroxenes and feldspars form the major mineral components of basalts and gabbros. |
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| [images, olivine gabbro with small, wavy, pegmatitic stringers containing abundant large pyroxenes, wavy pyroxene stringers, large pyroxenes in wavy pyroxene rock, wavy pyroxene rock inside pluton margin; augite, 1, 2, 2wp, 3wp, diopside, 1, hypersthene, jadeite, spodumene, pyroxene structure, pyroxene in gabbro, pyroxene crystal, pyroxene in andesite, rapid cooling - animation, thin-section, pyroxene chondrule from L3 Chondrite Boudnib, radial pyroxene chondrule from H3 Chondrite Brownfield, large twinned pyroxene (aegirine) crystal, PP, pyroxene relique]; structure: diopside, orthopyroxene, pyroxene group (UC), (crystals UC) | ||
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