The existence of these compositional gaps is believed to result from changes in Al–Si ordering behavior (next section) as the Ab:An ratio changes. Norton combined units (5) and (6) of Cameron et al. Field pictures and photomicrographs of HP granulite at Bashiwake, SAT. Intrusions of the Perthite Monzonite Suite are suggested to be co-magmatic with the Kiirunavaara Group volcanic rocks (Witschard, 1984; Martinsson, 2004a). The unmixed feldspars result in perthite (albite in orthoclase), or more rarely, antiperthite (orthoclase in albite). The modes of homogenous pegmatites of the Crystal Mountain district show a well-defined maximum at 20 percent quartz, 30 percent plagioclase, and 50 percent perthite. It is notable that both zoning schemes recognized assemblages of quartz plus lithium aluminosilicates or equivalent phosphates (zones (7) or [6]) as primary, despite their distinctly non-granitic compositions (cf. Neither zonation model addresses the textural features of pegmatites that are so prominent (e.g., London et al., 2012b). Complex, two-phase field, of triclinic potash feldspar and triclinic soda feldspar solid solutions (microcline-perthite and antiperthite). (H) Cr-spinel needles in Opx from the Lüliangshan garnet lherzolite (North Qaidam UHP metamorphic belt, China). (Answer: The solvus is a curve within the solid field. kinetics of the plagioclases, antiperthite texture has not received as great attention as the perthite textures from research workers. the pink phenocrysts in Fig. Cameron et al. c. Eutectic point of monoclinic orthoclase-rich solid solution and triclinic albite- They are generally colourless, white or grey, but may be yellow, red, or green; colourless in thin section. Reaction point R represented approximately by 5 KAlSi2O6 (leucite) + 3 KAlSi3O8 + 5 SiO2 (melt) → 8 KAlSi3O8. Syntectonic emplacement of granitoid magma is supported by: (1) concordance of magmatic and metamorphic foliation within the pluton; (2) continuity of foliations across intrusive contacts; and (3) general obliquity between foliation surfaces and pluton contacts (Riller et al., 2006). (C) Zoned plagioclase, with preferential alteration of Ca-rich zones (soil on andesite, western Java) (PPL). This zoning sequence was later revised by Norton (1983) on the basis of exposures in the large lithium-rich pegmatites of Tanco, Manitoba (Crouse and Černý, 1972), Bikita, Zimbabwe (Cooper, 1964), and similar pegmatites in the Black Hills, South Dakota: Plagioclase–quartz–spodumene (or petalite or montebrasite or both), Quartz–spodumene (or petalite or montebrasite or both). S.K. (A) Antiperthite with K-feldspar (Kfs) exsolution lamellae in two directions from a ~ 1.95 Ga high-pressure granulite (Hengshan, China). An even greater effect is shown on the addition of SiO2 to the system at 2 kbar (curve 3). Modal analyses by these three methods of the same thin section of a sandstone plot differently (Fig. (A) Orthoclase remains of perthite grain after dissolution of albite lamellae (soil on granite, Akagera National Park, Rwanda) (PPL). (K,Na)[(AlSi3O8]. With the addition of H2O to the system, at PH2O = 5 kbar, the liquidus and solidus temperatures are over 300° lower than for the anhydrous system (Figure 5, curve 2). Stewart, 1978). It can also occur by hydrothermal process and contains little natrium (sodium) components and is known as adular. Monophase quartz constitutes a central core, except for those pegmatites in which miarolitic cavities form the last primary units adjacent to the quartz cores (Fig. Below the solvus the solid solution breaks down to 2 phases - one Na-rich, the other K-rich. This if the composition of the initial liquid (and hence the feldspar) is in the two-feldspar range, resolution will occur when the temperature of the system reaches the solvus. They may also develop as authigenic minerals occurring as rims on detrital feldspar, often in optical continuity. (Or 83) Plag. Three formal, fairly rigorous, but different definitions (≈counting methods) have been erected (Suttner et al., 1981; Folk, 1974; Dickinson, 1970; see Appendix I). The 2.33 Ga Creighton (Frarey et al., 1982) and 2.47 Ma Murray (Krogh et al., 1996) granitoid plutons cut the Huronian Supergroup footwall rocks in the South Range of the SIC (Fig. Creek district; in these, the "perthite" of the diagrams is actually perthite plus muscovite. In alkali feldspars the intergrowths are called, Interpretation of Micromorphological Features of Soils and Regoliths (Second Edition), – colourless, often cloudy, low negative relief, perfect cleavage in one direction, less perfect in the others; first-order grey interference colours, simple Carlsbad twinning; can occur as part of exsolution features composed of orthoclase and albite lamellae (, Introduction to Mineralogy and Petrology (Second Edition), A synthesis of the geology of the Sudbury Structure. The T-controlled exsolution textures are exemplified by perthite/antiperthite (Fig. (a) and (b): Mafic granulites occur as lenses within felsic granulites; (c) garnet + kyanite + perthite (former ternary feldspar) + quartz assemblage in felsic granulite (CPL); (d) garnet + clinopyroxene + plagioclase + quartz + rutile assemblage in mafic granulite (PPL); (e) spinel grows between kyanite and garnet in felsic granulite (PPL); (f) sapphrine + plagioclase symplectite in mafic granulite (PPL). There are very clear differences in grain size, color, inclusion content, and diversity in rock types between the Creighton and the Murray granites, and the different U–Pb ages (Fig. Haldar, in Introduction to Mineralogy and Petrology (Second Edition), 2020. See text for criticism of Mikinen's diagram. Perthite is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar (near K-feldspar, KAlSi 3 O 8, in composition) includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar (near albite, NaAlSi 3 O 8, in composition).Typically the host grain is orthoclase or microcline, and the lamellae are albite. But see Bangs-Rooney and Basu (1994) for a possible alternative. Some cryptoperthites produce iridescent colors by the diffraction of light, the most familiar being the brilliant blue and silver iridescence of the otherwise black or dark green feldspars in the rock-type larvikite, used as an architectural stone in innumerable shopping malls. It can be described as a ‘strain-free’ solvus. Exsolution is a process by which a former fairly homogeneous phase separates into two or more solid solution phases under subsolidus conditions. The alkali feldspar phase diagram. (2015). (1949) extracted a general sequence of internal mineralogical zonation, given below, from margin to center, or border to core. (c) High (PH2O > 500) water-vapour pressure, depresses the alkali feldspar solidus to the point where it intersects the solvus. (E) Plagioclase phenocrystal in basalt fragment, showing complex pattern of alteration (volcanic ash soil, Isla Santa Cruz, Galápagos) (PPL). 11. K-feldspar melts incongruently at ∼1150° to leucite (KAlSi2O6) under anhydrous condition at 1 atm (Figure 3). The pyroxene lamellae suggest that previous supersilicic or majoritic garnet with substitutions of Mg (or Fe) + Si → 2Al and Na + Si → Ca + Al formed under ultrahigh-pressure conditions (~ 6 GPa or above) (Ye et al., 2000). 2. 1996), but they were also reported in low-pressure igneous plutons. In others cases, exsolution occurs by nucleation and growth of the exsolved phase. (b) At low (PH2O = 200 MPa) water-vapour pressure, complete solid solution exists at T > 700 ° C between KAlSi3O8 (Or) and NaAlSi3O8 (Ab). Plagioclase is not stable in the P–T environment of the eclogite facies; under these conditions the albite and anorthite components enter the compositions of omphacite and garnet, respectively. The phase diagram for the plagioclase series was one of the first to be determined experimentally (Figure 5). olivine, pyroxene). Though the sequences of zonation differed in detail from district to district, Cameron et al. perthite is strongly related to the string perthites, but do, however, ... in relation to the probable phase diagram of Laves (9, p. 561) this ... ratio the feldspar should be recognised as an antiperthite. 37 Phase diagram for the alkali feldspars. 1994. The nepheline in leucocratic syenite and pegmatite shows deviation from the ideal composition with respect to Si and K contents. Objective and reproducible modal analyses of sandstones, however, were hampered for over a 100 years because “rock fragments” defied the traditional description of “two or more minerals in a grain of sand.” Would a grain of rutilated quartz or a grain of perthite be counted as a rock fragment? K-feldspar albite inter-growths is known as perthite and albite intergrowths K-feldspar as antiperthite. Georges Stoops, Florias Mees, in Interpretation of Micromorphological Features of Soils and Regoliths (Second Edition), 2018, Optical characteristics – colourless, often cloudy, low negative relief, perfect cleavage in one direction, less perfect in the others; first-order grey interference colours, simple Carlsbad twinning; can occur as part of exsolution features composed of orthoclase and albite lamellae (perthite, antiperthite), Occurrence in rocks – in saturated intrusive rocks (e.g., granite), Occurrence in soils – almost absent in strongly weathered material but more stable than plagioclase, pyroxenes and amphiboles in soil environments (Taboada & García, 1999a, 1999b), Pedogenic alteration – along fractures, and with cross-linear patterns related to cleavage and twinning directions, followed by cavernous alteration (Delvigne et al., 1987); replaced by gibbsite in conditions with strong leaching; upon alteration of perthite or antiperthite, albite will disappear first, leaving a set of parallel orthoclase lamellae separated by weathering products (Fig. In contrast, primary precipitate plagioclase may be absent from the ferromagnesian-rich bands in which the feldspar occurs as a product of the crystallization of intercumulus liquid. The K–Na feldspars are essential constituents of alkali and acid igneous rocks and are particularly abundant in syenites, granites, granodiorites, and their volcanic equivalents. The basic QFL diagram to plot modal composition of sandstones, counted following Gazzi (1966) and Dickinson (1970). By continuing you agree to the use of cookies. With further cooling under equilibrium conditions, both liquid and crystals change their composition along the liquidus and solidus, respectively, until at 1285°C the crystals reach a composition of Ab50An50(D) as the last of the liquid now of composition (C) is used up. Both the relief and birefringence are low and similar to quartz. Development of antiperthite both by exsolution and replacement processes is accepted (Deer et el., t963). + MELT Two feldspars Or-rich feld. Ab-rich feld. It typically consists of 10%−36% of (KAlSi3O8), and 64% to 90% of (NaAlSi3O8). Composition of Plagioclase The plagioclase feldspars, consist ... solution occurs across the whole composition rangecomposition range. 19). Such sections may be recognised by the sharpness of the composition plane between albite twin lamellae, by the equal interference colours of the twin lamellae when the twin plane is parallel to the vibration direction of the polarizers, and by adjacent twin lamellae giving equal extinction angles on either side of the twin plane. The disordered solid solution can only exist at high temperatures. Values for glasses of plagioclase composition are also shown. In addition to those minerals noted by Norton (1983), the assemblage of zone [9] in lithium-rich pegmatites also contains abundant beryl, elbaite (lithium tourmaline), numerous species of phosphates and Sn–Nb–Ta oxides, and pollucite (CsAlSi2O6). Under these conditions no 1-feldspar field exists and therefore no homogeneous intermediate alkali feldspars can form; two separate feldspar species (one K-rich and one Na-rich) crystallize directly from the melt. Modal data, collected by the G-D method and plotted in the Dickinson diagram (Fig. Pyroxene/rutile/amphibole lamellae in garnet are observed in eclogites, orogenic garnet peridotites, and peridotite xenoliths in kimberlites. antiperthite . The maximum extinction angles normal to {010} or in the so-called symmetrical extinction zone are diagnostic (Figure 7). Phase compositions, the It is usually assumed that the bulk composition of the feldspar remains constant during unmixing and that it represents the composition of the crystal when it grew, but some perthites form by non-isochemical replacement (e.g. Many alkali feldspar crystals from plutonic igneous rocks (e.g. In meteorites, a whole range of plagioclase compositions occur, but there are two main clusters at around An90 and An15. This continuous change in composition of the plagioclase crystals with falling temperature occurs only if there is sufficient time for the earlier-formed crystals to react with the liquid: if there is insufficient time for this interchange of material, the crystals will be zoned. Thus, Cameron et al. Though most of the same zones are present in shallowly dipping pegmatites, their distribution tends to be layered and not concentric about a center. The peak assemblages are represented by garnet + kyanite + perthite (former ternary feldspar) + quartz in felsic granulite, and garnet + clinopyroxene ± perthite ± kyanite + plagioclase + quartz in mafic granulite (Zhang et al., 2005c) (Fig. Illustrations of models for internal zonation of pegmatite. Intergrowths develop within single crystals by a process that is variously called phase separation, unmixing and exsolution. For a sample This rock type is likely developed within the metamorphic halo the SIC, and it likely reflects recrystallization and possibly partial melting. Another feature of this intrusive suite that supports such a connection, and a mantle plume origin, is the abundant occurrence of ultramafic-mafic complexes northwest of Kiruna whose existence possibly indicates the location of the plume centre (Martinsson, 2004a). It is possible to form mixed-crystals of isomorphic replacement of Or-component and Ab-component at high temperatures. 3D) and the eclogite in North Qaidam, indicating that high concentrations of K and [OH]− 1 incorporate in pyroxenes at very high pressure (~ 6 GPa or above, Zhu and Ogasawara, 2002). ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. Intergrowths develop within single crystals by a process that is variously called phase separation, unmixing and exsolution. Optical microscopy has been and continues to be the mainstay of provenance investigations for identification of mineral grains as small as ∼20 μm in siliciclastic rocks. In a conventional phase diagram (Fig. 5 in Zuffa, 1985). Names can be used in conjunction, as in cryptomesoperthite. All the common members of the feldspars have D 2.55–2.5, with H 6–61/2. A. Equilibrium diagram of the binary system KAlSi2O6 (leucite)–SiO2. 2.7). 6) the solvus curve shows the compositions of structurally separate Ab- and Or-rich phases. (F) Idem in XPL, showing transformation of plagioclase to optically isotropic colloids, from which fan-like gibbsite aggregates have crystallised. 3D). In thin section, the alternate twin lamellae give symmetrical extinction angles on either side of the twin plane. Photograph of thin section microcline showing the grid structure of twining under cross-polarized light. 2.12C). T2 T3 % solid solid comp. 2.7). Marc's legend: a. Melting point of orthoclase. Figure 6. 11). If the host phase is K rich feldspar then this mineral is called perthite. These intergrowths are coherent and usually below the resolution of an optical microscope. The Perthite Monzonite Suite (PMS) is represented by large peraluminous to metaluminous intrusions in the northwestern part of Norrbotten, but such rocks are rare in the eastern part (Geijer, 1931; Witschard, 1984; Bergman et al., 2001). Address: R.P.-9, Sector 71, NOIDA, U.P. The optical properties of the plagioclases are directly related to their anorthite content. 38 It is an important ingredient of all acidic (granite, granodiorite) to neutral (syenite) igneous rocks (Table 5.1). 3I) in olivine were thought to be exsolved from olivine of ultradeep origin (Dobrzhinetskaya et al. Monoclinic or triclinic. 3.21). The color is usually white and sometimes changes from pale pink to reddish due to admixtures of iron (especially microcline). It often occurs in the pegmatite stage crystallization of magma and is the essential ingredient of pegmatite. The plutons are generally undeformed and the silica content shows three major peaks at 38–52, 57–66 and 70–76 wt.%, respectively (Martinsson, 2004a). Coesite/quartz lamellae in garnet, titanite, and chromite are thought to indicate ultrahigh-pressure conditions of greater than 6 GPa (Ye et al., 2000; Ogasawara et al., 2002; Zhang et al., 2005). (1949), Norton observed that most pegmatites culminate in pure quartz cores. Many pyroxenes from igneous or mantle rocks show exsolution textures of thin clinopyroxene lamellae in the host orthopyroxene, and vice versa, reflecting the originally high temperature of ~ 1000 °C or above (Robinson, 1977; Song et al., 2009b). In sedimentary rocks, plagioclase feldspars may occur via a variety of sources, including detrital minerals, authigenic minerals, and the products of volcaniclastic activity.

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