6) the solvus curve shows the compositions of structurally separate Ab- and Or-rich phases. (D) Quartz (Qtz) lamellae in omphacite from a UHP eclogite (East Kunlun, China). 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. 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. Perthite: Lab 101, 101A Perthitic alkali feldspars were initially a single alkali feldspar at high temperature (see lecture notes for relevant Ab-Or phase diagram). 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. The Na-rich and K-rich parts of perthitic intergrowths should be called the Ab- and Or-rich phases respectively. Marc's legend: a. Melting point of orthoclase. Many alkali feldspar crystals from plutonic igneous rocks (e.g. Microcline (KAlSi3O8) is the triclinic low-temperature K-feldspar stable at temperatures lower than 500°C. The exsolution lamellae occur in parallel groups, each being strictly oriented in one of the four directions corresponding to the isometric form {111}, i.e., octahedral planes of the host garnet (Fig. Like Cameron et al. Creek district; in these, the "perthite" of the diagrams is actually perthite plus muscovite. REFRENCES Fasset, J. E. and Durrani, N. A. Field relations and petrographic observations show that the formation and/or post-peak evolution of the Bashiwake granulite involved some degree of melting (Zhang et al., 2005c, 2014b), but the extent of melting and its relation to the granulite P–T path are still unclear. Perthite and antiperhite. They are generally colourless, white or grey, but may be yellow, red, or green; colourless in thin section. 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. Depending on the original composition we can end up with K- … Jianxin Zhang, ... C.G. This rock type is likely developed within the metamorphic halo the SIC, and it likely reflects recrystallization and possibly partial melting. Draw the arrows on the cotectics or reaction lines. Perthite unmixing in microcline. Figure 5. Fig. Sanidine, as a high-temperature alkali feldspar, is not stable in rocks on the surface or at shallow depth under the surface of the Earth, and gradually recrystallize in orthoclase over time. Orthoclase (KAlSi3O8) is the monoclinic medium temperature K-feldspar. (1949), Norton observed that most pegmatites culminate in pure quartz cores. Crystallization of melt under these conditions generates a single homogeneous feldspar phase of intermediate composition, which exsolves to form perthite or antiperthite as it cools through the solvus into the 2-feldspar field, characteristic of hypersolvus granites. Names can be used in conjunction, as in cryptomesoperthite. The alkali feldspar phase diagram. Triclinic. can be illustrated on a phase diagram exhibiting a miscibility gap (see the diagram below). Equilibrium diagram of the plagioclase feldspars: (1) in anhydrous conditions, (2) at 5 kbar water pressure, (3) eutectic liquidus and solvus in the albite–anorthite–quartz–H2O system at 2 kbar. For a sample (2015). The plutons intrude volcanic and sedimentary rocks of the Elsie Mountain and Stobie Formations and enclose roof pendants of both Formations, as well as rafts of basement gneiss and fragments of plagioclase glomeroporphyritic Matachewan diabase (Fig. The optical properties of the plagioclases are directly related to their anorthite content. Phase diagram of the alkali feldspar system which exhibits such exsolution behavior at low temperatures. The alkali feldspars include monoclinic feldspars (orthoclase and sanidine) and triclinic feldspars (microcline and anorthoclase). 38 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). Figure 4. Figure 2.3. 450 Ma which is interpreted as timing of the post-peak reheating under MP/MT granulite-facies conditions, in combination with Zr-in-rutile thermometry (Zhang et al., 2014b). (1949) consist of lenticular or sheet-like pegmatite bodies of generally steep dip. Mattinson, in Gondwana Research, 2017. An exsolution origin for the antiperthites in granulite facies rocks has been considered by Sen (1959). 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.

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