Exsolution occurs only in minerals whose compositions vary between two or more pure endmember compositions. At lower temperature the solid solution becomes progressively more limited, and as a result lamellae of albiterich feldspar begin to grow in the orthoclaserich alkali feldspar. In other plutonic igneous rocks, there may be two alkali feldspars. This paper shows afm results on twin domains and exsolution lamellae in a well characterized feldspar crystal. The miscibility gap is narrower at high temperatures, so that under slow cooling conditions, early formed augites and lowca pyroxenes develop exsolution lamellae as lowca pyroxene and augite respectively are expelled from the structures. Coarsegrained alkali feldspar bits a perthitic exhi texture, which was not observed among alkli feldspar a in the finegrained clasts, and tends to have more cracks and small pores. Microtextures related to sial ordering patterns in the framework, such as albite twins, are not eliminated, forming ghostlike lamellar strain patterns in. Perthites and moonstones cryptoperthites are a mi croscopic or submicroscopic association of units of albite, naa1si308 and kfeldspar, ka1si308 cf. Tem and sfm of exsolution and twinning in an alkali feldspar h uifang x u, 1, d avid r. Carstens 0967a, for example, discusses the textures displayed by antiperthites from south. Routine characterization and interpretation of complex.
In backscattered electron bse images, euhedral dolomite crystals were composed of several compositionally distinct domains. R amakrishna 3 1 department of earth and planetary sciences, the university of new mexico, albuquerque, new mexico 8711116, u. Exsolution and alteration microtextures in alkali feldspar. Revision 1 stem investigation of exsolution lamellae and creflections in carich dolomite from the platteville formation, west wisconsin zhizhang shen, hiromi konishi, philip e. Myrmekite formed by na and cametasomatism of k feldspar. The rates of homogenization of coherent exsolution lamellae in cryptoperthites have been used to evaluate kna interdiffusion in alkali feldspars. Thus the interference colors may range up to 1 o white.
Clinopyroxenes in apollo 15 quartznormative basalts qnbs are chemically complex recorders of thermal history. An experimental method for the determination of abor exsolution and alsi ordering in alkali feldspar is described. Moonstone is an alkali feldspar that displays beautiful iridescence produced by light interacting with submicroscopic exsolution lamellae that act as diffraction grates. Feldspar in felsic orthogneiss as indicator for uht. Stress and strain in cryptoperthite lamellae and the goherent. Pdf alkali feldspars usually exhibit optical or suboptical. While alkali feldspar in ocs is predominantly albitic, kfeldspar exsolution has been reported in a number of meteorites. Routine characterization and interpretation of complex alkali feldspar intergrowths.
Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. Schematic diagram showing coherent exsolution microtextures, as they appear. Cations which have a large difference in radii are not likely to readily substitute. In most cases, it occurs upon cooling below the temperature of mutual solubility or stability of the solution. The immiscibility gaps in the plagioclase solid solution are complex compared to the gap in the alkali feldspars. The most common kfeldspar rods are oblique to the c axis by about 5 and obviously nucleated at preexisting albitetwin boundaries. Quick navtop about alkali feldspar age distribution other languages common associates other information alkali feldspar in petrology references internet links significant localities locality list mineral andor locality. A perthite with approximately equal amounts of alkali feldspar and plagioclase.
A variety of intergrowth and exsolution textures can. The magnitude of the effect is great enough to explain unexpectedly large exsolution lamellae in some rhyolites, and also calls into question the effects of. The answer lies in the overall energetics of the system. Exsolution lamellae in pyroxene exsolution is a process by which a solid solution phase unmixes into two separate phases in the solid state. Potassiumrich alkali feldspar kfeldspar makes up 20 40% by vol of granites and is a. Striations are used to distinguish plagioclase feldspar which has striations from potassium feldspar which does not. Ronrlrl department ol earth and planetary sciences, m assachus etts i nstitute ol t echnology, cambridge, massachusetts 02 i 3 9 abstract transmission electron microscope observations of a cryptoperthitic alkali feldspar from. Lamellar and patchy intergrowths in feldspars mineralogical.
While alkali feldspar in ocs is predominantly albitic, k feldspar exsolution has been reported in a number of meteorites. Since this is the same interference color we expect for quartz, care must be taken to avoid. The alkali feldspar phase diagram the disordered solid solution can only exist at high temperatures. The lamellar spacing al is largely dependent on the rate of cooling during the first 70c after the sample has crossed the coherent spinodal. They are used in the manufacture of glass and ceramics. The kfeldspar is brilliantly luminescent, and the regular albite lamellae show. Perthite is used to describe an intergrowth of two feldspars. Threedimensional cooling pattern of a granitic pluton i 63croscopy sem have shown that alkali feldspar pheno crysts in the shap granite of cumbria, northwest england, have undergone both an exsolution reaction to form co. Examination of kfeldspar crystals in the sharpners pond tonalite, massachusetts, usa lorence g. This exsolution resulted in the formation of secondary albitetwin lamellae in the host plagioclase adjacent to the kfeldspar. The nature of potassium feldspar, exsolution microtextures. The spectra were analyzed using reference spectra of uniform samples with various abor. Exsolution is complicated by the large number of different polymorphs of alkali feldspar. Individual alkali feldspar phenocrysts are 23 cm in size, and often rimmed by plagioclase.
Surface characters of twin domains and exsolution lamellae in. Short survey of theories and some additional remarks on exsolution in feld spar. Threedimensional cooling pattern of a granitic pluton i. The orientation relations in exsolution lamellae of perthite and moonstone have been selected as an ap plication of the theory of phase boundaries to minerals. Exsolution, in mineralogy, process through which an initially homogeneous solid solution separates into at least two different crystalline minerals without the addition or removal of any materials. Microtextures related to sial ordering patterns in the framework, such as albite twins, are not eliminated, forming ghostlike lamellar strain patterns in chemically homogeneous feldspar. The sodiumrich feldspar albite naalsi 3 o 8 and the potassiumrich feldspar orthoclase kalsi 3 o 8, for example, may exist in a homogeneous solid solution above 650 c 1,200 f, but below that temperature exsolution will occur. Typically the host grain is orthoclase or microcline, and the lamellae are albite. Powder infrared spectroscopy was used to measure spectra. Striations appear when the minerals cleavage surface reflects the. With the exception of rare, very rapidly cooled volcanic alkali feldspars and some near endmember crystals that grew at diagenetic temperatures, almost all natural alkali feldspar crystals exhibit various intracrystal exsolution and twin microtextures that developed during their geological history.
Phase transitions and exsolution phenomena in pyroxenes. The regular perthites are straincontrolled intergrowths of albite andor pericline twinned albite exsolution lamellae within tweed orthoclase. Pdf routine characterization and interpretation of. Bsebright unreplaced residual calcite, gray febearing dolomite grains and the dark host dolomite crystal. This exsolution process results in a 2phase intergrowth, called perthite m m t t composition 0 2040 6080 100 nafeldspar kfeldspar 200 400 600 800. Physical aspects of exsolution in natural alkali feldspars.
The equilibrium precipitate formed by exsolution in alkali feldspar containing calcium in solid solution is a twophase intergrowth of plagioclase and quartz. Striations are parallel lines on a minerals cleavage surfaces that look like tiny hairline grooves like on an lp or cd. When albite and pericline twins are visible they are. However, in the case of exsolution and perthite development during cooling of a hightemperature, homogeneous alkali feldspar anorthite, just the opposite effect happenssegregated domains of albite and microcline become more stable at lower temperatures. Exsolution in feldspars of the granodiorite takes the form of irregular and often diffuse patches, whereas in feldspars of the qttartz monzonite, exsolution is more obvious, taking the. The orientation of exsolved lamellae in alkali feldspar expected from the spinodal model corresponds to the most common orientation observed in perthites. Texturally distinctive augite rims wo3sena2 mantle the clinopyroxene phenocrysts and contain submicroscopic exsolution lamellae of pigeonite and augite near the 001 and 100 orientations. Exsolution during metamorphism with particular reference to. Felspar wikipedia bahasa indonesia, ensiklopedia bebas.
This is mainly caused by difference in cation size. These arise mainly due to the ordering of al atoms within the structure. Effect of fluorine on solidstate alkali interdiffusion. Semicoherent straight film albite lamellae in orthoclase. To assess the compositional variation within single crystals, such as lamellaerich core and lamellaefree rim, preexsolution onephase original compositions have been calculated separately for feldspar wholegrain w and for core c where exsolution lamellae is concen. Therefore, an albite rim was formed on the border of the myrmekite, and albite lamellae were produced in the alkalifeldspar.
An average interdiffusion coefficient 1d is between2 and 4 x l017 cm2sec for a synthetic cryptoperthite at 600c in the composition range or23or53, and between 6 and 12 x 1016 cm2lsec for a. Alkali feldspars of intermediate composition that are cooled slowly from high temperatures may unmix or exsolve to form perthite, a. Similar effect also occurs in plagioclase feldspars. The play of colours visible in some feldspar of labradorite composition is due to very finegrained exsolution lamellae. Exsolution and alteration microtextures in alkali feldspar phenocrysts from the shap granite article pdf available january 1995 with 103 reads how we measure reads. Crystallographically oriented pyroxene and amphibole exsolution lamellae in garnet document decomposed supersilicic uhp majoritic garnet originally stable at diamondgrade conditions, but majoritic precursors have only been.
All of the alkali feldspars have low relief and low birefringence. The sodiumrich feldspar albite naalsi 3 o 8 and the potassiumrich feldspar. These relations together with experimental coarsening rates can be used to interpret the thermal histories of rapidly cooled cryptoperthites which are characterized by a coherent, lamellar microstructure. For a quantitative measurement, the relative absorbance a 114 of the ko band at 114 cm21 and the absorbance a 184 of the nao band at 184 cm21 for the constructed secondary reference spectra were measured. Feldspar ternary showing compositions from fine and coarsegrained clasts. In extended lamellae the periodicity of albite twins is typically 20. The coherent solvus and coherent spinodal for sanidinehigh albite are reasonably well known from experimental studies. It is a megacryst 0 cm x 10 cm x 10 cm that displays a greenish yellow interference color. We have also shown k feldspar exsolution in type 4 h, l. The size and morphologic variations of exsolution textures were characterized using the. Batuan yang hampir seluruhnya terbentuk dari felspar plagioklas kalsium dikenal sebagai anortosit. Diamond and coesite are classic indicators of ultrahighpressure uhp. Therefore, an albite rim was formed on the border of the myrmekite, and albite lamellae were produced in the alkali feldspar.
Exsolution lamellae article about exsolution lamellae by. Visible perthitic exsolution lamellae of albite in microcline. Exsolution and alteration microtextures feldspar phenocrysts from the shap granite in alkali martin r. Pdf alkali feldspar microtextures as provenance indicators in. Routine characterization and interpretation of complex alkali. There are four varieties of kfeldspar three of which we must be able to identify in this. Pdf routine characterization and interpretation of complex. Pdf the microstructures of perthitic alkali feldspars revealed by. This exsolution process results in a 2phase intergrowth, called perthite m m t t composition 0 2040 6080 100 na feldspar k feldspar 200 400 600 800. Tem and sfm of exsolution and twinning in an alkali feldspar. Anisotropy of magnetic susceptibility in alkali feldspar. Alkali feldspar, any of several common silicate minerals that often occur as variously coloured, glassy crystals. Feldspar and oxide thermometry of granulites in the. Revision 1 stem investigation of exsolution lamellae and c.
Exsolution lamellae are a feature of pyroxenes in slowly cooled plutonic and hyperbyssal igneous rocks. Perthite consists of lamellae of albite in an orthoclase or microcline host, while antiperthite is lamellae of kspar in an albite host see fig 1515, top. Dislocations have been reported previously in only two crystals, and periodic dislocations noted in only one, an orrich microperthite. In addition, in the plutonic rock types if the cooling takes place slowly enough, then perthitic exsolution lamellae may also form. Often where crystals of alkali feldspar and plagioclase feldspar are found in the same igneous rock, highk. Below the solvus the solid solution breaks down to 2 phases one narich, the other krich. Modeling exsolution and perthite formation as an example of. Revision 1 stem investigation of exsolution lamellae and. The intergrowths or exsolution lamellae of the albite are visible as white lines coursing through the k feldspar groundmass. Hightemperature alkali feldspars can, however, can be preserved metastably during rapid cooling without unmixing. The barium feldspars are monoclinic and comprise the following. Consequently, intermediate alkali feldspars often consist of an oriented intergrowth of kfeldspar and typically wormy or lamellar albiterich feldspar that forms by exsolution upon slow cooling, and is referred to as perthite.
The alkali feldspars albite naalsi 3 o 8 orthoclase kalsi 3 o 8 form a complete solid solution at high temperature. Exsolution during metamorphism with particular reference to feldspar solid solutions brian a. Exsolution in feldspars of the granodiorite takes the form of irregular and often diffuse patches, whereas in feldspars of the qttartz monzonite, exsolution is more obvious, taking the form of lamellae and relatively large patches fig. When a solid solution becomes unstabledue to a lower temperature, for exampleexsolution occurs and the two phases separate into distinct microscopic to megascopic lamellae. Powder infrared spectroscopy was used to measure spectra in the spectral range between 50 and 1500 cm21 using about 5 mg of sample material. Shape of the miscibility gap can get more na in kspar than k in albite. Stress and strain in cryptoperthite lamellae and the goherent solvus of alkali feldspars prnnneyvns f. Below the coherent solvus, exsolution lamellae thin on heating, leaving nanotunnels stranded in the orthoclase matrix. Twins on the albite law are parallel to 010 in all feldspars fig 2.
Quartz, mica, and amphibole exsolution from majoritic. Sample and expemment the feldspar specimen used for this study is from the johns hopkins university collection and comes from an unknown locality. Surface characters of twin domains and exsolution lamellae. Feldspar thermometry in ultrahightemperature metamorphic. When quartz is involved the relationships are more. In order to apply the feldspar thermometer to rocks from the adirondacks the exsolved albite must be reintegratedthe composition obtained is assumed to have equilibrated with its coexisting plagioclase at or near the peak of metamorphism. An intergrowth of albite or oligoclase with a microcline host, occasionally also with a orthoclase host. While the same charge, the size of k and na differ by 30 %. Microperthitic textures in crystals are visible using a light microscope, whereas. Exsolution during metamorphism with particular reference. Alkali feldspar is the third most abundant mineral in earths crust. Where no primary twin boundaries existed, exsolution lamellae of kfeldspar developed. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye. Pdf etching of alkali feldspar cleavage fragments with hydrofluoric acid.
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