A schlieren could be, for example, a tabular zone in a granite with either more or less of some of the minerals in the surrounding granite, typically the dark (mafic) minerals. The origins of schlieren are not always clear; they may be produced by differential magma flow, or disaggregation of xenoliths, or by other mechanisms.
A review of mesostructural and microstructural evidence favours the hypothesis that Kfeldspar augen and megacrysts in felsic gneisses and mylonites are generally, if not always, residual phenocrysts, rather than porphyroblasts that grew either during or after the deformation.
Another aspect of the crystalloblastic series is that minerals high on the list tend to form porphyroblasts (the metamorphic equivalent of phenocrysts), although Kfeldspar (a mineral that occurs lower in the list) may also form porphyroblasts. Porphyroblasts are often riddled with inclusions of other minerals that were enveloped during growth of the porphyroblast.
Jan 12, 2014· Basic Igneous Rock Textures Phaneritic this term applies to igneous rock in which the constituent minerals are megascopic in size. 4. Aphanitic – this term is used to describe te texture of an igneous rock composed entirely of microscopic crystals.
granites with an unusually large proportion of sodalime feldspar), of various grey shades, sometimes tinged with blue, pink or buff, and always marked with black mica; the finer varieties take a high polish and are used for monuments, and the coarser grades are used for construction, especially of railway bridges, and for paving and curbing.
A model of all three phenocryst phases (quartz, plagioclase, Kfeldspar) with 217 crystals, and a larger model containing 1599 Kfeldspar crystals was reconstructed in three dimensions. The first model revealed a nontouching framework of crystals in three dimensions, suggesting that individual crystals grew freely in the melt prior to quenching of the texture.
the liquids either the Becke or inclined illumination method can be used. The determination of feldspars from rock powders in this manner is especially valuable in cases where the feldspars are partly altered, where the rock is fine grained, when the feldspar content .
The distribution of elements with distinctly different diffusion rates and partition coefficients (, Ti versus Sr and K) within the plagioclase phenocrysts will be used as a means of determining the extent to which diffusive reequilibration has affected the composition of the observed phenocryst/inclusion.
"Kfeldspar," as used herein, refers to all structural variants (microcline, orthoclase, and sanidine), and "megacrystic granites" is used for simplicity to refer to both granites and granodiorites. "Megacrysts" is understood to refer to Kfeldspar megacrysts.
Plagioclase feldsparsEdit. The play of colours visible in some feldspar of labradorite composition is due to very finegrained exsolution lamellae . The specific gravity in the plagioclase series increases from albite ( g/cm 3) to anorthite (– g/cm 3 ).
The central part is calciumrich and the outside part is sodiumrich: [Sandra Johnstone, used with permission] Finally, if the magma is quite silicarich to begin with, there will still be some left at around 750° to 800°C, and from this last magma, potassium feldspar, quartz, .
4. Feldspars are normally lightcolored, including pink, white, tan, gray or green. The colour of feldspar varies because of the presence of impurities. On the other hand, quartz looks colourless or appears clear. 5. Quartz is used as a gemstone and has electronic applications. Feldspar is widely used in the making of dinnerware and tiles.
Texture description of the degree of crystallinity, grain size and shape, and arrangement of the minerals • Phaneritic crystals visible to the naked eye "you can see the bits" – Coarsegrained 3 cm to 5 mm – Mediumgrained 15 mm – Finegrained < 1 mm • Aphanitic crystals so small that they cannot be seen with the naked eye
INrnouucrroN crysts in the granite itself did not grow from a melt. Such Conflicting theories for the origin of Kfeldspar mega arguments are commonly inconclusive in that other in crysts in granitic rocks are principally (1) the megacrysts terpretations may be equally plausible.
Mineral properties Feldspar is the most common rockforming mineral. Feldspar is a name for a group of minerals with a general chemical formula of x Al(Al,Si) 3O. 8, where x can be one or more elements. Feldspar minerals are usually white or very light in color and have a .
The composition of the Plagioclase rims and of nonreplaced phenocrysts is An7981; the composition of relics is An83. The An and Ab contents of the Kfeldspar is higher than in Kfeldspar from altered basalt in Hole 418A in the Atlantic Ocean near the Bermuda Rise.
feldspar, quartz, and micas (Klein and Hurlbut 1993:512). Most of the nonclay components are identifiable in thin section, but clay minerals are so small (< 2 m in spherical diameter) that they can only be recognized on the basis of their crystalline structures. Four groups of clay
Dec 16, 2013· Geocache Description: Microcline is a form of feldspar and microcline phenocrysts can be identified by its reddish, rusty appearance. Notice the phenocrysts are made up of the top two most abundant minerals in the world feldspar (microcline) and quartz (blue quartz). The phenocrysts are enclosed in a matrix of other minerals (see Mineralogy of Llanite).
absence of Kfeldspar phenocrysts or megacrysts, are firstorder observations and a standard means typically used to distinguish between distinct geologic events such as intrusive pulses. However, if latestage textural coarsening Communicated by J. Blundy. B. R. Johnson () A. F. Glazner Department of Geological Sciences, University of North
Sandstone is a sedimentary rock mainly composed of small sand size grains of quartz and/or feldspar. It is often used for construction and also to make items used in the house.
calcic cores of many of the plagioclase phenocrysts are replaced to varying degrees by Kfeldspar with the optical properties of sanidine. This feldspar contains as much as 99 per cent orthoclase by composition, with only trace amounts of Na 2 O and CaO (Stewart et al., 1973). This replacement is probably due to Krich alka
3. phenocrysts A. 35, 36, 40, 41, 43, 47 B. 35, NaPlagioclase, hornblende 36, Kfeldspar (white orthoclase) 40 qtz, hornblende 41 Kfeldspar (Orthoclase) 43 Kfeldspar (Orthoclase); this sample is from near Mt. Rogers in SW VA and has likely been metamorphosed. It is quite dark for its type.