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Phase Diagrams
Published in Zainul Huda, Metallurgy for Physicists and Engineers, 2020
Plagioclase is one of the most common minerals in the earth’s crust. The phase diagram for the albite-anorthite ceramic system is shown in Figure E-6.4(a). A 50%Ab-50%An material is cooled from 1,500oC to room temperature. (a) Which type of phase diagram is shown in Figure E-10.5(a)? (b) What is the melting temperature of albite? (c) What is the melting temperature of anorthite? (d) At what temperature of the liquid does the first solid form? (e) At what temperature of the two-phase mixture does the last liquid solidify? (f) What is the composition of the first solid to form? (g) And what is the composition of the last liquid to solidify?
Volcanoes and Their Products
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Although sometimes absent, when present, phenocrysts help distinguish different kinds of volcanic rocks. Figure 7.47 shows the common phenocrysts in rocks ranging from felsic to ultramafic. These phenocryst minerals are the same minerals that may be present as microscopic crystals in the groundmass. From felsic to ultramafic, the volcanic rocks are rhyolite, dacite, andesite, basalt, and komatiite. Quartz and K-feldspar are generally restricted to relatively felsic rocks, olivine and pyroxene to relatively mafic rocks, and the other minerals to intermediate rocks. Plagioclase is a solid-solution mineral in volcanic rocks of many sorts and varies from being Na-rich for felsic rocks to being Ca-rich for mafic and ultramafic rocks. Figure 7.47 is only an approximation, because the minerals present as phenocrysts depend on many different things.
Recognising the different types of building stone
Published in John A. Hudson†, John W. Cosgrove, Understanding Building Stones and Stone Buildings, 2019
John A. Hudson†, John W. Cosgrove
As can be seen from Figure 3.6, the textures and colours of granites are varied. Their colour is generally controlled by the colour of the feldspar, the main mineral constituent; feldspars are aluminium silicates and their colour is determined by the alkali metals they contain. Orthoclase is a pink feldspar and contains potassium; plagioclase is a white to grey feldspar and contains sodium and calcium. The Peterhead granite in Figure 3.6 is dominated by orthoclase which gives it its characteristic pink colour, and the example illustrated contains a small black inclusion of the rock into which the granite was intruded. Depending on their composition and the time they spent in the magma before it cooled and solidified, these xenoliths (literally foreign rocks) may become completely assimilated by the magma or may remain intact as in the example illustrated here.
Production of Pottery from Esfandaghe and Jiroft, Iran, late 7th - early 3rd Millennium BC
Published in Materials and Manufacturing Processes, 2020
The matrix of Konar-Sandal pottery consists of mainly andesitic rock fragments. They present single as well as polycrystalline quartz, oligoclase, anorthite and muscovite as the main crystalline phase constituents in this group. Quartz grains are scattered in all parts of the matrix, and the grains are angular, because they were crushed during forming and processing. Plagioclases, mainly anorthite and oligoclase are easily detectable because of their clear structure of poly-synthetic twinning structure and extinction effect of the crystals (Fig. 2a). [14] Plagioclases are characterized as isomorphic series from Na-rich ((Na, Ca)AlSi3O8) to Ca-rich Anorthite (CaAl2Si2O6) (Fig. 2b). They also occur as crushed minerals from the rock fragments. Plagioclases are mostly weathered and are interpreted as the sericitization reaction, i.e. as the last stage of decomposition of plagioclase to illitic clay.[10,18] Muscovite shows the typical cleavage on the surface, and the yellow color due to the firing temperature above 900°C (Fig. 2c). [7,14,18] The observations show that the main aplastic additives in the matrix are rock fragments and grogs or crushed pottery. The grogs are mainly angular in shape and brown in color (Fig. 2d). The uses of grogs are quite exotic in the study of such materials, because they reveal the great know-how of the potters on the properties of crushed pottery.[16] At this temperature, Fe+2 is oxidized and lost from the crystal structure, therefore the color of muscovite changes from orange to yellow. In Fig. 2c and d, the matrix of the samples looks reddish in color, and this comes from the oxidation reaction through firing.
Basanite cobbles in Pleistocene sediments in Central Otago and their implications for intraplate volcanism and Clutha River paleo-drainage
Published in New Zealand Journal of Geology and Geophysics, 2023
James M. Scott, Alan F. Cooper, Dave Craw, Petrus J. le Roux, Hayden B. Dalton, Marshall C. Palmer
Pilotaxitic flow-aligned textures of plagioclase occur in some specimens (Figure 5b). Plagioclase compositions are typically labradorite (as calcic as An56Ab42Or3) but are zoned to rims of andesine (An43Ab53Or4) (Supplementary Table 1). Interstitial material is invariably present and has an intergranular to intersertal texture. In most specimens, the interstices consist of a pale brown, apparently isotropic glass phase containing feathery, in places dendritic (quench?), microlites, some of which are acicular needles of an opaque phase (ilmenite?) (Figure 5c). These dendrites are cut by hollow needles of apatite. In some specimens, the interstitial material is more yellow-brown and birefringent than the intersertal glasses and may have resulted from devitrification/recrystallisation. Compositions of this pale brown interstitial phase are highly variable, with low analytical totals probably reflecting variable degrees of hydration and devitrification. Some specimens contain coarser leucocratic patches, with crystals of isotropic analcite on the margins and finer-grained feldspathic or feldspathoidal crystal aggregates in the interior. The analcite may result from devitrification and recrystallisation of glass. The least hydrated glass compositions are typically rich in Na2O (e.g. 9.71%), Al2O3 (e.g. 23.37%), with K2O typically < 1%, SiO2 ∼54% and FeO < 1% (Supplementary Table 1) and contain Cl, although this element was not analysed quantitatively. Such compositions are similar to glasses analysed in Alpine Dike Swarm lamprophyres, which Cooper (1979) likened to compositions of phonolites that accompany lamprophyres in the evolved part of the Alpine Dike Swarm. Wilkinson (1966) determined that trachytic and phonolitic intersertal glasses in basalts are dependent on the degree of undersaturation of the host magma. A very similar conclusion was also reached by Tsypukova et al. (2014) who scanned vitreous groundmass areas, 800 mm square, by a focused beam. For basanites of similar composition to the Clutha samples, this method determined phonotephrite residual magma compositions. One specimen each from the Upper Clutha and Galloway suites contain nepheline (Ne75.7-79.5Ks12.26-12.25Qtz12.05-8.26), and the same Galloway sample also showed the presence of interstitial pools of sodalite (Supplementary Table 1). The presence of sodalite, nepheline and Cl-bearing glasses in the Upper Clutha-Galloway basanites indicates a very similar fractionation sequence to that observed in the Alpine Dike Swarm.