China
Africa
Explore chapters and articles related to this topic
Metamorphic Rocks
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
The kind of rock that forms under blueschist or eclogite facies conditions depends on the protolith. Pelitic rocks metamorphosed at high pressure contain kyanite and garnet, often with chloritoid, zoisite, an Na-rich amphibole, and phengite (a mica similar to muscovite but containing magnesium). Granites may contain the same minerals that pelites contain but in different proportions. Marbles may contain aragonite, a high-pressure polymorph of calcite, although aragonite tends to turn into calcite when high-pressure rocks are brought to the surface. The rock type called blueschist forms when basalt and other mafic compositions reach blueschist facies conditions. Blueschists contain glaucophane, a distinctive inky blue amphibole (but despite their name do not display schistosity). Figure 10.30a shows a blueschist, partially changed to a green eclogite, from an outcrop on the beach just north of Jenner, California, that contains blue glaucophane, green pyroxene, and red garnet. Glaucophane, an Na-rich amphibole, forms in blueschists because at high pressure, the Na-component of plagioclase reacts to make amphibole. Similarly, at high pressure, the Ca-component of plagioclase is unstable and reacts to form a pyroxene. So, high-pressure rocks commonly contain glaucophane and pyroxenes that are rich in Na and Al and other nonstandard components. The pyroxenes, such as the green ones seen in Figure 10.30a, are called omphacite. Many blueschists also contain lawsonite, a high-pressure mineral with composition equivalent to hydrated anorthite, and epidote.
Production of Pottery from Esfandaghe and Jiroft, Iran, late 7th - early 3rd Millennium BC
Published in Materials and Manufacturing Processes, 2020
The two groups of material have mostly similar crystalline phase constituents. The most important differences are determined by muscovite (K, Fe bearing), calcite (Ca bearing) and oligoclase (Na, K bearing). Crystalline phases within the matrix seem to have two origins, one from the raw materials before firing and from high-temperature phases after firing.[8,9] Quartz, oligoclase and anorthite (± calcite) come from the metamorphism setting of the region near Esfandaghe.[6] Augite, fassaite and omphacite are high-temperature phases formed by heat. Augite and omphacite appear in pottery making over 900°C and fassaite are stable before 1250°C depending on ƒO2.[11] With decreasing of ƒO2 the stability field of fassaite will decrease as well; therefore, the ƒO2 could not be high since, in that case, the amount of fassaite would not be so high within the matrix. Omphacite is an intermediate mineral between calcium-rich augite and sodium-rich jadeite (isomorphic series), formed after the reaction Albite ↔ Jadeite + SiO2. The negative correlation between the quantitative amount of omphacite and oligoclase in the samples is visible. On the base of the low amount of pyroxenes and their pattern, scattered under the background, we can say that the temperature range of manufacturing varied between 850°C and 920°C.[9] Hematites are not particularly high and this indicates that the ƒO2 and firing temperature were not high enough to form magnetite.[12] Among inclusions of the pottery-forming minerals, micaceous minerals, such as illite, can be noted. Clay minerals such as illite were commonly utilized in antiquity, but they cannot be detected by X-ray diffraction, because of their very low crystallinity or the overlapping of X-ray pattern of other minerals such as muscovite, calcite, dolomite and plagioclases.[13] Clay minerals like illite can be observed firstly as unaltered clay residual that can be expected in low-fired pottery.[12] Secondly as clay residuals that can particularly occur as secondary illite, after the alteration process of plagioclase (via sericitization reaction).[10,18]