Mineral Chemistry of Chlorite Replacing Biotite from Granitic Rocks of the Canadian Appalachians
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Abstract:
Chlorite flakes, as a product of alteration of biotite, the dominant ferromagnesian mineral in the Paleozoic granitic rocks of the Canadian Appalachians, have been analyzed by electron microprobe for major elements and by 57Fe M?ssbauer spectroscopy for the coordination and oxidation state of Fe. Comparison of M?ssbauer Fe3+/Fe ratios obtained from chlorite and its host biotite indicates that chloritization might have occurred under relatively oxidizing conditions. Based on 54 analyzed samples, Si cation totals of these sheet silicates are less than 6.25 atoms per formula unit (apfu), and the sum of octahedral cations is very close to 12 both an indication of trioctahedral chlorite. The calculated mole fraction of chlorite in interlayered phase, Xc, ranges from. 0.72 to 0.98 confirming that the chlorites are completely free of any smectite layers. Compositional variations in chlorite are strongly controlled by host biotite and rock type. Fe/(Fe+Mg) ratio ranges from 0.35 to 0.93 and Si contents from 5.18 to 6.11 apfu lead to the classification of chlorites mainly as ripidolite and brunsvigite. All major elements in the chlorite are strongly correlated with each other. Fe/(Fe+Mg) ratio in biotite is well preserved by chlorite. Chlorite thermometry based on the variation in tetrahedral Al content within the chlorite structure shows a large variation in temperatures from 200 to 390 °C with an average of 340 °C. The chlorite from igneous rocks could also be used to detect reheating events and reveal the thermal history of the rocks.Cite
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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1981)
In the northeastern part of the Kanto Mountains, central Japan, in basic schists, green biotite widely occurs in a lower-grade part where garnet is common in pelitic and psammitic schists, while brown biotite is present in a higher-grade part where brown biotite commonly occurs in them. Comparing with brown biotite from higher-grade part of the Sanbagawa metamorphic terrain in this area and central Shioku, green biotite in lower-grade part of this area contains lower Al2O3 and higher Fe2O3, and slightly lower TiO2. On the basis of the paragenetic relation of the green biotite-bearing assemblages, the occurrence of green biotite is explained by high Fe2O3, (FeO+MgO), and low Al2O3 contents of the host rocks. Brown biotite in basic schists in a higher-grade part is formed by the following reaction; green biotite+ (Fe, Mg-rich) chlorite+epidote+albite+quartz=brown bitite+hornblende+(Al-rich) chlorite+magnetite+H2O. This reaction enlarges the stability field of biotite, and in a higher-grade part, brown biotite-bearing assemblages widely occurs in the common basic schists.
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Abstract The relationships between the composition and the crystallization temperature of chlorites and illites have been investigated in different geothermal fields and in particular the Los Azufres system in Mexico, considered to be a natural analogue to experimental laboratories, as the main changes in physical and chemical conditions and mineralogy are related to progressively increasing temperature with depth. Temperature was estimated from combined geothermometric approaches, and especially from fluid inclusion studies on quartz coexisting with clays. The Al (IV) content in the tetrahedral site of chlorites, and the K content and total interlayer occupancy of illites increase with temperature. These chemical changes are mainly related to the marked decrease in the molar fraction of the Si (IV) -rich end-members (kaolinite for chlorites, and pyrophyllite for illites) which become negligible at ∼300°C. Other chemical changes, such as the variation in Fe and Mg contents, are partly influenced by temperature, but are strongly dependent on the geological environment, and consequently on the solution composition. The empirical relationships between chemical variables and temperature were calibrated from 150–300°C, but extrapolations at lower and higher temperatures seem possible for chlorites. Such geothermometers provide tools for estimating the crystallization temperature of the clays, and are important for the study of diagenetic, hydrothermal and low-T metamorphic processes.
Pyrophyllite
Illite
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Biotite, the dominant ferromagnesian mineral in Paleozoic granitic rocks of the Canadian Appalachians, has been analyzed with an electron microprobe (wavelength dispersion) for major elements and by 57Fe Mossbauer spectroscopy. We sampled a wide variety of rock types, ranging from gabbro, diorite, syenite to granite, but by far mostly granitic ( sensu lato ). The most pronounced variations are in total Al contents and Fe/(Fe + Mg) values. In the biotite quadrilateral (annite –siderophyllite –phlogopite – eastonite), biotite from A-type granites of the Humber and Avalon zones in Gaspe (Quebec) and New Brunswick is characterized by low mean Al contents, ~1.15 atoms per formula unit ( apfu ), and variable Fe/(Fe + Mg) values in the range 0.4 to 0.9. In the granites of the Notre Dame arc of the Dunnage zone in Newfoundland, biotite has moderate mean values of Al (~1.40 apfu ) and Fe/(Fe + Mg) (~0.58). In granites of the Gander zone of New Brunswick and Newfoundland, biotite has a mean Fe/(Fe + Mg) value of 0.6 and shows a pronounced trend of increasing total Al (1.05 to 1.75 apfu ), confirming significant contributions of aluminous supracrustal material to the magmas, either by assimilation or anatexis. Finally, in granites of the Meguma zone, derived entirely from metasedimentary material, biotite exhibits a remarkable increase in total Al (1.30 to 2.00 apfu ) and considerable iron-enrichment [Fe/(Fe + Mg) in the range 0.4 to 1], with compositions nearing the siderophyllite end-member. The biotite from most zones plots on or above the NNO buffer, indicating moderately oxidizing conditions, whereas that from the Meguma zone plots mainly between the QFM and NNO buffers, implying fairly reducing conditions during crystallization. Assuming a reasonable range of crystallization temperatures of 750 to 900°C, oxygen fugacities ranged from 10−10 to 10−16.9 bars during crystallization. The composition of biotite reflects primarily the nature of the host magmas. It cannot readily be used for tectonomagmatic characterization of these rocks without the aid of other types of data.
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Titanite
Hornblende
Pyroxene
Amphibole
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Igneous and hydrothermal biotite in the Santa Rita stock, which hosts the Santa Rita porphyry copper deposit, has been analyzed by electron-microprobe techniques for K, Na, Ca, Ba, Mg, Mn, Ti, Al, Si, Cl, and F; biotite separates were analyzed for ferrous and ferric iron by wet-chemical analysis and atomic absorption. Igneous biotite occurs as phenocrysts and microphenocrysts. Hydrothermal biotite occurs as secondary disseminated flakes, as aggregates of flakes resulting from recrystallization of igneous biotite or replacement of igneous hornblende, and as vein fillings. Included in the analyses are biotite in the least-altered part of the Santa Rita stock and in potassic, argillic, phyllic, and propylitic alteration associated with the stock. Of the 12 elements analytically sought, Ba, Fe, Mg, Ti, Al, Si, Cl, and F show the greatest variations.Systematic variations in the composition of biotite are apparent in the Santa Rita stock and in each type of alteration except phyllic. Mole fraction phlogopite, F, and Si increase progressively from igneous to secondary to vein biotite; the reverse relationship holds for Ti, Cl, and Al. Mole fraction phlogopite correlates positively with F and Si and negatively with Ti, Cl, and Al. These correlations, with the exception of that involving Ti, are well developed in the phyllic alteration. In phyllic alteration, however, the systematic relationship between the different varieties of biotite and their composition is obliterated. In all types of alteration, igneous biotite is distinguished from hydrothermal biotite by its higher Ba content. The Fe (super +2) /Fe (super +3) ratio increases progressively from igneous to secondary to vein biotite in the least-altered and potassically altered parts of the stock. The composition of overgrowths and protruding parts of igneous phenocrysts is intermediate between that of phenocryst and secondary biotite. The Al content of both igneous and hydrothermal biotite associated with phyllic alteration is notably higher than in biotite associated with other types of alteration; also, in general, the higher Al content correlates with a lower phlogopite content.Interpretation of the compositions of the several varieties of biotite in the Santa Rita porphyry copper deposit with various experimental and theoretical investigations and thermodynamic relationships suggests that an aqueous fluid phase exsolved from a partially crystallized melt at a T [asymp] 745 degrees C and a P [asymp] 1 kb. Subsequent hydrofracturing of the solidified part of the stock resulted in a lower T and P environment of potassic alteration which is suggested to range from about 670 degrees to 580 degrees C and 220 to 20 bars.The variable but systematically related content of Fe, Mg, Ti, and Al in relic biotite in the phyllic and argillic alteration of the Santa Rita deposit have resulted mainly from reequilibration with a solution that had variable ratios of Fe, Mg, and Ti to pH; this hydrothermal fluid encroached upon the potassic alteration zone at a temperature estimated to have ranged from 310 degrees to 370 degrees C. A postulated trend of biotite compositions across the phyllic-potassic transition zone of alteration and into the argillic zone indicates a progressive increase in pH and in a Mg (super +2) /a Fe (super +2) of the aqueous phase in that direction.
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Pseudomorph
Grossular
Muscovite
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In the Malanjkhand Granitoid (MG), chlorite occurs in micro-domain along with mineral assemblage biotite, hornblende, quartz, K-feldspar and plagioclase. Chloritization of biotite is the most dominant processes during the hydrothermal alteration in MG ore system followed by alteration of hornblende. Chlorite composition revealed two major types of substitution mechanism, i.e. couples Tschermak (TK) and di-trioctahedral (DT) which correspond to the coupled exchange of Mg and Fe for Al between end-members clinoclore-daphanite and amesite. TK substitution is more prominent than DT substitution between endmembers of chlorite solid solution during hydrothe rmal alteration in MG. Temperature estimates for chloritization using chlorite geothermometry range from 110°C to 400°C and are consistent with the temperature of hydrothermal mineralization (200-375°C) at Malanjkhand. The chloritization process incorporates K + and Ca 2+ ions in the hydrothermal fluids. Therefore, it is inferred that the chloritization in granitic plutons is due to alteration of biotite and hornblende which increases the oxygen fugacity and activities of K + and Ca 2+ ions in the hydrothermal fluid.
Hornblende
Ilmenite
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