https://clok.uclan.ac.uk/1139/1/1._The_thermal_decomposition_of_huntite_and_hydromagnesite_-_A_review.pdf

The thermal decomposition of huntite and hydromagnesite—A review☆

The densities of two allophane samples measured by displacement with water were 2·72–2·78 g/cm3, while those measured by a float-sink test in CH3OH-, CH3 COCH3-, C6H6- and CCl4-C2H2Br4 mixtures were 1·84, 1·94–1·98, 2·35 and 2·35–2·39 g/cm3, respectively. The results of the density determinations for montmorillonite, kaolinite, halloysite, imogolite and volcanic glass are also reported. The data were interpreted in terms of the accessibility of the displacement liquids to void spaces within and between the layers or ‘structure units’ of the minerals and a model of the ‘structure units’ of allophane is proposed.

Density and structure of allophane

/pdf/olivines-their-pseudomorphs-and-secondary-products-388mfjtq77.pdf

Olivines, their pseudomorphs and secondary products

Monoclinic K-feldspars and triclinic Na-feldspars at high temperatures form a "continuous" series of mix crystals. The apparent lack of a discontinuity in the monoclinic-triclinic solid-solution series has been puzzling. A transition from monoclinic to triclinic symmetry was found to exist and is here described. The composition at which the transition occurs was found to be a function of temperature: Or4oAbo at ~Oo C.; OrsAb9 at ~1,000 C. (extrapolated). The transition considered at a fixed composition as a function of temperature has the characteristics of a displacive transformation. The investigation was carried out with natural crystals homogenized at high temperature and quenched. Winchell's term "analbite" is redefined and used for the high-temperature modification of NaA1Si30s. Besides the displacive transformation, another type, or diffusive transformation, is considered. A detailed discussion of available data led to the conclusion that the series KA1Si3Os-NaAlSi30s, stable at high temperature (as a sanidine-barbierite, analbite series), differs from those members stable at low temperature (microcline-albite) by a different distribution of the Al and Si ions-disordered at high and ordered at low temperature. A slow and (at least on the potash side) continuous process of ordering in crystals originally disordered is considered to be responsible for the variable appearance of intermediate states (orthoclase, adularia). The process of K/Na exchange is also discussed in connection with an investigation of different types of perthites, such as those with exsolved albite and those with exsolved analbite. The meaning of the mineral name "anorthoclase" is considered in connection with some remarks on nomenclature of the alkali feldspars. The extension of this nomenclature to ternary feldspars is illustrated, with some results of an investigation of feldspars from rhombporphyries.

Phase Relations of the Alkali Feldspars: I. Introductory Remarks

The metasedimentary rocks in the Qnadrilatero Ferrifero have been divided into the lower Precambrian (pre-2,700 m.y.) Rio das Velhas Series and the middle Precambrian Minas (pre1,350 m.y.) and Itacolomi Series (pre-1,000, perhaps pre-1,350 m.y.). Five thermal events occurred, four of which may have been accompanied by granitic intrusion or formation. All regional metamorphisms were, in general, Barrovian-type lowgrade greenschist facies, either chlorite or biotite isograd except near granitic intrusions where alinandine-aniphibolite facies conditions obtained and the staurolite isograd was developed. The last thermal event, 500 m.y. was accompanied by pegmatite formation and probable retrograde nietamorphism of many of the earlier formed higher grade minerals. In a general way, the metamorphic grade increases to the south and east. Metamorphic aureoles developed around the Bagao Complex and the granite north of the Serra do Curral, where the metamorphic grade falls from staurolite isograde adjacent to the granite to the regional chlorite isograde away from it. In the chlorite zone, the most widespread assemblages developed in pelitic rocks include: chlorite-sericite (inuscovite)quartz, which is by far the most common, and chlorite-sericite (muscovite)-quartz-cliloritoid, with or without kyanite. Locally abundant are varieties rich in fuchsite, graphite, carbonate, iron oxides, or tourmaline. Because of the extremely high alumina (average 20.1 percent) and potash (average 4.5 percent) content compared to the low lime (0.15 percent and soda (0.35 percent) content, albite and epidote are not as common here as in low-grade rocks elsewhere. They occur with the chlorite-sericite-quartz assemblage. With increasing nietamorphism, biotite apparently replaces chlorite and sericite. Chlorite changes from an Feto Mg-Alrich variety as metamorphic grade increases, but the compositions of associated biotite also appear to change from Feto Mg-rich. Grain size becomes coarser in the biotite zone. Both albite and epidote, as well as spessartite, are found locally. In the upper part of the biotite zone, oligoclase and almandine occur. Staurolite is generally the highest metamorphic zone present in the area, although sillimanite and cordierite are found in the metamorphic aureole south of the Bagao Complex. In this zone, staurolite replaces chloritoid, almandine replaces chlorite and spessartite, and biotite is more abundant, although some chlorite persists. Kyanite is found in this as in all metamorphic zones; oligoclase and andesine occur in places. With increasing metamorphic grade, the only significant changes in "pure" itabirite are an increase in size of the quartz and hematite and a change from hematite to specularite. Rare accessory minerals in low-grade itabirite are kaolinite, talc, chlorite, and pyrophyllite; as the grade of nietamorphism increases, tremolite-actiuolite and finally, at the highest grade, cummingtonite-grunerite are found. In dolomitic itabirite, in addition to increase in grain size of magnetite, hematitespecularite, and quartz, with increasing nietamorphism, dolomite disappears, diopside and tremolite-actinolite form in medium-grade rocks, and pyroxene, andradite, and cummingtonite-grunerite form in high-grade rocks. Carbonate rocks generally do not change their mineralogy with increasing metamorphisni, although they do become coarser grained. Some tremolite and clinozoisite formed near si ear zones; in higher grades cummingtonite also developed. Quartzo-feldspathic rocks developed suites similar to tl ose in the pelitic rocks, except that plagioclase and microcline are much more abundant. Amphibolites are found in areas where metamorphism exceeded the greensehist facies. Most of these amphibolites appear to be metamorphosed mafic igneous rocks, although some may be altered "dirty" carbonate rocks. A typical assemblage in these rocks includes hornblende, almandine, andesine, quartz, and some biotite and actinolite. Major minerals of the Quadrilatero Ferrifero include: Chlorite. One of the most abundant minerals of the region, found in all rocks types and in all metamorphic zones. In changing from lower to higher grade rocks, chlorite appears to lose Fe and become more Mg-Al-rich, as well as coarser grained. It occurs with hematite in higher grade rocks; in rocks containing magnetite, biotite of a composition related to chlo^ite also appears. Chromian chlorite is associated with gold rtineralization. White mica. Includes mostly muscovite but also some fuchsite and pyrophyllite. No paragonite was found in any rock. Muscovite is one of the most abundant minerals of the area and is found in every rock type, except iron-formation, and in every metamorphic grade. In higher grade rocks, it is coarser grained; in the lowest grade, it is fine-grained "sericite." Biotite. Found in mediumto high-grade pelitic schists and in other rock types, but rarely found in low-grade pelites and itabirite. High-Mg biotite is found in carbonate-bearing and

/pdf/metamorphic-rocks-of-the-quadrilatero-ferrifero-minas-gerais-4s027vb6ce.pdf

Alonzo W. Quinn

Papers

Elements of optical mineralogy. Part II: Description of Minerals

Dana's System of Mineralogy. Volume II (Palache, Charles)

Dana's Manual of Mineralogy (Hurlbut, Jr.; Cornelius S.)