Grunerite

Volume properties in the system Mg7Si8022(OH)2-Fe7Si8022(OH)2 (cumming-tonite-grunerite) show well-defined linearity in the compositional range = 0.60 1.00 (Klein, 1964 Viswanathan and Ghose, 1965) Ve-... 

The chemistry of many natural amphiboles is conveniently described by a compositional quadrilateral like that seen for pyroxenes. The quadrilateral components are anthophyllite [Mg7Sig022(0H)2], grunerite [Fe7Sig022(OH)2], tremolite [Ca2Mg5Sig022(OH)2], and ferro-tremolite [Ca2Fe5Sig022(OH)2] (figure 5.36). 

Klein C. Jr. (1964). Cummingtonite-grunerite series A chemical optical and X-ray study. Amer. Mineral, 49 963-982. 

Intermediate members of the series that contain both Mg and Fe, may be identified as iron-rich cummingtonite, for example. Alternatively, a fibrous species whose composition and structure resembles grunerite occurs in South Africa. It is mined as asbestos and known commercially as amosite. Amosite is a varietal mineral name that has been used in the trade to describe mineral materials within the cummingtonite-grunerite series. The name is an acronym for asbestos mines of South Africa. Because it is not a proper mineral name, it may not be found in lists of minerals in standard miner-alogical texts. 

Over the past several decades six different silicate minerals have been mined as asbestos and processed for industrial and commercial applications. The most commonly encountered asbestos mineral today is chrysotile. The five other minerals are tremolite, actinolite, anthophyllite, grunerite, and rie-beckite. All five are members of the amphibole group of minerals, and each can occur as chunky, acicular, or equant crystals, as well as in fibrous form. When found as fine fibrous aggregates, in quantities appropriate for mining, they are usually distinguished as a special variety—for example, tremolite-asbestos. 

Amosite A commercial term for a type of amphibole-asbestos from South Africa consisting chiefly of fibrous members of the cummingtonite-grunerite series. Variable amounts of several fibrous members of the amphibole group and other minerals may be included in a particular sample. The name is an acronym derived from asbestos mines of 5outh Africa. 

The silicate facies is of a black finely laminated type, consisting mainly of magnetite with up to 15 vol. % grunerite and up to 5% carbonaceous matter. The facies is found in layers up to a few dm thick. Common to the silicate facies is the scarcity of quartz and the complete absence of carbonates. The sulphide facies of Isua consists of up to 60 vol. % sulphides (pyrite, pyrrhotite) together with grunerite or actinolite and magnetite (Appel, 1980) 118). 

Since Fe2+ ions are concentrated in the acentric M4 sites of cummingtonite, the intense band around 10,000 cm-1 in the p spectrum (fig. 5.19a) arises from absorption by Fe2+ ions in this very distorted site. The increased intensity and width of absorption bands in the a and y spectra of grunerite and the broadening of the intense band in the P spectra result from increased occupancies of Fe2+ ions in the more regular Ml, M2 and M3 octahedral sites. 

The breadth of the band near 10,100 cm-1 in the a and y spectra of grunerite and the shoulder at 8,500 cm-1 are attributed to absorption by Fe2+ ions in the Ml, M2 and M3 positions. Assuming a lower-level splitting of 500 cm-1 for the t2g orbitals of Fe2+ ions located in these relatively undistorted octahedral sites, an energy level diagram similar to that for the orthopyroxene Ml site (fig. 5.16) may be constructed leading to the approximate crystal field parameters... 

Thermodynamic analysis of mineral equilibria was first made by us (Mel nik and Siroshtan, 1973) on the basis of the new system of thermodynamic constants of minerals that is consistent with experiments (Mel nik, 1972b), which also includes important hydrous minerals like grunerite and minnesotaite. These data were subsequently amended (Mel nik and Radchuk, 1977b), and supplemented by the thermodynamic constants of greenalite and of some diagenetic iron and magnesium minerals (see Appendix). 

In addition, new experimental data were given on the conditions of formation of goethite, hematite, grunerite, and iron-magnesian amphiboles and pyroxenes. 

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