Cummingtonites

Cummingtonite (Mg,Mn,Fe)7Si8022(OH,E,Cl)2 CALCIC AMPHIBOLES P2Jm C2lm... 

Table 5.44 Compositions (in weight %) of some natural amphiboles (from Deer et al., 1983) (1) = anthophyllite from a serpentinite (2) gedrite from a gedrite-kyanite-garnet paragenesis (3) cummingtonite from a oligoclase-biotite schist (4) common hornblende from a tonalite (5) pargasite from a metamorphic limestone (6) basaltic hornblende from a lathe (7) glaucophane from a glaucophane schist. ...

Heating experiments on P2ilm amphiboles (cummingtonite) show transition to the C2lm polymorph at low T (50 to 100 °C Prewitt et ah, 1970 Sueno et ah, 1972) in analogy to the behavior of pigeonitic pyroxenes (P2jlc), which show transition to the form C2/c (cf. section 5.4.3). Phase stability limits are markedly conditioned by the chemistry of the system and, particularly, by the partial pres-... 

Site M4 is occupied in one of the two components by a cation with a large ionic radius (e.g., Ca ) and by cations of much shorter radii (e.g., Mg and Fe ) in the other component (hornblende-cummingtonite or actinolite-cummingtonite). 

Site A is partially or fully occupied in one component and completely empty in the other (hornblende-actinolite, hornblende-cummingtonite, gedrite-anthophyllite, richterite-riebeckite). 

To our knowledge, direct experimental data on amphibole mixtures have been obtained only for the (pseudo)binary system actinolite-cummingtonite (Cameron, 1975) at Ptotai = -Phjo = 2 kbar and for the (pseudo)binary system tremolite-pargasite at Ptotai = PhjO = 1 kbar (Oba, 1980). In both cases, an extended miscibility gap (or solvus field in the second case), is evident at low T(i.e., 600 to 800 °C), which is indicative of strong positive interactions in the solid mixtures. Unmixing of other compositional terms is also evident in microprobe investigations (see Ghose, 1982 for an appropriate discussion). 

Information on amphibole mixing properties comes from intracrystalline and intercrystalline Fe +-Mg distribution studies. Mueller (1960, 1961) described the coexistence of actinolite and cummingtonite in ferriferous metamorphosed bands of Bloom Lake (Quebec, Canada) based on the exchange equilibrium... 

Figure 539 Intracrystalline distribution of Fe +-Mg + between Y(Ml, M2, M3) and M4 sites in natural cummingtonites. (A) Regular site-mixing model. (B) Ideal site-mixing model. (C) Arrhenius plot of equilibrium constant.

We recall to this purpose that ideal site mixing does not necessarily mean that the mixture as a whole is ideal, but that the intracrystalline site distribution may be described by a constant (without the necessity of introducing site-interaction parameters cf equation 5.109, for example). Generally speaking, if the adopted constant differs from 1 (as in the case of sodic amphiboles in figure 5.40 and of cummingtonites in figure 5.39), the mixture as a whole is obviously nonideal because site permutability does not reach the maximum value attained by the ideal condition (cf section 3.8.1). 

Cameron K. L. (1975). An experimental study of actinolite-cummingtonite phase relations with notes on the synthesis of Fe-rich anthophyllite. Amer. Mineral, 60 375-390. 

Hafner S. S. and Ghose S. (1971). Iron and magnesium distribution in cummingtonites. Zeit. Kristallog., 133 301-326. 

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

Presnall D. C. (1969). The geometrical analysis of partial fusion. Amer. Jour. Scl, 267 1178-1194 Prewitt C. T, Papike J. J. and Ross M. (1970). Cummingtonite. A reversible, non-quenchable transition from P2i/m to C2/m symmetry. Earth. Planet. Scl Letters, 8 448-450. 

Viswanathan K., and Ghose S. (1965). The effect of Mg -Fe substitution on the cell dimension of cummingtonite. Amer. Mineral, 30 1106-1112. 

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. 

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. 

Cummingtonite-asbestos The type of amphibole-asbestos composed of the mineral cummingtonite. 

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