Alkali amphiboles

Ungaretti L., Smith D. C. and Rossi G. (1981). Crystal-chemistry by X-ray strncture refinement and electron microprobe analysis of a series of sodic calcic to alkali-amphiboles from the Nybo eclogite pod, Norway. Bull Mineral, 104 400-412. 

Transition metal ions most susceptible to large Jahn-Teller distortions in octahedral coordination in oxide structures are those with 3d4, 3d9 and low-spin 3(f configurations, in which one or three electrons occupy eg orbitals. Thus, the Cr2+ and Mn3+, Cu2+, and Ni3+ ions, respectively, are stabilized in distorted environments, with the result that compounds containing these cations are frequently distorted from type-structures. Conversely, these cations may be stabilized in distorted sites already existing in mineral structures. Examples include Cr2+ in olivine ( 8.6.4) and Mn3+ in epidote, andalusite and alkali amphiboles ( 4.4.2). These features are discussed further in chapter 6. 

The polarized spectra of alkali amphiboles such as those of glaucophane illustrated in fig. 4.15, have been studied extensively (Chesnokov, 1961 Littler and Williams, 1965 Bancroft and Bums, 1969 Faye and Nickel, 1970a Smith and Strens, 1976) and are dominated by Fe2+ —> Fe3+ IVCT bands ( 4.7.2). However, Fe2+ crystal field bands occurring at 10,000 cm-1 and 8,600 cm-1 are distinguishable in fig. 4.15, indicating that glaucophane provides CFSE s comparable to those of Fe2+ ions in Ml and M3 octahedral sites of other amphiboles, eq. (5.17). 

In alkali amphiboles, in which sodium fills the M4 positions, trivalent cations (Fe3+, Al3+) are largely confined to the smaller distorted octahedral M2 sites, so that the ordering pattern for Fe2+ ions becomes M3 > Ml M2. In the manganese-bearing alkali amphiboles winchite and juddite, Mn3+ ions are ordered in the M2 sites (Ghose et al., 1986), in which they are stabilized by the Jahn-Teller effect ( 6.3). 

In the case of Cu2+ and Cr2+, compounds of which are susceptible to Jahn-Teller distortions ( 6.3 table 6.1), these cations are predicted to show strong preferences for the most distorted orthopyroxene M2 and amphibole M4 sites. A similar explanation accounts for the observed enrichments of Mn3+ ions in the distorted andalusite Ml, alkali amphibole M2, epidote M3 and, perhaps, epidote Ml sites (table 6.1). The presence of significant amounts of chromium in olivines from the Moon and as inclusions in diamond may be due to the presence of Cr2+ ions, and not Cr3+, in the distorted Ml and M2 sites of the olivine structure (Bums, 1975b), in which Jahn-Teller stability may be attained. A similar factor accounts for the stability and site occupancy of the Cr2+ ion in the orthopyroxene M2 site (table 6.1). 

The Co2+, Ti3 and V3 ions are expected to prefer either distorted or small octahedral sites. Thus, Co2+ should be slightly enriched in the orthopyroxene M2 and cummingtonite M4 sites, favour the pseudo-tetragonally distorted olivine Ml site, and be randomly distributed over the amphibole Ml, M2 and M3 sites. The V3+ and Ti3+ ions are expected to occupy the orthopyroxene Ml and alkali amphibole M2 sites, and to be enriched in distorted epidote M3 sites. As noted earlier, the occurrence and stability of Ti3+ ions in lunar and mete-oritic clinopyroxenes ( 4.4.1) may be explained by the availability of the distorted octahedal Ml site in the calcic clinopyroxene structure. 

Bancroft. G. M. Bums, R. G. (1969) Mossbauer and absorption spectral study of alkali amphiboles. Mineral. Soc. Amer., Spec. Pap. 2,137-48. 

Chigoreva, O.G. Fedoseev, A.D. (1966). Synthesis and investigations of some properities of fibrous alkali amphiboles. In Investigations of Natural and Technical Minerahforrnation, pp. 169-174, Science, Moscow. 

Puliev, Kh. N., 1964. Infrared spectra of two alkali amphiboles, svidneite and richterite. Spisanie Bulgar. Geol. Druzhestvo 25 193. 

Figure 12 continued, (d)-(g) Mineral isochrons for rhyolites from the Olkaria volcanic center (Kenya), (d) and (f) alpha spectrometry resnlts from Black et al. (1997). (e) and (g) TIMS resnlts from Henmaim and Davies (2002). All the rhyolites have eraption ages between 3.3 and 9.2 ka. Note that the same sample (570) analyzed in both stndies gives rather different ages (f and g). Same abbreviations of mineral names as in Fignre 10 + Qz qnartz KF alkali feldspar Amph amphibole Bt biotite. 

Chrysotile is a noncombustible fibrous solid that has been widely used as a fireproof thermal insulator, for brake linings, in construction materials, and for filters under the name of asbestos. It decomposes with loss of water at 600-800 °C, eventually forming forsterite and silica at 810-820 °C. Because it is more resistant to attack by alkalis than are the amphibole asbestoses, chrysotile has been used in chloralkali cell membranes and in admixture with Portland cement for making sewer pipes (Chapter 11). 

ALKALI ROCKS. Igneous rocks which contain a relatively high amount of alkalis in the form of soda amphiboles, soda pyroxenes, or felspathoids, are said to be alkaline, or alkalic. Igneous rocks in which the proportions of both lime and alkalis are high, as combined in tire minerals, feldspar, hornblende, and angile, are said to be calcalkalic. 

In summary, the mechanism for dissolution of feldspars, pyroxenes, and amphiboles appears to involve a rapid hydrogen exchange for alkalis and alkaline earths, which creates a thin layer of hydrolyzed aluminosilicate. This residual layer ranges in thickness from several to a few tens of nanometers and is responsible for the initial nonstoichiometric release of alkalis and alkaline earths relative to Si and Al (Velbel, 1985). 

Asbestos fibers are basically chemically inert, or nearly so. They do not evaporate, dissolve, bum, or undergo significant reactions with most chemicals. In acid and neutral aqueous media, magnesium is lost from the outer bmcite layer of chrysotile. Amphibole fibers are more resistant to acid attack and all varieties of asbestos are resistant to attack by alkalis (Chissick 1985 WHO 1998). Table 4-2 summarizes the physical and chemical properties of the six asbestos minerals. 

---