Riebeckite

Fig. 1. Asbestos fibers (chrysotile, crocidoHte, and amosite) as separated from host rock and their massive varieties (antigorite, riebeckite,...

Table 5.32 Compositions (in weight %) of natural pyroxenes (samples 1-5 from Deer et al., 1983 samples 6 and 7 from Bonatti et al., 1986) (1) enstatite from a pyroxenite (2) ferrosilite from a thermometamorphic iron band (3) hedembergite (4) chromian augite from a gabbroic rock of the Bushveld complex (5) aegirine from a riebeckite-albite granitoid (6) diopside from a mantle peridotite (7) enstatite from a mantle peridotite. ...

Na2Mg3Fe2Si8022(OH)2-Na2Fe3Al2Sig022(OH)2 (magnesio-riebeckite-ferro-glaucophane Borg, 1967)... 

For the first four systems, the experimental data cover the entire compositional field, whereas for magnesio-riebeckite-ferro-glaucophane, the data are limited to the field ANa2Mg3Fe2Si8022(0H)2 = Within the studied compositional... 

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

Site M4 is occupied by Ca in one component and by Na" in the other (actinolite-riebeckite). 

It is important to emphasize here that, theoretically, if a solid mixture is ideal, intracrystalline distribution is completely random (cf section 3.8.1) and, in these conditions, the intracrystalline distribution constant is always 1 and coincides with the equilibrium constant. If the mixture is nonideal, we may observe some ordering on sites, but intracrystalline distribution may still be described without site interaction parameters. We have seen in section 5.5.4, for instance, that the distribution of Fe and Mg on Ml and M3 sites of riebeckite-glaucophane amphiboles may be approached by an ideal site mixing model—i.e.. 

Borg I. Y. (1967). Optical properties and cell parameter in the glaucophane-riebeckite series. Contrib. Mineral Petrol, 15 67-92. 

Riebeckite (variety crocidolite)—Na2(Fe, Mg)3Fe Anthophyllite—Mg7Sig022(OH)2 Tremolite—Ca2Mg5Sig022(OH)2 Actinolite—Ca2(Mg,Fe )5Sig022(OH)2... 

Crocidolite This mineral name was originally proposed in 1831 for what was thought to be a distinct mineral, but was later shown to be a fibrous variety in the amphibole mineral series riebeckite-magnesioriebeckite. Crocidolite continues in trade use and, to some extent, in mineralogical and biomediccd scientific use. 

Opacity of mixed-valence minerals. The opacities of many end-member Fe2+-Fe3+ oxide and silicate minerals result from electron hopping between neighbouring cations when they are located in infinite chains or bands of edge-shared octahedra in the crystal structures. Opaque minerals such as magnetite, ilvaite, deerite, cronstedtite, riebeckite and laihunite owe their relatively high electrical conductivities to thermally activated electron delocalization, contributing to intermediate valence states of iron cations which may be detected by Mossbauer spectroscopy. 

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