Pyroxenoids

Figure 8. High-resolution image of a disordered synthetic pyroxenoid (of composition MnFeSi206) not amenable to x-ray crystallographic study ( see text). The region shown consists of intergrown, recurrent strips of the pyroxenoid structures known as rhodonite (with 5 linked SiOk tetrahedra) and pyroxmangite (with 7...

Jefferson s studies of the pyroxenoids has added greatly to our application of the way in which, through the intermediary of planar - or planar and Kinke - faults one structure is converted into another (45). And Audier, Jones and Bowen (46) have revealed how unit cell strips of Fe C may be accommodated as extended defects in the Fe C structure. Both these carbidic phases can be readily identified by HREM at the interface of iron catalysts used for the disproportionation of CO (to yield C j+CC ). 

Another pyroxenoid, bustamite, [(Mn,Ca,)3Si309], whose stmcture closely approximates that of wollastonite, has also been identified in fibrous form, but no detailed examination has been undertaken to check the possibility that a structural segregation similar to wollastonite exists in this mineral, and might contribute to the formation of fibers. 

Mn2+ rhodonite, pyroxmangite and bustamite (pyroxenoids), johannsenite (pyroxene), spessartine (garnet), tirodite (amphibole)... 

Atomistic simulations have also been performed on pyroxenes and pyroxenoids by Catlow et al. (1982) and predict the stable Mg end member to have the diopside structure and the stable Ca end member the wol-lastonite structure, as observed. However, only pair potentials were employed in this study, and Post and Burnham (1986) have found that diopside structures are poorly described within such a model. It would clearly be of interest to repeat the simulations using the three-body silicate potentials, which have yielded accurate Si-O-Si angles in quartz (Catlow et al., 1985). 

Figure 8 Log (rate of release of Si) versus X/Si for the dissolving mineral. X/Si equals 2 for orthosilicates, 1 for pyroxenes and pyroxenoids, 0.875 for amphiboles, 1.5 for chrysotile, and 0.75 for talc. Data sources are...

Banfield J. F., Ferruzzi C. G., Casey W. H., and Westrich H. R. (1995) HRTEM smdy comparing naturally and experimentally weathered pyroxenoids. Geochim. Cosmochim. Acta 59(1), 19-31. 

Ferruzzi G. G. (1993) The character and rates of dissolution of pyroxenes and pyroxenoids. MS, University of California, Davis. 

Oelkers E. H. and Schott J. (2001) An experimental study of enstatite dissolution rates as a function of pH, temperature, and aqueous Mg and Si concentration, and the mechanism of pyroxene/pyroxenoid dissolution. Geochim. Cosmochim. Acta 65(8), 1219-1231. 

Andrault D, Itie JP, Farges F (1996) High-temperature stmctural study of germanate perovskites and pyroxenoids. Am Mineral, 81 822-832... 

The pyroxenoid family of silicates (MSiOs, where M can be Mg " ", Ca , Fe " ", Mn , or mixtures thereof) illustrate this point. Good progress has also been made theoretically in using the Ising model or the anisotropic nearest-neighbor Ising (ANNI) model to interpret polytypism. 

The band s width may include two or three octahedra. If the width of a band is equal to 3 octahedra, the chain period is equal to 3 tetrahedra (wollastonite). If the width of an octahedral band is equal to 2 octahedra, the chain period is equal to 2 tetrahedra (pyroxenes). Several intermediate structures of the so called pyroxenoids are listed in Table 9. 

A new pyroxenoid mineral, cascandite, was described by Mellini et al. (1982), and its crystal structure was analyzed by Mellini and Merlino (1982). The structure of cascandite is similar to those of pectolite, NaCa2Si3 0g(0H), and serandite, Na(Mn,Ca)2Si3 08(0H). The structure is composed of octahedral double chains, formed by two strands of edge-sharing octahedra, and tetrahedral single chains with... 

Ohashi Y. and Finger L.W., "The Role of Octahedral Cations in Pyroxenoid Crystal Chemistry. I. Bustamite, Wollastonite, and the Pectolite-Schizolite—Serandite Series," Am. Mineral., 63, 274-88 (1978). 

Beside the clino- and orthopyroxenes there exists a group of minerals with similar chemical compositions of pyroxenes called pyroxenoids. There structures are based on twisted chains of silica tetrahedra. An example is rhodonite (Mn,X)Si03 (X = Fe, Ca, Mg), which contains five different sites fliree moderately and one strongly distorted octahedral sites and one seven-fold coordinated site. Spectra of rhodonite and fowlerite (Zn-rich rhodonite) needed to be adjusted with five doublets showing Fe " to be present in all the available sites [217, 218],... 

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