Pyroxenes, pyroxenoids

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. 

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...

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

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. 

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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