Pyroxenes diopside

The cations located between the chains hold them together. Some pyroxenes are diopside (CaMgSi206), hendenbergite (Ca(Fe,Mg)Si206), and spodumene (LiAlSi206). 

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. ...

Figure 5JO (A) Simplified Gibbs free energy curves for various polymorphs along enstatite-diopside join at T = 1300 °C. (B) Resulting solvus, spinodal field, and miscibility gap compared with experimental data of McCallister and Yund (1977) on pyroxene unmixing kinetics (part B from Ganguly and Saxena (1992). Reprinted with permission of Springer-Verlag, New York).

Gasparik T. (1985). Experimentally determined compositions of diopside-jadeite pyroxene in equilibrium with albite and quartz at 1200-1350 °C and 15-34 kbar. Geochim. Cosmochim. Acta, 89 865-870. 

Both single- and double-stranded chains are found. The most important members of single chains are the pyroxenes including diopside. The most important double-chained minerals are the amphiboles. Some of these contain hydroxyl and fluoride ions, bonded directly to the metal cation and not to the silicon atom. 

X-ray diffractograms (Fig. 4) and petrographic analyses (scanning electron microscopy also used for estimating the surface roughness and micro-heterogeneity of samples) indicate the presence of diverse silicates and oxides on the surface and in the bulk of the vitrocrystalline samples. In addition to the nearly ubiquitous quartz, other minerals were found in several samples gehlenite, albite, diopside, portlandite, pyroxenes... 

Si04 units share two corners to form infinite chains (Figure 1.52(c)). The repeat unit is SiOs . Minerals with this structure are called pyroxenes (e.g., diopside (CaMg(S 103)2) and enstatite (MgSiOs)). The silicate chains lie parallel to one another and are linked together by the cations that lie between them. 

Jadeite, Na AlSi2C>6, is a pyroxene similar to diopside with Na (CN 8) replacing Ca in the same layers with A1 octahedral sites. Figure 10.30 shows a projection of the oxygen layer above the center of the cell and the Si, A1 and Na bonded to the oxygen atoms. We see SiC>4 chains along c separated by one A1C>6 chain and two NaO chains. 

The lava has a poorly porphyritic hypocrystalline texture and contains phenocrysts of Al-poor diopside (AI2O3 0.3-0.4 wt%) and phlogopite set in a groundmass of kalsilite, melilite and glass. The rock has been petro-graphically classified as a pyroxene-kalsilite melilitite, and is also known by the local name of coppaellite. The Cupaello lava has been affected to... 

Examples of pyroxenes are enstatite (MgSi03), diopside [CaMg(Si03)2], and spodumene [LiAl(Si03)2]. A typical amphibole is tremolite Ca2Mg5(Si4On)2(OH)2. 

Trivalent titanium has been positively identified by optical spectral measurements of a green calcic pyroxene from the meteorite that fell near Pueblo de Allende, Mexico, in 1969. The chemical analysis of this titanian pyroxene (Dowty and Clark, 1973) revealed it to be an iron-free subsilicic diopside (fas-saite) containing coexisting Ti3+ and Ti4+ ions and having the chemical formula Ca1.0lM 0.38,n3+0.34,n4+0.14Alo.87Sil.2606-... 

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