Feldspathoid minerals

Subclass Tectosilicates (framework silicates) The tectosilicates or framework silicates have a structme where all of the four oxygens of an Si04" tetrahedron are shared with four adjacent tetrahedra. The ratios of Si to 0 is thus 1 3. The basic structural group then becomes SiOj. The minerals - quartz, cristobalite, tridymite etc. - are based on this structure. If some of the Si ions are replaced by AF, a charge imbalance is produced which is neutralized by incorporation of other cations in different coordination sites within the framework structure. Thus, the feldspar and feldspathoid minerals are also based on the tectosilicate framework. 

There are four feldspathoid minerals that are key constituents in certain glass-ceramics. Nepheline solid solutions and kalsilite have already been dis-... 

Silicates. This mineral classification encompasses the largest group of mineral species and includes most of the important rock-forming minerals, such as the feldspars, feldspathoids, pyroxenes, amphiboles. 

Brown. W.L., Editor Feldspars and Feldspathoids, Re idol. Boston, 1984. Campbell. A N. el ol. "Recognition of u Hidden Mineral Deposit by an Artificial Intelligence Program,". Viemr. 217. 927-029 tl982). 

The principal secondary minerals in CM CAIs are diverse phyllosilicates (cronstedite, serpentine, and the Fe-Ni-S-O phase tochilinite see review by Tomeoka et al, 1989) and calcite. Feldspathoids have only rarely been reported (MacPherson et al, 1983 El Goresy et al, 1984). 

In systems G and H, sodium sulfate was used as a sodium source mixed with kaolinite and calcium acetate. Sodium sulfate melts at approximately 884 C. Therefore, at 750 C no interaction between kaolinite and sodium sulfate was seen (systems G and H). In system G nepheline was formed at 1000 C, with none of the sulfur released from the sodium sulfate involved in the formation of any new high temperature minerals. However, in system H additional feldspathoids (i.e., gehlenite and hauyne) other than nepheline were formed. In this case, sulfur was involved in forming new high temperature minerals. 

In systems A through I the major aluminosilicate minerals produced were silica-deficient. However, in systems J where silica was provided in excess in the form of quartz the system still produced only feldspathoids containing only two-thirds as much silica as their silica-rich counterparts (i.e. alkali feldspars). In addition, the quartz peaks in system J were not substantially reduced at higher temperatures. These two facts suggest that SIO2 in the form of quartz is inactive up to and at 1000 C. This idea is supported by the fact that quartz peaks are also quite evident in diffractometer patterns of ash samples generated between 750% and 1000 C. It appears that temperatures in excess of 1000 C are required for quartz to contribute to the formation of silica-rich minerals. 

The principal primary mineral groups are the silica minerals (including quartz), feldspars, feldspathoids, olivines, pyroxenes, amphiboles, and micas. All are silicates and can be classified structurally based on the arrangement of connected silica (Si04) tet-rahedra. Table 2.2 summarizes the classification system for these common rockforming minerals. 

Olivine ((Mg,Fe)2Si04) and the feldspathoids (amphiboles, mainly ham-blende, and pyroxenes). In these minerals Fe(II) can be oxidized, increasing the weathering rate. 

The most important rock-forming minerals are often referred to as felsic and mafic, depending on whether they are light or dark coloured, respectively. Felsic minerals include quartz, muscovite, feldspars and feldspathoids, whereas olivines, pyroxenes, amphiboles and biotite are mafic minerals. The colour index of a rock is an expression of the percentage of mafic minerals that it contains. Four categories have been distinguished ... 

Deer, W.A.,Howie, R.A., Wise, W.S. and Zussman, J. (2004). Rock-forrning rninerals. Volume 4B. Framework silicates Silica minerals, feldspathoids and the zeolites (2nd ed.). Geological Society of London, London. 

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