Pyroxenes spodumene

FIGURE 7.11 The crystal structure of a pyroxene (spodumene) shows layers of Li+ (larger) and AP (smaller) ions in octahedra alternating with layers of Si in tetrahedra giving a nominal formula of LiAISisOe. 

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

Spodumen is a monoclinic pyroxene, space group C C2 c), with two not equivalent metal cation sites Ml and M2. The aluminum occupies the smaller Ml site, which is approximately octahedral (actual symmetry C2) with an average metal-oxygen distance of 1.92 A. The M2 site, occupied by Li, is also six-fold coordinated with an average metal-oxygen distance of 2.23 A. Both A1 and Li sites may be substitutionally replaced by ions of the transitional metals in various proportions. Both Mn " " and Cr " centers have been identified in luminescence spectra by steady-state spectroscopy (Tarashchan 1978 Walker et al. 1997). At room and lower temperatures only one emission band of Mn + occurs and the excitation spectra taken for the different wavelengths of the luminescence bands are always the same. So it is very probable that Mn + ions in the spodumen matrix present only in one site. The calculated values of 10D,j and B are consistent with the occupation of larger M2(Li) weak-field site. Mn + is mainly in Li-sites rather than Al-sites. 

For descriptions of members of the pyroxene group, see also Acmite-Aegerine Augile Diallage Diopslde Enslalite Hypersthene Jadeiie and Spodumene. 

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. 

The deformation microstructures of monoclinic pyroxenes are considered in Section 9.9.2. Optical microscope observations (Griggs, Turner, and Heard 1960 Raleigh 1965) indicate that the dominant slip system in monoclinic pyroxenes is also (100)[100]. However, van Duysen and Doukhan (1984) found by TEM that in naturally deformed a-spodumene the activated slip systems are (110) [001] and [lT0) <110>. In specimens of a-spodumene deformed by scratching, they also observed interesting microstructures of dislocations and faults that may be related to the twins observed in deformed diopside by Kirby and Christie (1977). 

Spodumen or triphane LiAl (8103)2 belongs to the catena silicates or pyroxenes. It is found in pegmatites in the United States and also in Scotland and Austria. 

Single SiOf- 2 Pyroxenes, e.g.. MgSiO,. enslatile LiAKSiO,), spodumene CsAHSiO). pollucite... 

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