Particular forsterite

Chrysotile (serpentine) occurs in both clino and ortho stmctures. Both one-layer ortho and clino, and six-layer ortho (as in nacrite) stmctures have been observed. Chrysotile transforms at high temperature to forsterite [15118-03-3] and siHca. Particularly fibrous varieties are called asbestos (qv). 

The temperatures of appearance of various solid phases as a cooling solar gas condenses are illustrated in Figure 7.1. This particular condensation calculation was performed at a pressure of 10 4 atm (thought to be appropriate for parts of the nebula). It is important to note that not every mineral in the sequence actually condenses directly from the vapor. Instead, some minerals form by reaction of previously condensed solids with the vapor. For example, FeS forms by reaction of already condensed Fe metal with sulfur in the gas phase, and olivine first condenses as the magnesium end member forsterite and then becomes progressively more iron-rich by reaction with vapor as temperature decreases. 

The technique of channeling-enhanced X-ray emission (CHEXE) has enabled cation site occupancies to be determined in various minerals, including transition metal ions in spinels and ferromagnesian silicates (Taftp, 1982 Taftp and Spence, 1982 Smyth and Taftp, 1982 McCormick etal., 1987). The method, which is based on relative intensities of X-ray peaks measured on crystals with diameters as small as 50 nm under the electron microscope, is particularly useful for determining site occupancies of minor elements with concentrations as low as 0.05 atom per cent in a structure. The most important criterion for the determination of element distribution in a mineral by this technique is that the cation sites should lie on alternating crystallographic planes. In order to make quantitative site population estimates, additional information is required, particularly the occupancy of at least one element in one of the sites or in another site that lines up with one of the sites of interest (McCormick et al., 1987). For example, cation site occupancies by CHEXE measurements have been determined from X-ray peak intensity ratios of Si to Ni, Mn, Cr and Fe in forsterite, as well as thermal disordering of these cations in heated olivines (Smyth and Taftp, 1982). 

Thus, although the average metal-oxygen distance of the forsterite M2 site is 3.4 pm larger than the Ml site, there are two individual M2-0 distances that are actually shorter than the smallest Ml-O distance (Appendix 7). These differences are comparable to ionic radii differences between Mg2+-Ni2+ or Mg2+-Co2+ pairs. The situation is even worse for the orthopyroxene structure in which there is a large spread of metal-oxygen distances particularly for the M2 site (table 5.6 Appendix 7). 

The observation that Cr and Ni are enriched in earliest minerals crystallizing from magma such as chromite and forsteritic olivines discussed in earlier sections ( 8.5) has led to the widespread use of the abundances of these particular... 

It is particularly intriguing that Fo and En are also seen in the IR spectra of young stars with disks and also in dust formed from gas outflows from evolved stars (Molster et al., 2002a,b Waters and Molster, 2002 Jaeger et al., 1998). Forsterite and enstatite are common minerals around stars, in LP comets and in the most primitive meteorites even though they are not seen in the diffuse interstellar medium or in SP Kuiper Belt comets. 

The 10 p.m feature of chondritic IDPs has been compared with the 10 p.m feature of astronomical silicates. No particular IDP IR class consistently matches the —10 p.m feature of solar system comets or silicate dust in the interstellar medium (Sandford and Walker, 1985). However, the —10 p.m features of CP IDPs composed mostly of GEMS and submicrometer enstatite and forsterite crystals generally resemble those of comets and late-stage Herbig Ae/Be stars in support of the hypothesis that some CP IDPs are of cometary origin (Figure 10). 

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