Crystalline olivine

Fine structure on the silicate feature provides more specific mineralogical information. A small bump at 11.2 pm on the 10 pm feature (Fig. 12.4a), observed for several long-period comets, is generally interpreted as indicating crystalline olivine. Another shoulder at... 

As the peaks observed near 33 and 69 p.m are due to phonon transitions in the crystalline silicate lattice, the ISO observations are an unambiguous indication of the presence of crystalline silicates in cometary dust. More recently, crystalline olivine and orthopyroxene have been identified in Comet C/2001 Q4 (NEAT) via 10 pm observations (Wooden et al. 2004). There is considerable controversy concerning the origin of these crystalline grains. 

Fine structure on the silicate feamre was first seen in observations of comet Halley. It consisted of a small 11.2 pm bump on the silicate feature detected in high spectral resolution and good signal-to-noise ratio observations. This small bump has now been seen on the silicate feature of several comets and it is widely interpreted as evidence for the presence of olivine because IR studies of powdered olivine samples show fine stmcmre at 11.2 pm. In the astronomical literature this feature is considered to be crystalline olivine as compared to amorphous silicates that cannot produce the pronounced 11.2 pm bump on the overall 10 pm silicate feamre. 

The 11.2 pm fine structure on the Si-O silicate feature has provided interesting insight into the relationship between cometary and interstellar materials, because IR observations of silicates in the diffuse interstellar medium and molecular clouds do not show the feature (Molster et al., 2002a,b). Searches for the 11.2 pm fine structure towards the Galactic center indicates that less than 0.5% of interstellar silicates are crystalline (Kemper and Tielens, 2003). The crystalline olivine feature is, however, seen in certain astronomical objects, stars surrounded with disks. It has been seen in Beta Pictoris (Knacke et al., 1993) and Herbig Ae/Be stars... 

Campins H. and Ryan E. (1989) The identification of crystalline olivine in cometary silicates. Astrophys. J. 341, 1059-1066. 

Figure 7. Spectropolarimetry of the lOpm silicate feature [41] (a) upper panel along the line of sight to the source AFGL 2136. The middle-box shows the polarization spectrum is composed of an emissive component (long-dashed line) and an absorptive component (short-dashed line). Note the change in PA of polarization across the feature (right-hand box) (b) lower panel along the line of sight to the source AFGL 2591. Note that a purely absorptive component is sufficient to account for the polarization spectrum. Note also, the feature at 11.2pm attributed to crystalline olivine.

Pallasites. These meteorites are mixtures of olivine and FeNi metal that formed deep in the core-mantle boundary of a small, differentiated asteroid. As the overlying cumulate olivine cooled and contracted, the still slightly molten metal was injected into the crystalline olivine forming a continuous matrix. Later collisions exposed this layer and delivered samples to Earth. There are three compositional clusters representing separate parent bodies. PAL... 

The weathering of silicates has been investigated extensively in recent decades. It is more difficult to characterize the surface chemistry of crystalline mixed oxides. Furthermore, in many instances the dissolution of a silicate mineral is incipiently incongruent. This initial incongruent dissolution step is often followed by a congruent dissolution controlled surface reaction. The rate dependence of albite and olivine illustrates the typical enhancement of the dissolution rate by surface protonation and surface deprotonation. A zero order dependence on [H+] has often been reported near the pHpzc this is generally interpreted in terms of a hydration reaction of the surface (last term in Eq. 5.16). 

Not all crystalline compounds exhibit slope modifications similar to those shown for olivine in figure 4.7. For instance, figure 4.8 shows dilfusivity plots for feldspars according to the revision of Yund (1983). The temperature ranges investigated by various authors are sufficiently wide, but in any case we do not observe any slope modifications imputable to modified defect regimes. 

The volume properties of crystalline mixtures must be related to the crystal chemical properties of the various cations that occupy the nonequivalent lattice sites in variable proportions. This is particularly true for olivines, in which the relatively rigid [Si04] groups are isolated by Ml and M2 sites with distorted octahedral symmetry. To link the various interionic distances to the properties of cations, the concept of ionic radius is insufficient it is preferable to adopt the concept of crystal radius (Tosi, 1964 see section 1.9). This concept, as we have already noted, is associated with the radial extension of the ion in conjunction with its neighboring atoms. Experimental electron density maps for olivines (Fujino et al., 1981) delineate well-defined minima (cf figure 1.7) marking the maximum radial extension (rn, ,x) of the neighboring ions ... 

Scattering media to which this matrix applies include randomly oriented anisotropic spheres of substances such as calcite or crystalline quartz (uniaxial) or olivine (biaxial). Also included are isotropic cylinders and ellipsoids of substances such as glass and cubic crystals. An example of an exactly soluble system to which (13.21) applies is scattering by randomly oriented isotropic spheroids (Asano and Sato, 1980). Elements of (13.21) off the block diagonal vanish. Some degree of alignment is implied, therefore, if these matrix elements... 

Chondrules exhibit a bewildering variety of compositions and textures (F ig. 6.1 a,b). Most are composed primarily of olivine and/or pyroxene, commonly with some glass. (For a crash course in mineral names and compositions, see Box 6.1.) If melt solidifies so quickly that its atoms cannot organize into crystalline minerals, it quenches into glass. Iron-nickel metal and iron sulfide occur in many chondrules, often clustered near the peripheries. The textures of... 

The Hostrock and Backfill Material. Most crystalline igneous rocks, including granite and gneiss, are composed of a comparatively small number of rock forming silicate minerals like quartz, feldspars (albite, microcline, anorthite etc.) micas (biotite, muscovite) and sometimes pyroxenes, amphiboles, olivine and others. Besides, there is a rather limited number of common accessory minerals like magnetite, hematite, pyrite, fluorite, apatite, cal cite and others. Moreover, the weathering and alteration products (clay minerals etc.) from these major constituents of the rock would be present, especially on water exposed surfaces in cracks and fissures. 

Solid solution A group of minerals or other compounds with the same crystalline structure and whose chemical compositions vary somewhat due to substitutions between two or more different elements or pairs of elements. As examples, Mg2+ and Fe2+ substitute for each other to form a solid solution in olivine ((Fe,Mg)2SiC>4) and (CaAl)5+ and (NaSi)5+ substitute for each other in plagioclase. 

From a mineralogy viewpoint, IDPs are aggregates of mostly sub-micron-sized crystalline silicates (olivine and pyroxene), amorphous silicates, sulfides, and minor refractory minerals, held together by an organic-rich, carbonaceous matrix. Large fractions, 30-60 wt%, of these IDPs are amorphous silicates, known as glass with... 

Figure 5.3 The amount of order in silicates can vary dramatically. A. The crystalline backbone structures for olivine, pyroxene, and quartz. The charge of the silicon tetrahedra is neutralized by metal cations in olivine and pyroxene. B. Silicate melts contain a mix of unaligned crystalline structures with metal cations randomly distributed in the melt. C. Chaotic condensates have not formed silicate tetrahedra rather, they appear more like a frozen gas state. These materials are typically under-oxygenated and contain more metals than a glass. Annealing supplies the chaotic silicate with the energy needed to rearrange into the more stable silicate tetrahedra. D. The gas phase largely consists of SiO. Metals are typically present as atoms or simple monoxides while excess oxygen can be found as OH (Nuth et al. 2002).

ALHA773071 CO3.0 Si-Fe-Mg amorphous material, olivine, troilite, Fe,Ni metal, magnetite Mainly submicron crystalline phases embedded in amorphous silicate. Distinct compositional domains... 

Silicates with olivine composition (MgxFe(i x))2Si04 are common in chondrites, comets, IDPs, and in protoplanetary disks. The Mg-rich end-member of the olivine family is forsterite, also often termed as Foioo the Fe-rich end-member is fayalite (Foo). The interstellar medium contains a similar concentration of the FeO- and MgO-rich silicates (see Chapter 2). Correspondingly, amorphous silicate grains frequently have similar magnesium and iron abundances in protoplanetary disks, in cometary dust, and in chondritic IDPs. In stark contrast, crystalline dust is almost always dominated by Mg-rich grains in protoplanetary disks (e.g. Malfait et al. 1998 Bouwman etal. 2008), comet tails (e.g. Crovisier el al. 1997 Wooden et al 2004 Harker et al. 2005 Lisse et al. 2006), in the most primitive and least processed chondritic matrices, and IDPs (for a review, see Wooden et al. 2007). 

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