Silicates, amorphous

Table 2. Solution Rates of Amorphous Silicate Powders ...

Amorphous silicate powders, solution rates of, 22 464t... 

Powder(s). See also Amorphous silicate powders Group A powders... 

In addition to the inorganic hydroxyl groups which are exposed on many mineral surfaces (metal oxides, phyllo-silicates and amorphous silicate minerals) we need to consider the particular features relating to charge on the silica surfaces of layer silicates. 

The hydroxyl (OH) group is the dominant reactive functional group on the surface of many solid phase particles, amorphous silicate minerals, metal oxides, oxyhydroxides, and hydroxides [17,25,160]. In the case of various organic pol-... 

Opal (or opaline silica) An amorphous silicate formed through the polymerization of silicic acid molecules. Though most is biogenic in origin, some forms as a result of diagenesis. 

Far infrared emission decreasing as 1 /A Amorphous silicates or carbon possible shape effects... 

In all but one type of chondrite, small millimeter-sized spherical silicate grains are trapped in the mineral matrix. These small grains consist mainly of amorphous silicates and are called chondrules. This latter term gives its name to the meteorite class containing chondrules, i.e. the chondrite class. The chondrites themselves are divided into three subclasses enstatite, ordinary chondrites and carbonaceous chondrites. Carbonaceous chondrites themselves are classified into four groups I (for Ivuna), M (for Mighei), O (for Omans) and V (for Vigarano). 

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... 

Infrared absorption spectroscopy of interstellar clouds shows that the interstellar dust population varies with the line of sight, yet it maintains a similar character. In particular, submicron-sized amorphous silicate grains are the dominant component in every direction. The absence of crystalline grains is likely the result of rapid amorphization by the interstellar radiation field. 

Sensitive observations enable comparative surveys of silicate emission features from disks around low-mass, intermediate-mass, and Sun-like stars. While no strong correlations have been found with disk properties, flatter disks and disks around the coolest stars more often show crystalline silicate features. Cool stars and very low-mass disks display prominent crystalline silicate emission peaks (Apai et al. 2005 Merfn et al. 2001 Pascucci et al 2009). Thus, whatever processes are responsible for the presence of crystals around Sun-like stars must be capable of very efficiently producing crystals around low-mass stars, too. Interferometric measurements suggest that the amorphous/crystalline dust mass fraction is higher in the inner disk than at medium separations (van Boekel et al. 2004 Ratzka et al. 2007). The surveys also show that amorphous silicate grains frequently have similar magnesium and iron abundances in protoplanetary disks. In contrast, those with crystalline silicates are always dominated by Mg-rich grains (e.g. Malfait et al. 1998 Bouwman et al. 2008). 

Some of the chondritic meteorites contain grains (including crystalline and amorphous silicates, diamonds, silicon carbide, graphite, metal oxides, and metal nitrides) that have been identified as presolar based on non-solar isotopic ratios (Zinner 1988 Anders Zinner 1993 Bematowicz et al. 2006), particularly for... 

Other amorphous silicates are also found in IDPs. The compositions range from ferromagnesian silica, with variable Mg/Fe ratios, containing Ca and Al (Bradley 1988) to Fe-Mg-bearing aluminosilica(Klock Stadermann 1994). Bradley (1988) reported pyroxene glass in one IDP. 

From infrared spectroscopy it is very difficult to obtain the composition of the amorphous silicates. This is because the spectral signature observed is a combination of grain composition, shape, size, and structure, making it difficult to isolate the pure amorphous silicate signal. This, in combination with the relatively small spectral changes caused by the composition of the silicates, makes it hard to get a definitive answer in most cases. In the case of interstellar dust we have a unique opportunity the grains are very small and (almost) all silicates are amorphous. 

Both the anhydrous, porous IDPs and Tempel 1 appear to have a higher crystalline to amorphous silicate ratio than is inferred from infrared spectroscopy of interstellar or most circumstellar grains. This could simply be a result of grain alteration in interstellar space, with radiation processing converting crystalline silicates to amorphous silicates. 

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... 

Acfer 0942 Unique Type 3 Si-Fe-Mg amorphous material, olivine, enstatite, pyrrhotite Mainly submicron olivines and pyroxenes embedded in groundmass of amorphous silicate... 

Y-7911984 CM2 Si-Fe-Mg amorphous material, pyrrhotite, pentlandite, olivine, phyllosilicates Distinct micron-sized sulfide-rich and sulfide poor domains, with sulfide embedded within amorphous silicate material... 

Adelaide5 Unique Type 3 Iron-bearing olivine, amorphous silicate, enstatite, pentlandite, magnetite Well-shaped and irregular olivine grains, with interstitial amorphous material... 

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