Silicates dust particle sizes

The darkness associated with dense interstellar clouds is caused by dust particles of size =0.1 microns, which are a common ingredient in interstellar and circum-stellar space, taking up perhaps 1% of the mass of interstellar clouds with a fractional number density of 10-12. These particles both scatter and absorb external visible and ultraviolet radiation from stars, protecting molecules in dense clouds from direct photodissociation via external starlight. They are rather less protective in the infrared, and are quite transparent in the microwave.6 The chemical nature of the dust particles is not easy to ascertain compared with the chemical nature of the interstellar gas broad spectral features in the infrared have been interpreted in terms of core-mantle particles, with the cores consisting of two populations, one of silicates and one of carbonaceous, possibly graphitic material. The mantles, which appear to be restricted to dense clouds, are probably a mixture of ices such as water, carbon monoxide, and methanol.7... 

Dust Micron-sized particles of silicate in the interstellar medium responsible for short-wavelength scatter - dust particles become covered with water-ice mantels. 

Toxicology. The toxicity of calcium silicate depends on particle size, aspect ratio, and amount of silica and respirable fiber. Synthetic nonfibrous calcium silicate is considered to be a nuisance dust. 

Figure 2.2 A GEMS particle found in an IDP, consisting of a glassy matrix with silicate-like composition and embedded nanometer-sized inclusions of iron metal and iron sulfide (dark patches). Some of them are possibly interstellar silicate dust grains. (From NASA http //stardust.jpl.nasa.gov/science/sci2.html.)...

Examples of the dust cross-section distributions for astronomical silicate at different wavelengths and for different power-law particle size distributions are shown in Fig. 7.1. The 9.7 pm absorption cross-section distribution for particles with p = — 3.5 actually peaks at 1 pm, but is otherwise dominated by particles <3 pm. Thus, for this distribution, observations at 9.7 pm are not very sensitive to large particles and their presence cannot be ruled out. For only a slightly shallower size distribution with p = —3.0, the absorption cross-section suddenly becomes dominated by large particles, and particles smaller than 1 pm will not contribute much to the total opacity. At 1 mm, the picture is different. Here, the absorption... 

The spectral profiles of solid-state resonances from small particles depends on both the shape and the size of the particles. This is particular true for the strong mid-infrared Si-0 stretching and O-Si-O bending modes, characteristic of circumstellar silicate dust. In the dielectric spectrum, the stretching mode is roughly centered on 10 pm, while the bending mode is centered on 18 pm. Both resonances are very broad and strongly dependent on the structure of the molecules in which the SiO ... 

Silicate dust in both the diffuse ISM as well as in dense protostellar envelopes has been observed to exhibit a nearly constant band profile dominated by small particles (Bouwman et al. 2001 Kemper et al. 2005). In sharp contrast, silicate bands observed in emission from protoplanetary disks exhibit a wide range of silicate band profiles, indicating a dominant presence of dust particles larger than the Rayleigh limit (Bouwman et al. 2001 van Boekel et al. 2005 Kessler-Silacci et al. 2006). The interpretation of this observation is that the characteristic size of the dust particles has grown either by a move to a shallower dust size distribution, or by the removal of smaller particles (cf. Fig. 7.1). Either way, the inference usually made is that small particles have been removed by a coagulation process not occuring in the ISM. 

Fluidized-bed chlorination was started in 1950. The titanium raw material (with a particle size similar to that of sand) and petroleum coke (with a mean particle size ca. five times that of the Ti02) are reacted with chlorine and oxygen in a brick-Uned fluidized-bed reactor (c) at 800-1200 °C. The raw materials must be as dry as possible to avoid HCl formation. Since the only losses are those due to dust entrainment the chlorine is 98-100% reacted, and the titanium in the raw material is 95-100% reacted, depending on the reactor design and the gas velocity. Magnesium chloride and calcium chloride can accumulate in the fluidized-bed reactor due to their low volatility. Zirconium silicate also accumulates because it is chlorinated only very slowly at the temperatures used. All the other constituents of the raw materials are volatilized as chlorides in the reaction gases. 

The dust production is of the order of the total gas production (10 -10 g/s at 1 a. u. solar distance). Particle sizes probably range from less than a micron to several centimeters. Infrared observations show the silicate features of the dust. The dynamics of the particles is governed by solar radiation pressure. 

The interstellar medium constitutes 10 % of the mass of the galaxy. It can be subdivided into environments with very low-density hot gas, environments with warm intercloud gas, and regions with denser and colder material (23). H and He gas are the major components of interstellar clouds molecules and submicron dust particles are only present in small concentration (22). Through gas phase reactions and solid-state chemistry, gas-grain interactions can build up complex organic molecules. Silicate and carbon-based micron-sized dust particles provide a catalytic surface for a variety of reactions when they are dispersed in dense molecular clouds 24). In cold clouds such dust particles... 

The product is derived from a purified silicate liquor and has a much lower level of impurities than products made by the direct fusion route. By virtue of the granulating process, the spray-coated product has essentially an onion-type-layered structure and a superior particle size distribution and is relatively dust free and more readily soluble in water [1]. 

Solubility data were very confusing until it was found that traces of certain metal impurities and especially the presence of an amorphous or at least disturbed layer on the crystal surface caused variable results, especially at temperatures below 150 C. In 1952, Dempster and Ritchie (117) reported that siliceous dusts have a layer of high solubility that gradually blends into the solid core, which adsorbs basic dyestuffs (118). Alexanian (119) found by electron diffraction that quartz possesses a surface layer of amorphous silica about 100 A thick, which is removed by HF but is re-formed in ambient humidity. Waddams found that the quartz surface in water released mosaic silica, presumably as particles of colloidal size, since they scattered light (120). This was confirmed by Sakabe et al. (121), who found that in neutral or alkaline aqueous suspension, quartz released both soluble silica and colloidal particles of crystalline nature, 0.01-0.3 microns in size. Stober and Arnold (122) found that the amount of silica released was much more than a monomolecular layer, and that it decreased with successive changes of water. When quartz was intensively pulverized in water, the disturbed surface layer can amount to as high as 35%, with a specific surface area of 70 m g, and the solubility is increased from less than 10 to 70 ppm at 25 C (123). Paterson and Wheatley (124) made similar observations. 

The shredded PVC waste (20 - 30 mm particle size) is burnt continuously together with brown coal dust, sand and calcium-chloride solution. Brown coal dust and sand improve the burn-out and the recovery of chlorine sand also bonds with heavy metals etc. in the slag. The third additive is a 30% calcium chloride solution, recirculated from the waste water unit. Under incineration conditions calcium chloride is converted to calcium silicate and HCl. 

Continental soil dust is usually contained in the size fraction between 1 and 20 pm in diameter, with the mass median diameter about 2-4 pm. Larger particles fall out quickly because of their large sedimentation velocities, although individual dust particles of several 100 pm have been collected in South America that have been transported from the Sahara (46). The reported size spectra of mineral particles (41,47) indicate that the mass of fine clay particles is one to two orders of magnitude smaller than that of coarse silt particles (1-10 pm). Noll et al. (48) compared the composition of coarse atmospheric particles at a urban and rural site. At the rural site, the coarse particles contained predominantly cmstal material (limestone 89% and silicates 6%) and biogenic material. Limestone had a mean mass diameter of about 20 pm, and silicates 12 pm. In urban samples, a significant fraction (25%) of rubber tire was found with a mean mass diameter of 25 pm. 

The sizes of the cometary dust grains vary from less than a micron to probably several centimeters. Infrared observations near 10 pm show the silicate spectral features. In addition, there seems to be a black ingredient presumed to be carbon. Due to different accelerations from the solar radiation pressure, the larger particles follow the comet close in its orbit and are more concentrated to the orbital plane. They become sometimes visible in the anti-tails , narrow spikes which point towards the Sun by an effect of projection when the Earth crosses the comet s orbital plane. Non of the meteorites found so far on Earth seem to be of cometary origin. However, very fluffy micron sized interplanetary dust grains (Brownlee particles) which have been collected by high flying aircraft are possibly cometary debris. 

Compositions of the pervasive layered deposits seen in Mars Global Surveyor imagery (Malin and Edgett, 2000) have not been measured directly. However, TES spectra, which are dominated by sand-sized particles, indicate that igneous minerals (pyroxenes and plagioclase) are abundant on the martian surface. Quartz, which should be readily detectable in TES spectra, has not been observed. Deconvolutions of the spectra of soils and dust are dominated by framework silicates, either plagioclase or zeolites (Bandfield and Smith, 2003 McSween et aL, 2003). 

Thus, it is necessary to consider the role of the dust in the ISM. Dust comprises about 1% ofthe mass ofthe material in the ISM and can act as a surface upon which chemistry can occur (Fig. 4). Although we have yet to actually examine a piece of interstellar dust we believe it is similar to that found in the Solar System and recently collected by the Stardust mission. The dust is either carbonaceous or silicate in nature, comprising of small particles, typically sub-microns in size, probably with an irregular (fractal ) structure. Being so cold (around 10 K) the dust grains act as a depository for any gaseous molecules which "stick" to the surface. Hence H atoms may collide with the surface, and subsequent reaction between such H ... 

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