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Silicate minerals these are minerals consisting of si

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Unformatted text preview: the true ISM abundances are not solar, the pattern is representative. The most abundant elements (H and He) do not by themselves form solids under reasonable conditions, although H may be incorporated into molecules if combined with heavy elements. Of the metals, by far the most abundant are C and O. Ne does not participate in chemistry, and we will return to N later, but the subsequent metals (Mg, Si, Fe) do form solid minerals with oxygen (the silicates). Clearly many materials can be made out of these elements. The main cases that we will consider here, since they are observed in the diffuse ISM phases, are: Carbonaceous material: Ordinary carbon is found in the form of graphite, which consists of honeycomb ­like sheets of carbon atoms bonded to three others in a hexagonal pattern. The sheets are held together by van der Waals forces. Carbonaceous material may also consist largely of small honeycomb ­ like units but with impurities, defects, etc. rather than pure graphite sheets (soot!). The smallest carbonaceous grains are the polycyclic aromatic hydrocarbons (PAHs). As their name suggests, these are really molecules. The PAHs are characterized by honeycomblike2 carbon skeletons with hydrogen atoms bonded around the exterior. Silicate minerals: These are minerals consisting of Si atoms bonded in a tetrahedral pattern to 4 O atoms. There are many ways to connect these tetrahedral structures to each other. For example, the tetrahedral structures may share oxygen atoms, forming networks (O:Si = 2:1), long chains (O:Si = 3:1), or individual units (O:Si = 4:1). In the latter cases, the dangling oxygen 1 Asplund, Grevesse, Sauval, & Scott, ARA&A 47:481 (2009) 2 In a planar hydrocarbon, the 2p o...
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