The result is that the oxygen-to-silicon ratio is lower than in olivine instead of , and the net charge per silicon atom is less —2 instead of —4 , since fewer cations are necessary to balance that charge.
Pyroxene can also be written as Mg,Fe,Ca SiO 3 , where the elements in the brackets can be present in any proportion. In other words, pyroxene has one cation for each silica tetrahedron e. In olivine, it takes two divalent cations to balance the —4 charge of an isolated tetrahedron.
The diagram below represents a single chain in a silicate mineral. Count the number of tetrahedra versus the number of oxygen ions yellow spheres. Each tetrahedron has one silicon ion so this should give the ratio of Si to O in single-chain silicates e. The diagram below represents a double chain in a silicate mineral. Again, count the number of tetrahedra versus the number of oxygen ions.
This should give you the ratio of Si to O in double-chain silicates e. In amphibole structures, the silica tetrahedra are linked in a double chain that has an oxygen-to-silicon ratio lower than that of pyroxene, and hence still fewer cations are necessary to balance the charge. Amphibole is even more permissive than pyroxene and its compositions can be very complex.
Hornblende, for example, can include sodium, potassium, calcium, magnesium, iron, aluminum, silicon, oxygen, fluorine, and the hydroxyl ion OH —. In mica structures, the silica tetrahedra are arranged in continuous sheets, where each tetrahedron shares three oxygen anions with adjacent tetrahedra. There is even more sharing of oxygens between adjacent tetrahedra and hence fewer charge-balancing cations are needed for sheet silicate minerals.
Bonding between sheets is relatively weak, and this accounts for the well-developed one-directional cleavage Figure 2. Chlorite is another similar mineral that commonly includes magnesium.
In muscovite mica, the only cations present are aluminum and potassium; hence it is a non-ferromagnesian silicate mineral. When two oxygen of SiO 4 4- units share with other SiO 4 4- units, the silicates form a ring or an infinite chain. The stoichiometry of the silicates becomes SiO 3 n 2n-.
Benitoite BaTi SiO 3 3 contain three silica rings, but these are relaxed 6-atom rings. The precious stone beryl Be 3 Al 2 SiO 3 6 contain six-silica rings. Single chain silica are called pyroxenes.
Double chain silicates are called amphiboles , part of the double chain is shown here, same as the double chain shown in Inorganic Chemistry by Swaddle. These chains have a stoichiometry of Si 4 O 11 n 6n. You can easily identify one such unit in the diagram. The true asbestoses such as crocidolite or blue asbestos consist of double chain silicates. Asbestoses have been identified as carcinogens, and its application has since been limited due to a ban to limit its exposure to the public.
Most commercial asbestoses are chrysotile, which contain layers of silicate sheet as we shall below. Sheet slilicates are called phyllosilicates phyllo means leaflike. These silicates are easy to cleave as does graphite. Talc is a main ingredient of the soapstone steatite. The diagram below shows the arrangement of sheets in brucite, Mg OH 2 , in which the sheets consist of corner sharing octahedrons of Mg OH 6.
In chlorite, there are two types of sheets. Half of the sheets are the same as those of brucite, but half of the brucite-sheets are sandwiched between sheets of silicates. The talc consists of only the sandwiched sheets. The comercial asbestos chrysotile is a sheet silicate, but the sheets are rolled up like a tube. These tubes appear as fibers, and they are usually known as asbestos. As mentioned earlier, the SiO 4 4- units can share every oxygen with other units to form a three dimensional network, and quartz has such a structure.
A portion of such a framework is shown here. In this arrangement, the stoichiometry is reduced to SiO 2 , which is often called silica. A collection of small pieces of quartz is sand. Quartz is a group of minerals. Asbestos is the name applied to six naturally occurring minerals that are mined from the earth.
The different types of asbestos are:. Of these six, three are used more commonly. XRD revealed that even minerals with similar chemical formulas could have very different crystal structures, strongly influencing those minerals' chemical and physical properties. As scientists created XRD images of the atomic structure of minerals, they were better able to understand the nature of the bonds between atoms in the silicate and other crystals.
The result of this is that the silica tetrahedra can polymerize, or form chain-like compounds , by sharing an oxygen atom with a neighboring silica tetrahedron. The silicates are, in fact, subdivided based on the shape and bonding pattern of these polymers , because the shape influences the external crystal form, the hardness and cleavage of the mineral, the melting temperature, and the resistance to weathering. These different atomic structures produce recognizable and consistent physical properties, so it is useful to understand the structures at an atomic level in order to identify and classify the silicate minerals.
Identifying minerals in a rock may seem like an arcane exercise, but it is only by identifying minerals that we begin to understand the history of a given rock. The most common silicate minerals fall into four types of structures, described in more detail below: isolated tetrahedra, chains of silica tetrahedra, sheets of tetrahedra, and a framework of interconnected tetrahedra.
The link below opens a page in a new window, which contains 3-dimensional versions of these different structures. You can manipulate and compare the structures as you read about them. Olivine see Figures 2a and 2b below is the most common silicate of this type, and it makes up most of the mantle.
Because these minerals contain a relatively high proportion of iron and magnesium, they tend to be both dense and dark-colored. Because the tetrahedra are not polymerized, there are no consistent planes of internal atomic weakness, so they also have no cleavage. Garnet is another common mineral with this structure. When silicate anions polymerize, they share an oxygen atom with a neighboring tetrahedron.
Commonly, each tetrahedron will share two of its oxygen atoms, forming long chain structures. These metal cations commonly bond to multiple chains, forming bridges between the chains. Single-chain silicates include a common group called the pyroxenes, which are generally dark-colored see Figures 3a and 3b. Because the bonds within the tetrahedra are strong, planes of atomic weakness do not cross the chains; instead, pyroxenes have two cleavage planes parallel to the chains and at nearly right angles to each other.
Double chains form when every other tetrahedron in a single chain shares a third oxygen ion with an adjoining chain see Figure 4a. Like single chains, the double chains still maintain a net negative charge and bond to cations that can form bridges between multiple double chains. The most common amphibole is hornblende, a black mineral found in igneous rocks like granite and andesite see Figures 4b and 4c.
Amphiboles tend to form prismatic crystals with two cleavage planes at degrees to each other. Pyroxenes and amphiboles can be difficult to distinguish from one another, as they are both dark-colored, blocky minerals.
A careful examination of the angle between cleavage planes, described above, is required to identify them. The best way to tell single-chain silicate minerals from double-chain silicate minerals is by examining their. When every tetrahedron shares three of its oxygen ions with neighboring tetrahedra, sheets are formed see Figure 5a. Micas such as muscovite and biotite see Figure 5b are both common sheet silicates, notable for their one perfect cleavage.
This perfect cleavage results from the type of bonds that occur between sheets — van der Waals bonds.
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