Table 2.1.5 Important Minerals; Types of Minerals: Examples: Silicate minerals of common rocks: plagioclase feldspar, potassium feldspar, quartz, mica, amphibole, pyroxene and olivine: If a Na+ ion substitutes for a Ca+2 ion, it creates an unequal charge that must be balanced by other ionic substitutions elsewhere in the crystal. This texture, which indicates a very slow crystallization, is called pegmatitic. In pyroxene, the one divalent cation (2+) per tetrahedron balances that 2 charge. The result is that the oxygen-to-silicon ratio is lower than in olivine (3:1 instead of 4:1), and the net charge per silicon atom is less (2 instead of 4), since fewer cations are necessary to balance that charge. In addition to silica tetrahedra, feldspars include the cations aluminum, potassium, sodium, and calcium in various combinations. Felsic is a contraction formed from feldspar, the dominant mineral in felsic rocks. Classification of Igneous Rock Series. Rhyolite is a fine-crystalline felsic extrusive rock. Pyroxene compositions are of the type MgSiO3, FeSiO3, and CaSiO3, or some combination of these. This mineral group is composed of the carbonate ion and one or more kinds of positive ions. *Some of the formulas, especially the more complicated ones, have been simplified. Orthoclase feldspar (KAlSi3O8), also called potassium feldspar or K-spar, is made of silica, aluminum, and potassium. The metamorphic rock, amphibolite, is primarily composed of amphibole minerals. The three main feldspar minerals are potassium feldspar The simplest silicate structure, that of the mineral olivine, is composed of isolated tetrahedra bonded to iron and/or magnesium ions. The Henry Mountains of Utah are a famous topographic landform formed by this process. Within the cooling magma, the mineral crystals continue to grow until they solidify into igneous rock. Peacock, M. A. Members of the pyroxene family have a complex chemical composition that includes iron, magnesium, aluminum, and other elements bonded to polymerized silica tetrahedra. Chlorite is another similar mineral that commonly includes magnesium. Recall that for non-silicate minerals, we classified minerals into groups according to their anion or anionic group. Within these frameworks in feldspar are holes and spaces into which other ions like aluminum, potassium, sodium, and calcium can fit giving rise to a variety of mineral compositions and mineral names. In muscovite mica, the only cations present are aluminum and potassium; hence it is a non-ferromagnesian silicate mineral. Note that iron can exist as both a +2 ion (if it loses two electrons during ionization) or a +3 ion (if it loses three). When porphyritic, it often has either olivine or plagioclase phenocrysts. A fine-grained sheet silicate mineral that can accept water molecules into interlayer spaces, resulting is swelling. Plagioclase feldspar is not ferromagnesian, so it falls in the non-ferromagnesian (light minerals) region in Figure 7.16 even when it has a darker colour. In olivine, the 4 charge of each silica tetrahedron is balanced by two divalent (i.e., +2) iron or magnesium cations. All of the ions shown are cations, except for oxygen. Cut around the outside of the shape (solid lines and dotted lines), and then fold along the solid lines to form a tetrahedron. As already noted, the 2 ions of iron and magnesium are similar in size (although not quite the same). Significant examples include galena (lead sulfide), sphalerite (zinc sulfide), pyrite (iron sulfide, sometimes called "fool's gold"), and chalcopyrite (iron-copper sulfide). Biotite mica has more iron and magnesium and is considered a ferromagnesian silicate mineral. Hornblende, for example, can include sodium, potassium, calcium, magnesium, iron, aluminum, silicon, oxygen, fluorine, and the hydroxyl ion (OH). What are some non silicate minerals? As already noted, the +2 ions of iron and magnesium are similar in size (although not quite the same). Chlorite is another similar mineral that commonly includes magnesium. Diorite is identifiable by its Dalmatian-like appearance of black hornblende and biotite and white plagioclase feldspar. If the fragments accumulate while still hot, the heat may deform the crystals and weld the mass together, forming a welded tuff. To avoid these complications, the following figure presents a simplified version of igneous rock nomenclature focusing on the four main groups, which is adequate for an introductory student. (non-ferromagnesian) silicates with the dark (ferromagnesian) silicates and list three minerals common to each group. In olivine, unlike most other silicate minerals, the silica tetrahedra are not bonded to each other. Fe2+ is known as ferrous iron. Basalt is a fine-grained mafic igneous rock. The non-metallic and poorly-reactive mineral carbon is often found as a native element, such as graphite and diamonds. One type of clay, kaolinite, has a structure like an open-faced sandwich, with the bread being a single layer of silicon-oxygen tetrahedra and a layer of aluminum as the spread in an octahedral configuration with the top oxygens of the sheets. A number of minerals and their formulas are listed below. Ionic radii are critical to the composition of silicate minerals, so well be referring to this diagram again. Thats why pyroxenes can have iron (radius 0.63 ) or magnesium (radius 0.72 ) or calcium (radius 1.00 ) cations (see Figure \(\PageIndex{2}\) above). Silicate minerals are made of silicate groups that form rocks. This allows them to substitute for each other in some silicate minerals. Minerals in this solid solution series have different mineral names. In silicate minerals, these tetrahedra are arranged and linked together in a variety of ways, from single units to complex frameworks (Figure 2.9). The result is that the oxygen-to-silicon ratio is lower than in olivine (3:1 instead of 4:1), and the net charge per silicon atom is less (2 instead of 4). Pyroclastic texture is usually recognized by the chaotic mix of crystals, angular glass shards, and rock fragments. A very common family of framework silicate minerals. An angstrom is the unit commonly used for the expression of atomic-scale dimensions. The diagram below represents a single chain in a silicate mineral. Silica tetrahedra are bonded in three-dimensional frameworks in both the feldspars and quartz. This allows them to substitute for each other in some silicate minerals. The three main feldspar minerals are potassium feldspar, (a.k.a. Because of this size similarity, and because they are both divalent cations (both have a charge of +2), iron and magnesium can readily substitute for each other in olivine and in many other minerals. 2. Since the silicon ion has a charge of 4 and each of the four oxygen ions has a charge of 2, the silica tetrahedron has a net charge of 4. Laccoliths are blister-like, concordant intrusions of magma that form between sedimentary layers. Therefore, albite is NaAlSi3O8 (one Al and three Si) while anorthite is CaAl2Si2O8 (two Al and two Si), and plagioclase feldspars of intermediate composition have intermediate proportions of Al and Si. Ferromagnesian Silicates are silicates with iron (Fe) and/or magnesium (Mg) in their structure. A tetrahedron has a pyramid-like shape with four sides and four corners. Because each silicon ion is +4 and each oxygen ion is 2, the three oxygens (6) and the one silicon (+4) give a net charge of 2 for the single chain of silica tetrahedra. The large crystals are called phenocrysts and the fine-grained matrix is called the groundmass or matrix. The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Some pumice is so full of vesicles that the density of the rock drops low enough that it will float. Detrital sedimentary rocks are composed of mechanically weathered rock particles, like sand and gravel. The table below lists examples of oxides, sulphides, sulphates, halides, native elements and carbonates of economic value. Other rarer elements with similar properties to iron or magnesium, like manganese (Mn), can substitute into the olivine crystalline structure in small amounts. Halite, Calcite ________ exhibit a sheet-like silicate structure. If you are doing this in a classroom, try joining your tetrahedron with others into pairs, rings, single and double chains, sheets, and even three-dimensional frameworks. Olivine can be either Mg2SiO4 or Fe2SiO4, or some combination of the two (Mg,Fe)2SiO4. These are generally lighter-colored than the ferromagnesian silicates. Fe3+ is known as ferric iron. Pyroxene compositions are of the type MgSiO3, FeSiO3, and CaSiO3, or some combination of these, written as (Mg,Fe,Ca)SiO3, where the elements in the brackets can be present in any proportion. Silicate minerals are classified as being either ferromagnesian or non-ferromagnesian depending on whether or not they have iron (Fe) and/or magnesium (Mg) in their formula. The building block of all of these minerals is the silica tetrahedron, a combination of four oxygen atoms and one silicon atom. Common mafic rocks include basalt, diabase and gabbro. This is called a coupled-substitution.. They are. Figures 3.1.1, 3.1.2, 3.1.3, 3.1.4, 3.1.5, 3.1.6: Steven Earle. Feldspars are mostly silica with aluminum, potassium, sodium, and calcium. In addition to silica tetrahedra, feldspars include the cations aluminum, potassium, sodium, and calcium in various combinations. In addition to silica tetrahedra, feldspars include the cations aluminum, potassium, sodium, and calcium in various combinations. In this course, we will focus on just the isolated, single chain, double chain, sheet, and framework silicates. A silicate mineral that does not contain iron or magnesium (e.g., feldsspar). There is no need for aluminum or any of the other cations such as sodium or potassium. Physical Geology Lab Samples - Georgia Southwestern State University Feldspars, micas, and ferromagnesian minerals (pyroxenes, amphiboles, olivines, etc.) Quartz contains only silica tetrahedra.
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