3.3: Silicate Minerals

Minerals are categorized based on their composition and structure. Silicate minerals are built around a molecular ion called the silicon-oxygen tetrahedron. A tetrahedron has a pyramid-like shape with four sides and four corners. Silicate minerals form the largest group of minerals on Earth, comprising the vast majority of the Earth’s mantle and crust. Of the nearly 4,000 known minerals on Earth, most are rare. There are only a few that make up most of the rocks likely to be encountered by surface-dwelling creatures like us. These are generally called the rock-forming minerals.

Figure \(\PageIndex{1}\): Rotating animation of a tetrahedron. (By Cyp; CC BY-SA 3.0 via Wikimedia Commons.)

The silicon-oxygen tetrahedron (SiO₄) consists of a single silicon atom and four oxygen atoms located at the four corners of the tetrahedron. The silicon ion is much smaller than the oxygen ions and fits in the space in the center of the oxygen ions. Each oxygen ion has a -2 charge and the silicon ion has a +4 charge. The silicon ion shares one of its four valence electrons with each of the four oxygen ions in a covalent bond to create a symmetrical four-sided pyramid figure. Only half of the oxygen’s valence electrons are shared, giving the silicon-oxygen tetrahedron an ionic charge of -4. Because only one of the valence electrons of the corner oxygens is shared, the silicon-oxygen tetrahedron has chemically active corners available to form bonds with other silicate tetrahedra or other positively charged ions such as Al^₃₊, Fe^_2+,3+, Mg^₂₊, K^₊, Na^₊, and Ca^₂₊. Depending on many factors, such as the original magma chemistry, silicon-oxygen tetrahedra can combine with other tetrahedra in several different configurations. For example, tetrahedra can be isolated, attached in chains, sheets, or three-dimensional structures. These combinations and others create the unique chemical compositions forming the large group of silicate minerals.

Figure \(\PageIndex{2}\): Silicon-oxygen tetrahedron. (By Helgi; CC BY-SA 3.0 via Wikimedia Commons.)

The Dark Ferromagnesian Silicates

Olivine Family

Olivine is the primary mineral component in mantle rock such as peridotite and basalt. It is characteristically green when not weathered. The chemical formula is (Fe,Mg)₂SiO₄. As previously described, the comma between iron (Fe) and magnesium (Mg) indicates these two elements occur in a solid solution. Not to be confused with a liquid solution, a solid solution occurs when two or more elements have similar properties and can freely substitute for each other in the same location in the crystal structure. Chemically, olivine is mostly silicate, iron, and magnesium and therefore is grouped among the dark-colored ferromagnesian (iron=ferro, magnesium=magnesian) or mafic minerals, a contraction of their chemical symbols Ma and Fe. Ferromagnesian silicates tend to be more dense than non-ferromagnesian silicates. This difference in density ends up being important in controlling the behavior of the igneous rocks that are built from these minerals: whether a tectonic plate subducts or not is largely governed by the density of its rocks, which are in turn controlled by the density of the minerals that comprise them.

Figure \(\PageIndex{3}\): Green olivine in basalt. (By Vsmith; CC BY-SA 3.0, via Wikimedia Commons.)

Olivine is referred to as a mineral family because of the ability of iron and magnesium to substitute for each other. The iron and magnesium ions in the solid solution are about the same size and charge, so either atom can fit into the same location in the growing crystals. Within the cooling magma, the mineral crystals continue to grow until they solidify into igneous rock. The relative amounts of iron and magnesium in the parent magma determine which minerals in the series form. Olivine can form crystals of all iron as one end member (called fayalite), all magnesium as the other end member (called forsterite), and mixtures of iron and magnesium in between. Different mineral names are applied to compositions between the end members. Other rarer elements with similar properties to iron or magnesium, like manganese (Mn), can substitute into the olivine crystalline structure in small amounts. Such ionic substitutions in mineral crystals give rise to the great variety of minerals and are often responsible for differences in color and other properties within a group or family of minerals.

The crystal structure of olivine is built from independent silicate tetrahedra. Minerals with independent tetrahedral structures are called neosilicates (or orthosilicates). In addition to olivine, other common neosilicate minerals include garnet, topaz, kyanite, and zircon.

Figure \(\PageIndex{4}\): Tetrahedral structure of olivine. (By Matanya; public domain via Wikimedia Commons.)

Two other similar arrangements of tetrahedra are close in structure to the neosilicates and grade toward the next group of minerals, the pyroxenes. In a variation on independent tetrahedra called sorosilicates, there are minerals that share one oxygen between two tetrahedra and include minerals like pistachio-green epidote, a gemstone. Another variation are the cyclosilicates, which as the name suggests, consist of tetrahedral rings, and include gemstones such as beryl, emerald, aquamarine, and tourmaline.

Pyroxene Family

Pyroxene is another family of dark ferromagnesian minerals, typically black or dark green in color. Members of the pyroxene family have a complex chemical composition that includes iron, magnesium, aluminum, and other elements bonded to polymerized silicate tetrahedra. Polymers are chains, sheets, or three-dimensional structures, and are formed by multiple tetrahedra covalently bonded via their corner oxygen atoms. Pyroxenes are commonly found in mafic igneous rocks such as peridotite, basalt, and gabbro, as well as metamorphic rocks like eclogite and blue-schist.

Figure \(\PageIndex{5}\): Crystals of diopside, a member of the pyroxene family. (By Robert M. Lavinsky, iRocks.com; CC BY-SA 3.0, via Wikimedia Commons.)

Pyroxenes are built from long, single chains of polymerized silicate tetrahedra in which tetrahedra share two corner oxygens. The silicate chains are bonded together into the crystal structures by metal cations. A common member of the pyroxene family is augite, which forms in several solid solution series with a complex chemical formula (Ca,Na)(Mg,Fe,Al,Ti)(Al,Si)₂O₆ that gives rise to a number of individual mineral names.

Figure \(\PageIndex{6}\): Single chain tetrahedral structure in pyroxene. (By Bubenik; CC BY-SA 3.0 via Wikimedia Commons.)

This single-chain crystalline structure bonds with many elements, which can also freely substitute for each other. The generalized chemical composition for pyroxene is XZ(Al,Si)₂O₆. X represents the ions Na, Ca, Mg, or Fe, and Z represents Mg, Fe, or Al. These ions have similar ionic sizes, which allows many possible substitutions among them. Although the cations may freely substitute for each other in the crystal, they carry different ionic charges that must be balanced out in the final crystalline structure. For example, Na has a charge of +1, but Ca has a charge of +2. If a Na⁺ ion substitutes for a Ca²⁺ ion, it creates an unequal charge that must be balanced by other ionic substitutions elsewhere in the crystal. Note that ionic size is more important than ionic charge for substitutions to occur in solid solution series in crystals.

Amphibole Family

Amphibole minerals are built from polymerized double silicate chains and they are also referred to as inosilicates. Imagine two pyroxene chains that connect together by sharing the third oxygen on each tetrahedron. Amphiboles are usually found in igneous and metamorphic rocks and typically have a long-bladed crystal habit. The most common amphibole, hornblende, is usually black; however, they come in a variety of colors depending on their chemical composition. The metamorphic rock, amphibolite, is primarily composed of amphibole minerals.

Figure \(\PageIndex{7}\): Double chain structure of inosilicates. (By Bubenik; CC BY-SA 3.0 via Wikimedia Commons.)

Amphiboles are composed of iron, magnesium, aluminum, and other cations bonded with silicate tetrahedra. These dark ferromagnesian minerals are commonly found in gabbro, basalt, diorite, and often form the black specks in granite. Their chemical formula is very complex and generally written as (RSi₄O₁₁)₂, where R represents many different cations. For example, it can also be written more exactly as AX₂Z₅((Si,Al,Ti)₈O₂₂)(OH,F,Cl,O)₂. In this formula A may be Ca, Na, K, Pb, or blank; X can be Li, Na, Mg, Fe, Mn, or Ca; and Z can be Li, Na, Mg, Fe, Mn, Zn, Co, Ni, Al, Cr, Mn, V, Ti, or Zr. The substitutions create a wide variety of colors such as green, black, colorless, white, yellow, blue, or brown. Amphibole crystals can also include hydroxide ions (OH^–)^, which occur from an interaction between the growing minerals and water dissolved in the magma.

Figure \(\PageIndex{8}\): Left: Hornblende crystals. (By USGS; public domain via Wikimedia Commons.) Right: Elongated crystals of hornblende in orthoclase. (By Dave Dyet; public domain via Wikimedia Commons.)

Sheet Silicates

Sheet silicates are built from tetrahedra which share all three of their bottom corner oxygens thus forming sheets of tetrahedra with their top corners available for bonding with other atoms. Micas and clays are common types of sheet silicates, also known as phyllosilicates. Mica minerals are usually found in igneous and metamorphic rocks, while clay minerals are more often found in sedimentary rocks. Two frequently found micas are dark-colored biotite, frequently found in granite, and light-colored muscovite, found in the metamorphic rock called schist.

Figure \(\PageIndex{9}\): Left: Crystal of biotite mica. (By Fred Kruijen; CC BY-SA 3.0 NL via Wikimedia Commons.) Right: A stack of sheets of muscovite mica. (By USGS; public domain via Wikimedia Commons.)

Chemically, sheet silicates usually contain silicon and oxygen in a 2:5 ratio (Si₄O₁₀). Micas contain mostly silicate, aluminum, and potassium. Biotite mica has more iron and magnesium and is considered a ferromagnesian silicate mineral. Muscovite micas belong to the felsic silicate minerals. Felsic is a contraction formed from feldspar, the dominant mineral in felsic rocks, and silicate.

Figure \(\PageIndex{10}\): Sheet structure of mica, view perpendicular to the sheets. (By Benjah-bmm27; public domain via Wikimedia Commons.)

The illustration of the crystalline structure of mica shows the corner oxygen atoms (O) bonded with K, Al, Mg, Fe, and Si atoms, forming polymerized sheets of linked tetrahedra, with an octahedral layer of Fe, Mg, or Al, between them. The yellow potassium ions form Van der Waals bonds (attraction and repulsion between atoms, molecules, and surfaces) and hold the sheets together. Van der Waals bonds differ from covalent and ionic bonds, and exist here between the sandwiches, holding them together into a stack of sandwiches. The Van der Waals bonds are weak compared to the bonds within the sheets, allowing the sandwiches to be separated along the potassium layers. This gives mica its characteristic property of easily cleaving into sheets.

Figure \(\PageIndex{11}\): Left: Crystal structure of a mica, view parallel to the sheets. (By USGS; public domain via Wikimedia Commons.) Right: Mica silicate "sandwich” structure. (By Kindred Grey from Sandwich by Alex Muravev; CC BY 4.0 via Introduction to Earth Science.)

Clays minerals occur in sediments formed by the weathering of rocks and are another family of silicate minerals with a tetrahedral sheet structure. Clay minerals form a complex family and are an important component of many sedimentary rocks. Other sheet silicates include serpentine and chlorite, found in metamorphic rocks.

Clay minerals are composed of hydrous aluminum silicates. 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.

Figure \(\PageIndex{12}\): Structure of kaolinite. (By USGS; public domain)

Framework Silicates

Quartz and feldspar are the two most abundant minerals in the continental crust. There are two types of feldspar: one containing potassium and abundant in felsic rocks of the continental crust, and the other with sodium and calcium and abundant in the mafic rocks of oceanic crust. Together with quartz, these minerals are classified as framework silicates. They are built with a three-dimensional framework of silicate tetrahedra in which all four corner oxygens are shared with adjacent tetrahedra. Framework silicates are called tectosilicates and also include the alkali metal-rich feldspathoids and zeolites.

Figure \(\PageIndex{13}\): Pink orthoclase feldspar crystals. (By Didier Descouens; CC BY-SA 4.0, via Wikimedia Commons.)

Quartz is a felsic mineral composed of pure silica, SiO₂ with the tetrahedra arranged in a three-dimensional framework. Impurities consisting of atoms within this framework give rise to many varieties of quartz among which are gemstones like amethyst, rose quartz, and citrine.

Figure \(\PageIndex{14}\): Freely growing quartz crystals showing crystal faces. (By Didier Descouens; CC BY-SA 4.0 via Wikimedia Commons.)

Feldspars are mostly silica with aluminum, potassium, sodium, and calcium. The silica tetrahedra framework has 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. Aluminum, which has a similar ionic size to silicon, can substitute for silicon inside the tetrahedra. Because potassium ions are so much larger than sodium and calcium ions, which are very similar in size, the inability of the crystal lattice to accommodate both potassium and sodium/calcium gives rise to the two families of feldspar: orthoclase and plagioclase respectively. Orthoclase feldspar (KAlSi₃O₈), also called potassium feldspar or K-spar, is made of silica, aluminum, and potassium. In plagioclase feldspar, (Ca,Na)AlSi₃O₈, the solid solution (Ca,Na) indicates a series of minerals, one end with calcium CaAl₂Si₂O₈, called anorthite, and the other end with sodium NaAlSi₃O₈, called albite.

Figure \(\PageIndex{15}\): Crystal structure of potassium feldspar. (By Taisiya Skorina, Antoine Allanore; CC BY 3.0 via Research Gate.)

Feldspars are usually found in igneous rocks, such as granite, rhyolite, and basalt as well as metamorphic rocks and detrital sedimentary rocks. Detrital sedimentary rocks are composed of mechanically weathered rock particles, like sand and gravel. Quartz is especially abundant in detrital sedimentary rocks because it is very resistant to disintegration by weathering. While quartz is the most abundant mineral on the Earth’s surface due to its durability, the feldspar minerals are the most abundant minerals in the Earth’s crust, comprising roughly 50% of the total minerals that make up the crust.

Figure \(\PageIndex{16}\): Pie chart of mineral abundance in Earth's crust. (By Kindred Grey; CC BY 4.0 via Introduction to Earth Science.)