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 four thousand known minerals on Earth, most are rare. There are only a few that make up most of the rocks likely to be encountered by us. These are generally called the rock-forming minerals.

The silicon-oxygen tetrahedron (SiO₄) consists of a single silicon atom at the center and four oxygen atoms located at the four corners of the tetrahedron. Each oxygen ion has a -2 charge and the silicon ion has a +4 charge. The silicon ion shares one of its four electrons with each of the four oxygen ions in a covalent bond to create a symmetrical geometric four-sided pyramid figure. Only half of the oxygen’s electrons are shared, giving the silicon-oxygen tetrahedron an ionic charge of -4. This silicon-oxygen tetrahedron forms bonds with many other combinations of ions to form the large group of silicate minerals.

Figure \(\PageIndex{2}\): Ping pong ball model of a tetrahedron: balls are oxygen, small space in the center is silicon

The silicon ion is much smaller than the oxygen ions (see the figures) and fits into a small space in the center of the four large oxygen ions. The metal ball represents the silicon ion in the figure. Because only one of the electrons of the corner oxygens is shared, the silicon-oxygen tetrahedron has chemically active corners available to form bonds with other silica tetrahedra or other positively charged ions such as Al₊₃, Fe_+2,+3, Mg₊₂, K₊₁, Na₊₁, and Ca₊₂. Depending on many factors, such as the original magma chemistry, silica-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 chemical structure in which positively charged ions can be inserted for unique chemical compositions forming silicate mineral groups.

Figure \(\PageIndex{3}\): The silicon atom in the center of the tetrahedron (with the top oxygen removed) represented by a metal ball

The Dark Ferromagnesian Silicates

Figure \(\PageIndex{4}\): Green olivine in basalt

The Olivine Family

Olivine is the primary mineral component in mantle rock. 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 can freely substitute for each other in the same location in the crystal structure.

Figure \(\PageIndex{5}\): Tetrahedral structure of olivine

The crystal structure of olivine is built from independent silica tetrahedra. This arrangement, where the tetrahedra are not connected to one another, is also often referred to as having isolated tetrahedra.

Pyroxene Family

Figure \(\PageIndex{6}\): Crystals of diopside, a member of the pyroxene family

Pyroxenes are typically black or dark green in color and have a complex chemical composition that includes iron, magnesium, aluminum, and other elements bonded to the silica tetrahedra. They are built from long, single chains of silica tetrahedra formed as tetrahedra share two corner oxygens. The chains are bonded together into the crystal structures by metal positively charge ions.

Figure \(\PageIndex{7}\): Single chain

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.

Amphibole Family

Figure \(\PageIndex{1}\): Hornblende crystals

Figure \(\PageIndex{8}\): Elongated crystals of hornblende in orthoclase

Amphibole minerals are built from double chains of silica tetrahedra. Imagine two pyroxene chains that connect together by sharing the third oxygen on each tetrahedron.

Figure \(\PageIndex{9}\): Double chain structure

Amphiboles are composed of iron, magnesium, aluminum, and other positive ions bonded with silica tetrahedra. Their chemical formula is very complex and generally written as (RSi₄O₁₁)₂, where R represents many different ions. 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 equals Li, Na, Mg, Fe, Mn, or Ca; and Z is 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.

Sheet Silicates

Figure \(\PageIndex{10}\): (left) Sheet crystals of biotite mica. (right) A stack of sheets of muscovite mica

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.

Figure \(\PageIndex{11}\): Sheet structure of mica, view perpendicular to the sheets

Chemically, sheet silicates usually contain silicon and oxygen in a 2:5 ratio (Si₄O₁₀). Micas contain mostly silica, aluminum, and potassium.

Figure \(\PageIndex{12}\): (left) Crystal structure of a mica, view parallel to the sheets. (right) Mica “silica sandwich” structure related to layers in illite structure.

The illustration of the crystalline structure of mica shows the corner O atoms 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 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 breaking into sheets.

Framework Silicates

Figure \(\PageIndex{14}\): Quartz crystals

Quartz and feldspar are the two most abundant minerals in the continental crust. In fact, feldspar itself is the single most abundant mineral in the Earth’s crust. These minerals are classified as framework silicates. They are built with a three-dimensional framework of silica tetrahedra in which all four corner oxygens are shared with adjacent tetrahedra. 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.

Figure \(\PageIndex{15}\): Pink orthoclase crystals

Quartz is composed of pure silica, SiO₂. and lacks the holes and spaces that feldspar has. Impurities consisting of atoms within this framework give rise to many varieties of quartz among which are gemstones like amethyst (purple), rose quartz (pink), and citrine (yellow).