Mineralogy of Silicates: Nesosilicates and Sorosilicates

Introduction to Silicates

Silicates are the principal constituents of the Earth's crust and mantle. The majority of minerals that make up rocks are silicates, like olivines, garnets, pyroxenes, amphiboles, micas, feldspars, clay minerals, and quartzes. Therefore, it is necessary to conduct extensive research to gain a better understanding of silicate minerals.

Silicate minerals are abundant in igneous rocks because silicon and oxygen are the most prevalent elements in the Earth's crust. Additionally, these minerals often contain other elements such as Al, Fe, Mg, Ca, Na, and K. Therefore, we will definitely meet them in the mineral formulas of silicates.

There are many other reasons to study silicates except just their abundance. Silicates comprise a significant part of the soil; they are widely used to produce modern construction materials like bricks, cement, ceramic, and glass. Some silicates have properties that make them useful as thermal and electric insulators, complex nano filters, and absorbers for water treatment. Additionally, many silicates are found in gem quality and are used in the jewelry industry.

The silicate minerals can be divided into six groups based on crystal chemistry. Due to the enormous amount of material, we will only cover two groups in this article. By understanding the basic principles of silicate structures, we can see what is happening inside the minerals just by looking at their chemical formulas.

Crystal Chemistry of Silicates

The crystal chemistry of silicates is very complex. Let's look at silicates as building blocks for children's toys, not to overload you with advanced chemistry. A [SiO₄]^4- tetrahedron is the fundamental building block for silicate. The tetrahedron comprises one silicon atom in the center and four oxygen atoms at the corners. You can imagine it as a triangular pyramid. This [SiO₄]^4- tetrahedron is the essential building block we will gradually construct all silicates with.

Various silicate mineral species result from bonding [SiO₄]^4- tetrahedrons with other tetrahedrons and cations. Mineralogists refer to silica tetrahedra combining with other 'blocks' as polymerization, a term borrowed from organic chemistry. So, silicate groups, or subclasses, are named according to how silicon tetrahedra are linked (polymerized).

Silicate Groups

There are six groups or subclasses of silicates based on the increasing polymerization of the Si-O complex (synonym to [SiO₄]^4- tetrahedron). We will gradually add [SiO₄]^4- tetrahedra to construct all silicate subclasses. Each subclass's 'building block,' or unit, is set in square brackets for clarity.

Nesosilicates

The easiest way to create silicate is to combine isolated tetrahedrons with cations. In that case, [SiO₄]^4- tetrahedra stay isolated and do not touch each other. This group is called orthosilicates, or nesosilicates (from Greek nēsos - 'island'). Sometimes, you can hear the simplified name 'island silicates.' These three terms are synonyms.

Sorosilicates

Further, we can put two [SiO₄]^4- tetrahedra together by connecting them via one corner. In that case, two tetrahedra share one oxygen atom. By calculating all atoms in this simple connection, we receive two atoms of silicon Si₂ and seven atoms of oxygen O₇. As a result, we have a typical structure of sorosilcates - [Si₂O₇]^6-. Because only two tetrahedrons are connected, you may find a synonym for this silicate subclass, 'disilicates' or, even simpler, 'butterfly silicates.'

Cyclosilicates

Let's make our structure more complex. By joining three, four, or six tetrahedrons, we will receive a ring. So, this subgroup is called ring silicates, or cyclosilicates. The three-membered ring will have [Si₃O₉]^6- structural unit, a 4-membered ring - [Si₄O₁₂]^8-, and the most common 6-membered ring - [Si₆O₁₈]^12-.

Inosilicates

Imagine sorosilicates (pairs of tetrahedra) combined into one infinite chain, and you will receive inosilicates, sometimes simply called chain silicates. They are further subdivided into single chains with [Si₂O₆]^4- units and double chains with [Si₄O₁₁]^6-.

Phyllosilicates

Combining double chains will have an infinite layer with [Si₄O₁₀]^4- unit.

Sometimes, this subclass is called sheet or layered silicates, but mostly phyllosilicates (from Greek phýllon' leaf').

Tectosilicates

The last and most complex structure, where [SiO₄]^4- tetrahedra connect over all their four corners, forms a three-dimensional framework. Therofere is sometimes called 'framework silicates' with unit - [SiO₂]⁰.

Silicates Summary

We understand that it looks hard at first. To summarize, here is a simple list of silicate mineral subclasses:

1. Nesosilicates: [SiO₄]^4-
2. Sorosilicates: [Si₂O₇]^6-
3. Cyclosilicates: [Si₆O₁₈]^12-
4. Inosilicates:[Si₂O₆]^4- and [Si₄O₁₁]^6-
5. Phyllosilicate: [Si₄O₁₀]⁴ or sometimes written as [Si₂O₅]^2-
6. Tectosilicates: [SiO₂]⁰

Tetrahedron configurations are negatively charged, and therefore, they need to be balanced with positively charged cations to create a mineral. As a result, the mineral formula of silicates contains many other elements. However, if you quickly look at the Si-O complex, you can identify which group of silicates you are dealing with. This can provide hints about the mineral's structure and even some physical properties.

This article will cover the two first subclasses, nesosilicates and sorosilicates, and their most common mineral representatives.

Introduction to Nesosilicates

We have already learned that each [SiO₄]^4- tetrahedron is isolated in nesosilicates, meaning the oxygen corners are not shared with other [SiO₄]^4- tetrahedrons. In nesosilicates, tetrahedra are connected with other cations like Mg⁺², Fe⁺², or Ca⁺². The most common mineral example to illustrate nesosilicates is olivine (Mg,Fe)₂[SiO₄].

A Closer Look at Chemical Formula, Subclass, and Isomorphism

Let's take the olivine formula as an example and try to understand what it tells us.

Olivine: (Mg,Fe)₂[SiO₄]

First, pay attention to the [SiO₄] complex in square brackets. It gives us an indication that this mineral belongs to the nesosilicates subclass. Occasionally, square brackets are replaced with round brackets or omitted entirely, but we have chosen to keep them in entire text for your convenience.

The other part of the formula (Mg,Fe) provides information about isomorphism, also known as isomorphous replacement. Isomorphism occurs in minerals when certain elements are replaced by another element of the same valency without changing the crystal structure of the mineral. Olivine is an example of such a mineral, where magnesium and iron are replaced by each other. This makes it a solid solution between forsterite Mg₂[SiO₄] and fayalite Fe₂[SiO₄]. Magnesium and iron have similar characteristics, so they can easily replace each other in a crystal structure. As a result, it isn't easy to find pure forsterite or pure fayalite in nature.

When you see elements in round brackets separated by coma, it indicates possible isomorphic substitution in the mineral.

Garnets

Garnet is a group of minerals, not a single mineral species. The garnet group includes a lot of mineral species, like pyrope and almandine.

The garnets have a complex composition due to the high possibility of isomorphic substitution of cations. The end members are known as series: pyrope-almandine-spessartite (pyralspite) and uvarovite-grossular-andradite (ugrandite). Because of various possible isomorphic substitutions, the garnet group formula looks the following way:

A₃B₂[SiO₄]₃

A stands for divalent cations Ca²⁺, Mg²⁺, Fe²⁺, Mn²⁺

B stands for trivalent cations Al³⁺, Fe³⁺, Cr³⁺

Based on the information in the previous section on chemical formulas, it is also correct to write the garnet group formula this way:

(Ca,Mg,Fe,Mn)₃(Al,Fe,Cr)₂[SiO₄]₃

but in this case, it looks very complicated.

Other Nesosilicates

There are also a lot of other nesosilicates like phenakite Be₂[SiO₄], willemite Zn₂[SiO₄], sillimanite Al₂O[SiO₄], andalusite Al₂O[SiO₄] and many others.

Introduction to Sorosilicates

The sorosilicates' structure is represented by two linked [SiO₄]^4- tetrahedrons isolated from all other tetrahedrons. Hence, the basic structural unit is [Si₂O₇]^6-. A good example of a sorosilicate is the mineral hemimorphite - Zn₄[Si₂O₇](OH)*H₂O. Some sorosilicates are a combination of single and double tetrahedrons, like in epidote - Ca₂(Fe⁺³,Al)Al₂[SiO₄][Si₂O₇](OH).

Diagnostic Characteristics of Nesosilicates and Sorosilicates

Here, we would like to provide diagnostic characteristics to differentiate and identify some of the most common nesosilicates and sorosilicates. We are giving the most ubiquitous and economically significant ones like olivine, zircon, topaz, kyanite, staurolite, titanite, garnets, epidote, zoisite, and vesuvianite, emphasizing how to differentiate them from the most similarly looking minerals. The best diagnostic characteristics are highlighted in bold.

Some similar properties of silicates are their elevated hardness (generally more than 5), vitreous luster, high transparency, and white streak.

Nesosilicates

[SiO₄]⁴⁻

Olivine

Formula: (Mg,Fe)₂[SiO₄]

Forsterite: Mg₂[SiO₄]

Fayalite: Fe₂[SiO₄]

Olivine is easily recognizable due to its yellow-green color and granular occurrence. Olivine is typically found in granular aggregates in basalt rocks.

Differentiating forsterite and fayalite is impossible without at least a density test. Therefore, people agreed to call them olivine simply. But remember that olivine is a group, not a mineral species. Green, gem-quality olivines are known as peridots in the gem and jewelry world.

Zircon

Formula: Zr[SiO₄]

Zircon typically occurs in tabular to prismatic crystals and in brownish colors. Also, it has high density, so it feels heavier than many other silicates. Another characteristic of zircon is its luminescence in pale yellow to vivid orange colors.

Topaz

Formula: Al₂[SiO₄](F,OH)₂

Sometimes, it is challenging to tell apart topaz from quartz and aquamarine. Topaz's outstanding features are hardness - 8 on the Mohs scale and perfect basal cleavage, so you can usually see the flat bottom of crystals (nobody cut them, it is a cleavage plane). However, even these features commonly cannot be observed.

So here is a small tip on distinguishing brown topaz from smoky quartz that nobody usually lists in their guides. Prism faces of topaz are generally vertically striated, while quartz prism is striated perpendicularly to elongation.

Sometimes, it's challenging to tell apart blue topaz from aquamarine, so close observation of the prism will give you a hint. Topaz is orthorhombic, so its prism will be flattened in cross-section, while aquamarine hexagonal prism will have an excellent hexagonal shape.

Kyanite

Formula: Al₂O[SiO₄], or Al₂[Si₂O₅]

Identification of kyanite doesn't usually have any issues. Kyanite is typical for its bladed, tabular, elongated crystal forms, perfect cleavage, and blue coloration. Another outstanding feature of kyanite is its anisotropy of hardness. You can see two different hardness values within one crystal. Kyanite is softer (5-5.5) if you try to scratch it along with crystal elongation but is harder (7) perpendicular to crystal elongation.

When scratch tested along its crystal elongation, kyanite has a hardness of 5-5.5. However, when scratched by the same tool across its crystal elongation, the same kyanite crystal resists scratching. Photos © International Gem Society/Olena Rybnikova, PhD.

Staurolite

Formula: Fe²⁺₂Al₉O₆[SiO₄]₄(O,OH)₂

Staurolite is typical with its brownish colors and very common crystal twinning at 60^o and as 90^o cruciform twins.

Titanite

Formula: CaTi[SiO₄](O,OH,F)

Titanite (also known as sphene in gemology) commonly occurs in yellow-green or brown color and has a very high adamantine luster. Wedge-shaped forms of crystals are usually helpful for differentiating titanite from staurolite, sphalerite, or olivine. Unlike staurolite (7-7.5), titanite is softer (5-5.5). However, titanite is harder than sphalerite (3.5-4).

Garnet Group

Formula: A₃B₂[SiO₄]₃

It is easy to differentiate garnet minerals from others due to their crystal habitus, which is typically subhedral to euhedral crystals, showing the dodecahedron or trapezohedral forms. Garnets are minerals of cubic symmetry, so their crystals are never elongated but isometric; also, they never show pleochroism. To differentiate garnet mineral species, color, and density will come in handy. Uvarovite is quite an outstanding representative of the garnet group to be told apart as it forms a group of tiny crystals that are rarely more than half of cm.

We are listing the most common representative of garnet group and end-members of pyrope-almandine-spessartite (pyralspite) series and uvarovite-grossular-andradite (ugrandite) series.

These minerals are representative of one group, so their physical properties are mostly identical. By comparing the characteristics of various species, you can see how slight variations in chemical composition influence color, hardness, and density while the crystal system and morphology stay the same.

Pyralspite Series

Pyrope

Formula: Mg₃Al₂[SiO₄]₃

Almandine

Formula: Fe²⁺₃Al₂[SiO₄]₃

Spessartite

Formula: Mn²⁺₃Al₂[SiO₄]₃

Ugrandite Series

Andradite

Formula: Ca₃Fe³⁺₂[SiO₄]₃

Grossular

Formula: Ca₃Al₂[SiO₄]₃

Uvarovite

Formula: Ca₃Cr₂[SiO₄]₃

Sorosilicates

[Si₂O₇]^6-

Epidote

Formula: Ca₂(Fe³⁺,Al)Al₂O[SiO₄][Si₂O₇](OH)

Epidote is outstanding for its greenish colors and perfect cleavage. It occurs in prismatic crystals and commonly in groups making fibrous aggregates of dark-green color.

Zoisite

Formula: Ca₂Al₃O[SiO₄][Si₂O₇](OH)

Zoisite is also a pretty hard mineral for identification as it occurs in various colors and forms. The most common crystals you may see are greenish-brown in color. Crystals are characteristic of their chisel shape and are heavenly striated. One of the zoisite's distinguishing features is strong pleochroism (different colors when viewed from different sides).

People usually see zoisite in two different forms. The first is as a green-colored aggregate associated with hornblende and ruby. This is popularly known as "ruby-in-zoisite." The second one is tanzanite, a blue-purple color variety of zoisite. Green color and association with corundum and vivid blue and purple colors with strong pleochroism are also a hint for zoisite identification for you. (Gem dealers sometimes call gem-quality zoisites with other colors besides blue or purple "fancy colored tanzanites." However, the name tanzanite should be reserved for zoisites with blue-purple color only).

Vesuvianite

Formula: Ca₁₀(Mg,Fe)₂Al₄[SiO₄]₅[Si₂O₇]₂(OH)₄

It isn't easy to tell apart vesuvianite from many other minerals. Vesuvianite is commonly known for its yellow-green colors, similar to olivine group minerals. However, vesuvianite can surprisingly occur in different colors like brown, black, emerald green, and even violet. Therefore, it is always necessary to look at the crystal shape. Short columnar crystals in the form of aggregates are characteristic of vesuvianite.

References for Silicates

1. Anthony, J. W., Bideaux,R. A., Bladh, K. W., & Nichols, M C. (2001). Handbook of Mineralogy, Mineralogical Society of America, Chantilly, VA 20151-1110, USA. http://www.handbookofmineralogy.org/.

1. Armbruster, T., Bonazzi, P., Akasaka, M., Bermanec, V., Chopin, C., Gieré, R., … & Pasero, M. (2006). Recommended nomenclature of epidote-group minerals. European Journal of Mineralogy, 18(5), 551-567. link

1. Baxter, E. F., Caddick, M. J., Ague, J. J. (2013). Garnet: common mineral, uncommonly useful. Elements 9:415-419 link

1. Klein, C., & Dutrow, B. (2007). Manual of mineral science. John Wiley & Sons, 704 p.

1. Novak, G. A., & Gibbs, G. V. (1971). The crystal chemistry of the silicate garnets. American Mineralogist: Journal of Earth and Planetary Materials, 56(5-6), 791-825. link

1. Okrusch, M. & Frimmel, H. E. (2020). Mineralogy: An introduction to minerals, rocks, and mineral deposits. Springer Nature, 719 p. https://doi.org/10.1007/978-3-662-57316-7

1. Papike, J. J. (1987). Chemistry of the rock‐forming silicates: Ortho, ring, and single‐chain structures. Reviews of Geophysics, 25(7), 1483-1526. link

1. Ribbe, P. H., & Gibbs, G. V. (1971). The crystal structure of topaz and its relation to physical properties. American Mineralogist: Journal of Earth and Planetary Materials, 56(1-2), 24-30.

1. Strunz, H., & Nickel, E. H .(2001). Strunz mineralogical tables. Schweizerbart, Stuttgart. 869 p.

Olena Rybnikova, PhD

Olena Rybnikova is a gemologist and mineralogist. She has a PhD in mineralogy and petrology specializing in beryllium minerals and is a certified Applied Jewelry Professional accredited by the Gemological Institute of America. Her passion is actively promoting knowledge and appreciation of nature, geology, and gemstones.

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