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, such as olivines, garnets, pyroxenes, amphiboles, micas, feldspars, clay minerals, and quartzes. Therefore, it is essential for mineralogists to understand 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.

There are more reasons to study silicates than just their abundance. Silicates comprise a significant part of the soil. They are widely used to produce modern construction materials like brick, cement, ceramic, and glass. Some silicates make useful thermal and electrical insulators, complex nano filters, and water treatment absorbers. Additionally, many gem-quality silicates are used in the jewelry industry.

Silicate minerals can be divided into six groups based on crystal chemistry. By learning the basic principles of silicate structures, we can understand what is happening inside these minerals just by looking at their chemical formulas.

Crystal Chemistry of Silicates

The crystal chemistry of silicates is very complex. A [SiO₄]^4- tetrahedron is the fundamental building block for all silicates. The tetrahedron comprises one silicon atom in the center and four oxygen atoms at the corners. You can imagine it as a triangular pyramid.

Polymerization

Various silicate mineral species result from the bonding of [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 subclasses are named according to how silicon tetrahedra are linked (polymerized).

Silicate Subclasses

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

Nesosilicates

The easiest way to create a 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 the Greek nēsos for "island"). Sometimes, you may hear them referred to simply as "island silicates."

Sorosilicates

We can go further and 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 the typical structure for sorosilcates: [Si₂O₇]^6-. Because only two tetrahedrons are connected, this silicate subclass is sometimes called "disilicates" or "butterfly silicates."

Cyclosilicates

Let's look at more complex silicate structures. By joining three, four, or six tetrahedrons, we will get ring structures. Thus, this subgroup is called cyclosilicates or "ring silicates." The three-member ring will have a [Si₃O₉]^6- structural unit, the four-member ring will have a [Si₄O₁₂]^8- structural unit, and the most common six-member ring will have a [Si₆O₁₈]^12- structural unit.

Inosilicates

Imagine sorosilicates (pairs of tetrahedra) combined into one infinite chain. These are 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 create an infinite layer with [Si₄O₁₀]^4- units. This subclass is sometimes called "sheet" or "layered silicates" but is more commonly called phyllosilicates (from the Greek phýllon for "leaf").

Tectosilicates

The last and most complex silicate structure, where [SiO₄]^4- tetrahedra connect over all their four corners, forms a three-dimensional framework. Thus, this subclass is called tectosilicates or "framework silicates" with a [SiO₂]⁰ structural unit.

Silicates Summary

Here is a simple summary of the silicate mineral subclasses:

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

Tetrahedron configurations are negatively charged and, therefore, need to be balanced with positively charged cations to create a mineral. As a result, silicate mineral formulas contain many other elements. However, you can identify the silicate subclass by looking at the Si-O complex. This can provide hints about the mineral's structure and even some physical properties.

Introduction to Nesosilicates

In nesosilicates, each [SiO₄]^4- tetrahedron is isolated, meaning the oxygen corners are not shared with other [SiO₄]^4- tetrahedrons. Instead, the tetrahedra are connected with other cations like Mg⁺², Fe⁺², or Ca⁺². The most common example of nesosilicates is olivine (Mg,Fe)₂[SiO₄].

A Closer Look at Chemical Formulas, Subclasses, 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 indicates this mineral belongs to the nesosilicates subclass. Occasionally, the square brackets are replaced with round brackets or omitted entirely. However, for educational purposes, we will keep the square brackets in our articles.

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. (Note how isomorphism differs from both mineral polymorphism and pseudomorphism). 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 a comma, it indicates possible isomorphic substitution in the mineral.

Garnets and Isomorphism

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

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 is written as:

A₃B₂[SiO₄]₃

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

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

It is also correct to write the garnet group formula this way:

(Ca,Mg,Fe,Mn)₃(Al,Fe,Cr)₂[SiO₄]₃

However, this version looks very complicated.

Other Nesosilicates

Other nesosilicates include phenakite (Be₂[SiO₄]), willemite (Zn₂[SiO₄]), sillimanite (Al₂O[SiO₄]), andalusite (Al₂O[SiO₄]), and many others.

Introduction to Sorosilicates

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

Diagnostic Characteristics of Nesosilicates and Sorosilicates

Below, you'll find diagnostic characteristics to differentiate and identify some of the most common nesosilicates and sorosilicates. Included here are some of the most ubiquitous and economically significant members. We emphasize how to differentiate them from minerals with very similar appearances. The best diagnostic characteristics are highlighted in bold.

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

Nesosilicate Minerals

Structural unit: [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.

Distinguishing forsterite and fayalite is impossible without at least a density test. Regardless, they are considered olivines. Keep in mind that olivine is a mineral 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. It also 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

Distinguishing topaz from citrine (quartz) and aquamarine (beryl) can sometimes be challenging. Topaz's outstanding features are hardness (8 on the Mohs scale) and perfect basal cleavage. You can sometimes see this as flat bottoms on crystals. Though they may look faceted, no one has cut them. They are visible cleavage planes. However, these features aren't always observable in every crystal specimen.

One way to distinguish topaz from similarly colored quartz is to example the prism crystal faces of a specimen. Topaz faces are generally vertically striated, while quartz faces are striated perpendicularly to elongation. Observing crystal faces will also help distinguish topaz from aquamarine. Topaz has an orthorhombic crystal structure, so its prism form will be flattened in cross-section. Aquamarine has a hexagonal crystal structure, so its prism form will have a distinctive hexagonal shape.

Kyanite

Formula: Al₂O[SiO₄], or Al₂[Si₂O₅]

Identifying kyanite usually isn't difficult. Kyanite is known for its bladed, elongated crystal forms, perfect cleavage, and blue coloration. Another outstanding feature of kyanite is its anisotropic hardness. One crystal can have two different hardness values. Kyanite is softer (5-5.5) if you scratch it along its crystal elongation but harder (7) if you scratch it perpendicular to its 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 known for its brownish colors and cruciform twinning at 60^o or 90^o.

Titanite

Formula: CaTi[SiO₄](O,OH,F)

Titanite (also known as sphene in gemology) commonly occurs in yellow-green or brown colors and has a very high adamantine luster. Wedge-shaped crystal forms usually help distinguish titanite from staurolite, sphalerite, and olivine. Titanite is softer than staurolite (5-5.5 compared to 7-7.5). However, titanite is harder than sphalerite (3.5-4).

Garnet Group

Formula: A₃B₂[SiO₄]₃

You can usually easily distinguish garnets from other minerals due to their crystal forms, which are typically sub-euhedral to euhedral, showing dodecahedral or trapezohedral shapes. Garnets form in the cubic system, so their crystals are isometric, never elongated.

Differentiating between garnet mineral species can be difficult. You will need to examine a combination of colors, refractive index, and absorption spectrum. Uvarovite is perhaps the most unique species of the garnet group, since it forms as a cluster of tiny crystals rarely more than half a centimeter wide.

Below, you'll find the most common representatives of the garnet group and the end-members of pyrope-almandine-spessartite (pyralspite) series and uvarovite-grossular-andradite (ugrandite) series.

Remember, these minerals are all representatives of a single group, so many of their physical properties are almost identical. By comparing the characteristics of various garnet species, you can see how slight variations in chemical composition can influence color, hardness, and density, even though their 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

Sorosilicate Minerals

Structural unit: [Si₂O₇]^6-

Epidote

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

Epidote has greenish colors and perfect cleavage. It occurs in prismatic crystals and commonly forms in groups, which create fibrous aggregates with a dark-green color.

Zoisite

Formula: Ca₂Al₃O[SiO₄][Si₂O₇](OH)

Zoisite may be difficult to identify because it occurs in various colors and forms. However, most crystals are usually greenish brown. Crystals usually have a "chisel shape" and show heavy striation on their faces. Zoisites also have strong pleochroism, showing different colors when viewed from different sides.

Mineral and gem enthusiasts usually look for zoisite in two different forms. The first is as a green-colored aggregate associated with hornblende and corundum. This is commonly known as "ruby-in-zoisite." The second one is tanzanite, a very popular blue-purple color variety of zoisite. 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

Distinguishing vesuvianite from other minerals may prove difficult. Vesuvianite is well-known for yellow-green colors, similar to those of olivine group minerals. However, vesuvianite can also occur in brown, black, emerald-green, and even violet colors. Examine the crystal shape of potential vivianite specimens. Short columnar crystals in the form of aggregates are characteristic of vesuvianite.

References for Silicates

• 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/.

• 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

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

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

• 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

• 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

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

• 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.

• 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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