Mineralogy of Cyclosilicates and Inosilicates

Crystal Chemistry of Cyclosilicates and Inosilicates

This article delves deeper into the more complex combination of tetrahedrons found in cyclosilicates and inosilicates.

Cyclosilicate and inosilicate subclasses comprise numerous mineral groups and important mineral species. Cyclosilicates' most familiar representatives are beryl and the tourmaline supergroup. Inosilicates are subdivided into single-chain silicates with pyroxene supergroup and double-chain amphibole minerals.

Introduction to Cyclosilicates

Cyclosilicate units form by joining three, four, or six SiO₄ tetrahedrons closed into a ring. So sometimes you can run into ring silicates synonym names of this group. By joining three tetrahedrons, a three-membered ring is created, so there is a [Si₃O₉]^6- structural unit, a 4-membered ring will logically have [Si₄O₁₂]^8-, and the most common 6-membered ring - [Si₆O₁₈]^12-.

An example of a three-membered cyclosilicate with [Si₃O₉]^6- unit is a rare California state gem benitoite, BaTi[Si₃O₉].

An example of four-membered cyclosilicate with [Si₄O₁₂]^8- unit is a quite rare papagoite Ca₂Cu₂Al₂[Si₄O₁₂](OH)₆.

The six-membered ring's iconic example is beryl, which even exhibits its inner structure in the crystal habit of a hexagonal prism.

Tourmaline Supergroup

Tourmaline is not a single mineral species but rather a name given to a group of minerals. This group includes many different mineral species, with elbaite and fluor-liddicoatite being the most common ones. These minerals are known for occurring in a variety of colors, much like a rainbow, and they have high transparency, making them suitable for use as gemstones.

The general formula for tourmaline minerals can be written the following way:

XY₃Z₆[T₆O₁₈](BO₃)₃V₃W

X, Y, Z, T. V, and W are sites that can be occupied by different elements

According to Hawthorne and Henry (1999), they are:

X = Ca²⁺, Na⁺, K⁺, ⃞ (vacant site)

Y = Li⁺, Mg²⁺, Fe²⁺, Mn²⁺, Al³⁺, Cr³⁺, V³⁺, Fe³⁺, (Ti⁴⁺)

Z = Mg²⁺, Al³⁺, Fe³⁺, V³⁺, Cr³⁺

T = Si⁴⁺, Al³⁺, (B³⁺)

B = B³⁺, (⃞)

V = OH, O

W = OH, F, O

Si-Al Substitution: Aluminum Coordination

While checking tourmaline sites, you may wonder why there is a T site that stands for silicon and aluminum, not only for silica-making simple [Si₆O₁₈]^12- units as we used to see. This important crystal-chemical principle complicates silicate structure and gives us more diverse minerals.

Aluminum atomic radius is very similar to silicon's radius, so aluminum can easily substitute Si in silicon tetrahedra, creating some aluminum tetrahedra. This can also be called aluminum in [4]-coordination, meaning Al can be linked to four oxygen atoms. You may see Al^[4] — a number in square brackets means coordination, not charge.

Aluminum can also occur in [6]-coordination and can be linked to six oxygens creating octahedral units. This aluminum can substitute for cations like Mg and Fe.

Possible Substitutions

A wide range of possible substitutions gives rise to highly variable chemical composition. Based on the dominating cation in the X position, the following subgroups are distinguished:

• alkali tourmalines
• calcic tourmalines
• X-site vacant tourmalines

The primary tourmaline groups can be determined by the ternary system based on the dominant occupancy of the X site. A general series of tourmaline species based on the anion occupancy of the W site. You are welcome to check ternary diagrams.

Henry D. et al. (2011) provide a modern list of tourmaline supergroup species:

Alkali Subgroup Tourmalines

Calcic Subgroup Tourmalines

Vacant Subgroup Tourmalines

Identifying Tourmaline Subgroups

It is only possible to identify a member of the tourmaline supergroup with a chemical analysis. Do not confuse varietal names of tourmalines like rubellite or indicolite with mineral species. Varietal names are just color-based tourmaline names to emphasize color and have nothing in common with crystal structure and chemical composition.

Introduction to Inosilicates

When combining SiO₄ tetrahedra into infinite chains without making cycles or rings, we will receive inosilicates, sometimes simply called chain silicates. They are subdivided into single chains with [Si₂O₆]^4- units and double chains with [Si₄O₁₁]^6-. Double chains can be visualized as two chains connected.

To understand the difference between a single and double chain, you may think of a single chain as a shoelace and a double chain - as a ribbon.

Pyroxenes and Amphiboles

Pyroxenes and amphiboles are two important rock-forming mineral groups that are representatives of inosilicates. These two groups have a lot of common crystallographic, physical, and chemical properties. Most of them occur in monoclinic symmetry. These groups have similar cations, while amphiboles have additional (OH) anions. Pyroxenes and amphiboles have very similar colors, luster, and hardness, while because of (OH) in amphiboles, they have a lower density and lower refractive index. Pyroxene habit is short prismatic, but amphiboles are more elongated and needle-like.

Pyroxines crystalize under higher temperatures, forming during the early stages of magma cooling. When water is present in magma, pyroxenes can interact with water molecules, forming amphiboles. Pyroxenes and amphiboles are excellent indicators of pressure-temperature (P-T) conditions. During prograde metamorphism (temperature is gradually rising), amphiboles can be changed into pyroxenes with losing water, and under retrograde metamorphism (temperature is slowly decreasing), pyroxenes are changed into amphiboles with gaining (OH) anion.

Single Chain [Si 2 O 6 ] 4-

Single chain inosilicates are represented by pyroxene supergroup minerals. Similarly to tourmaline or garnet groups, pyroxene is not the name of a single mineral species but a massive group of mineral species with the same or very similar crystal structure and varying chemical composition.

Pyroxene Supergroup

A general formula of pyroxenes is the following

XY[T₂O₆],

with positions:

X = Na⁺, Ca²⁺, Fe²⁺, Mg²⁺, Mn²⁺

Y = Fe²⁺, Mg²⁺, Mn²⁺, Zn²⁺, Fe³⁺, Al³⁺, Cr³⁺, V³⁺, Ti⁴⁺,

T = essentially Si⁴⁺, partly also Al³⁺.

Pyroxenes are also solid solutions that form different series. They are the

• Mg-Fe pyroxene series (enstatite-ferrosilite)
• Mn-Mg pyroxene
• Ca pyroxene series (diopside-hedenbergite series)
• Na pyroxene series (jadeite-aegirine series)
• Li pyroxene

A modern list of pyroxene end-members mineral species provided by Morimoto, N. (1988) and simplified in the following table:

Mg-Fe Pyroxene Series

Mn-Mg Pyroxene

Ca Pyroxene Series (Diopside-Hedenbergite Series)

Na Pyroxenes

Ca-Na Pyroxenes

Li Pyroxene

Most pyroxenes are dark brown and green, almost black, with typical short prismatic habits. An essential distinctive feature between pyroxenes and amphibols is the angle between cleavage planes, 87° for pyroxenes and 124° for amphiboles. This feature is usually visible under the microscope; however, in some cases, it may be visible on a big, well-formed crystal.

Pyroxenoid Group

Sometimes, the SiO₄ tetrahedra are rotated within the chain, making it slightly distorted. Therefore, some minerals with distorted chains are classified as pyroxenoids.

The chemical formula of pyroxenoids is written the following way:

M[SiO₃] or a multiple of it,

where M site stands for Ca, Mg, Fe, and Mn.

The most common mineral representatives of pyroxenoids are wollastonite Ca₃[Si₃O₉], rhodonite (Mn,Ca,Fe)₅[Si₅O₁₅], pyroxferroite (Ca,Fe)(Fe,Mn)₆[Si₇O₂₁], and pectolite NaCa₂[Si₃O₈](OH).

Double Chain [Si 4 O 11 ] 6- or Doubled [Si 8 O 22 ]

When two chains connect from one side, they create a double chain. The chemical composition of amphiboles - representatives of double chain inosilicates, is shown in the following way:

A_0-1B₂C₅[T₈O₂₂](OH,F)₂.

The following cations may occupy various sites in the amphibole structure:

A = Na⁺, more rarely K⁺, ⃞ (vacant site)

B = Na⁺, Ca²⁺, Mg²⁺, Fe²⁺, Mn²⁺

C = Mg²⁺, Fe²⁺, Mn²⁺, Al³⁺, Fe³⁺, Ti⁴⁺

T = Si⁴⁺, Al³⁺

The cations are mostly the same as in pyroxenes and amphiboles. However, the amphiboles contain (OH)⁻ as additional anions. This explains the amphiboles' slightly lower densities and refractive indices compared to pyroxenes of similar composition.

Amphiboles develop more commonly elongate, columnar, or even acicular to fibrous habit. They also form in lower temperatures where water can be present.

The amphibole nomenclature is extremely complicated. They are subdivided into:

• Mg-Fe-Mn-Li amphiboles
• Ca amphiboles
• Na-Ca amphiboles
• Na amphiboles

A modern list of amphibole end-member mineral species provided by Leake et al. (1997) and is given in the following table:

Mg-Fe-Mn-Li Amphiboles

Ca Amphiboles

Na-Ca Amphiboles

Na Amphiboles

Diagnostic Characteristics of Cyclosilicates and Inosilicates

Here, we would like to provide diagnostic characteristics to differentiate and identify some of the most common cyclosilicates and inosilicates. We are giving the most ubiquitous and economically significant ones like beryl, tourmaline supergroup, enstatite-ferrosilite, diopside-hedenbergite series, jadeite, spodumene, rhodochrosite, pectolite, hornblende, and tremolite-actinolite series, 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.

Cyclosilicates [Si 6 O 18 ] 12-

Beryl

Formula: Be₃Al₂[Si₆O₁₈]

Beryl has numerous colored varieties like aquamarine, emerald, heliodor, morganite, and goshenite. Therefore, the color is not a beryl diagnostic feature. The hexagonal prismatic habit of beryl crystals is very characteristic and allows us to tell apart beryl from topaz, tourmaline, quartz, and numerous other silicates. Beryl is harder (7.5-8) than hexagonal apatite (5). Beryl also has weak pleochroism, although it is hard to observe pale crystals.

Elbaite (Tourmaline)

Elbaite Formula: Na(Al,Li)₃Al₆(BO₃)₃[Si₆O₁₈](OH)₄

As we already know, tourmaline is a term for a massive tourmaline supergroup. All members of the tourmaline supergroup share identical crystal systems and morphology but are slightly different in chemical composition and, consequently, in colors. Gem-quality tourmalines are mostly elbaite and fluor-liddicoatite. So, we are giving the mineral formula and physical properties of elbaite.

The typical characteristic of tourmalines is their crystal morphology of ditrigonal prism. Tourmaline has a very typical cross-cut that looks like a rounded triangle. Another typical feature of tourmaline is zoning. Elbaites and fluor-liddicoaties are commonly zoned. Color zoning occurs in two ways: parallel to crystal elongation and from the crystal core to the rim. Tourmalines are also vertically striated.

Moderate to strong pleochroism may also be helpful for tourmaline identification, especially if you are dealing with rough crystals without crystallographic features or with a faceted gemstone.

Inosilicates (Single Chain) [Si 2 O 6 ] 4-

Pyroxenes

Enstatite-Ferrosilite

Formula: Mg₂[Si₂O₆] - Fe²⁺₂[Si₂O₆]

A typical feature of pyroxenes is their short prismatic habit. It is only possible to differentiate between series end members with advanced mineralogical methods. Enstatite generally forms in brown semitransparent short prismatic crystals, while ferrosilite is commonly black because of Fe prevalence.

Also, a mutual feature for all pyroxenes that helps to differentiate them from similarly looking amphiboles is cleavage, with cleavage planes intersecting at 87° and 93°. In contrast, amphibole cleavage planes intersect at 56° and 124°.

Diopside-Hedenbergite

Formula: CaMg[Si₂O₆] - CaFe²⁺[Si₂O₆]

The diopside-hedenbergite series is another representative of the pyroxene group. Diopside is more known as it can occur in vibrant green color and gem quality and is used as a gemstone. In contrast, hedenbergite occurs in darker colors with lower luster and transparency because of the presence of Fe.

Like other pyroxenes, their distinguishing feature from amphiboles are angles of cleavage planes 87° and 93°.

Jadeite

Formula: NaAl[Si₂O₆]

Jadeite is a type of pyroxene often found as massive aggregates rather than in crystal form. This may be surprising to some, as jadeite crystals are typically too small to be seen with the naked eye. Jadeite can come in various colors, from apple green to white and occasionally lavender. One diagnostic feature of jadeite is its splintery fracture, which sets it apart from most other silicates that have conchoidal fractures.

Jadeite and nephrite are both popularly grouped together as the material known as "jade." However, thanks to modern analytical techniques, mineralogists can distinguish between the two types of jade. Pyroxene jadeite is usually the most expensive type of jade. Nephrite jade, which is made up of the tremolite-ferro-actinolite series of amphiboles, usually costs less than jadeite.

Spodumene

Formula: LiAl[Si₂O₆]

Spodumene is another pyroxene that can occur in gem quality. Its varieties are more commonly known under the trade names kunzite (pink-colored variety of spodumene) and hiddenite (green-colored variety). Spodumene is characterized by its crystal morphology with typically flattened and heavily striated crystals, moderate to strong pleochroism, and reaction to UV.

Pyroxenoids

Rhodonite

Formula: (Mn²⁺,Fe²⁺,Mg,Ca)[SiO₃]

Rhodonite is a pyroxenoid mineral. It typically occurs in massive compact aggregates of rose-pink color. Rhodonite can be mistaken for rhodochrosite both for its name and appearance. However, rhodochrosite is carbonate and is far softer than silicate rhodochrosite.

Pectolite

Formula: NaCa₂[Si₃O₈](OH)

Pectolite is a type of pyroxenoid mineral characterized by its acicular and radiating fibrous aggregates. These formations are relatively fragile and should be handled carefully to avoid breakage. One of the most well-known varieties of pectolite is known by the gem trade name Larimar, which is highly valued for its pleasing blue lagoon color and exciting patterns.

Inosilicates (Double Chain) [Si 4 O 11 ] 6-

Tremolite-Ferro-Actinolite

Formula: Ca₂Mg₅[Si₈O₂₂](OH)₂ - Ca₂Fe²⁺₅[Si₈O₂₂](OH)₂

This type of amphibole, consisting of small crystal clusters, can form a highly valued nephrite jade. Nephrite is known for its exceptional toughness. Distinguishing between nephrite jade and jadeite jade by appearance alone is impossible. However, nephrite jade commonly contains black inclusions.

Hornblende

Formula: Ca₂(Mg₄(Al,Fe))[Si₇AlO₂₂](OH)₂.

Hornblende is a term used to refer to all Ca-amphiboles. You can find a complete list of Ca-amphiboles in the table above. Ca-amphiboles share similar properties, making it impossible to distinguish between them without conducting a chemical analysis. So, we will be describing the features of the common Ca-amphibole magnesio-hornblende.

References for Cyclosilicates and Inosilicates

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/.
2. Ertl, A., Giester, G., Schüssler, U., Brätz, H., Okrusch, M., Tillmanns, E., & Bank, H. (2013). Cu-and Mn-bearing tourmalines from Brazil and Mozambique: crystal structures, chemistry and correlations. Mineralogy and Petrology, 107, 265-279.
3. Hawthorne FC, Dirlam DM (2011) Tourmaline the indicator mineral: From atomic arrangement to Viking navigation. Elements 7:307-312 link
4. Henn, U., Bank, H., Bank, F. H., Von Platen, H., & Hofmeister, W. (1990). Transparent bright blue Cu-bearing tourmalines from Paraíba, Brazil. Mineralogical Magazine, 54(377), 553-557. link
5. Henry, D. J., Novák, M., Hawthorne, F. C., Ertl, A., Dutrow, B. L., Uher, P., & Pezzotta, F. (2011). Nomenclature of the tourmaline-supergroup minerals. American Mineralogist, 96(5-6), 895-913. link
6. Klein, C., & Dutrow, B. (2007). Manual of mineral science. John Wiley & Sons, 704 p.
7. Leake, B. E., Woolley, A. R., Arps, C. E., Birch, W. D., Gilbert, M. C., Grice, J. D., … & Youzhi, G. (1997). Nomenclature of amphiboles; report of the subcommittee on amphiboles of the International Mineralogical Association, Commission on New Minerals and Mineral Names. The Canadian Mineralogist, 35(1), 219-246. link
8. Morimoto, N. (1988). Nomenclature of Pyroxenes. Mineralogy and Petrology, 39(1), 55-76. link
9. 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
10. Papike, J. J. (1987). Chemistry of the rock‐forming silicates: Ortho, ring, and single‐chain structures. Reviews of Geophysics, 25(7), 1483-1526. link
11. 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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