Mineralogy of Cyclosilicates and Inosilicates

Crystal Chemistry of Cyclosilicates and Inosilicates

Cyclosilicates and inosilicates are two subclasses of silicate minerals. They include numerous mineral groups and important mineral species. The cyclosilicates most familiar to gemologists are beryls and the tourmaline supergroup. Inosilicates are subdivided into single-chain silicates, which includes the pyroxene supergroup, and double-chain silicates, which includes the amphibole minerals. The gem material gemologists call jade actually includes both pyroxene (jadeite) and amphibole (nephrite) members.

Introduction to Cyclosilicates

Cyclosilicate units form by joining three, four, or six SiO₄ tetrahedrons into closed rings.

• Three joined tetrahedrons create a three-member ring with a [Si₃O₉]^6- structural unit.
• Four joined tetrahedrons create a four-member ring with a [Si₄O₁₂]^8- structural unit.
• Six joined tetrahedrons create a six-member ring with a [Si₆O₁₈]^12- structural unit. Six-member rings are the most common type of cyclosilicates.

Benitoite (BaTi[Si₃O₉]), the rare state gem of California, is an example of a three-member cyclosilicate.

The rare mineral papagoite (Ca₂Cu₂Al₂[Si₄O₁₂](OH)₆) is an example of a four-member cyclosilicate.

Beryl is an iconic example of a six-member cyclosilicate. This mineral's inner structure is even visible in its hexagonal prismatic crystal habit.

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 with high transparency. These qualities make tourmalines well-suited for use as gemstones.

The general formula for tourmaline minerals can be written as:

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

You may wonder why there is a T site that stands for silicon and aluminum and not just the silicate-making [Si₆O₁₈]^12- units we would expect to see.

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

Aluminum can also occur in [6]-coordination and can be linked to six oxygen atoms, creating octahedral units. This aluminum can substitute for cations like Mg and Fe. This important crystal-chemical principle complicates silicate structure and gives us more diverse minerals.

Possible Substitutions

A wide range of possible substitutions leads to highly variable chemical compositions for tourmaline. The following subgroups are distinguished based on the dominant cation in the X position:

• Alkali tourmalines
• Calcic tourmalines
• X-site vacant tourmalines

The primary tourmaline groups can be described based on the occupancy of the X site. For example, Henry D. et al. (2011) provide a modern list of tourmaline supergroup species divided into these three subgroups:

Alkali Subgroup Tourmalines

Calcic Subgroup Tourmalines

Vacant Subgroup Tourmalines

Identifying Tourmaline Species

Only advanced chemical analysis can identify the species of a specific tourmaline. Do not confuse varietal names of tourmalines like rubellite or indicolite with mineral species. Varietal names are just color-based tourmaline names. They don't correspond to any differences in crystal or chemical composition.

Introduction to Inosilicates

When SiO₄ tetrahedra combine into infinite chains without forming cycles or rings, inosilicates are formed. (They are sometimes called "chain silicates"). Inosilicates are subdivided into single chains with [Si₂O₆]^4- structural units and double chains with [Si₄O₁₁]^6- structural units. (You can visualize double chains as two connected chains).

Pyroxenes and Amphiboles

Pyroxenes and amphiboles are two important rock-forming mineral groups in the inosilicate subclass. These two groups share many crystallographic, physical, and chemical properties. Most of the minerals in these groups have monoclinic symmetry and similar cations. However, amphiboles have additional (OH) anions. Pyroxenes and amphiboles have very similar colors, luster, and hardness. Most pyroxenes have dark brown and green — almost black — colors. Because of additional (OH) anions, amphiboles have lower densities and refractive indices. Pyroxene crystals are short and prismatic, while amphibole crystals are more elongated, columnar, needle-like, or even fibrous.

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

Distinguishing Pyroxenes and Amphiboles

The angles between cleavage planes mark an essential difference between pyroxenes and amphiboles. Pyroxenes have cleavage planes intersecting at 87° and 93° angles; amphiboles have cleavage planes intersecting at 56° and 124° angles. These features are usually visible only under a microscope. However, in some cases, these features may be visible to the naked eye in large, well-formed crystals.

Single Chain Inosilicates

Single chain inosilicates include the pyroxene supergroup minerals. Similarly to tourmaline or garnet, pyroxene is not the name of a single mineral species. Instead, it is the name of a very large group of mineral species with the same or very similar crystal structure and varying chemical compositions.

Pyroxene Supergroup

A general formula for 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, such as:

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

Morimoto, N. (1988) has provided a modern list of pyroxene end-member mineral species. The following table simplifies that list:

Mg-Fe Pyroxene Series

Mn-Mg Pyroxene

Ca Pyroxene Series (Diopside-Hedenbergite Series)

Na Pyroxenes

Ca-Na Pyroxenes

Li Pyroxene

Pyroxenoid Group

Sometimes, the SiO₄ tetrahedra are rotated within the chain. This creates a slightly distorted chain. Some minerals with distorted chains are classified as pyroxenoids. The chemical formula for 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 pyroxenoid minerals are wollastonite (Ca₃[Si₃O₉]), rhodonite ((Mn,Ca,Fe)₅[Si₅O₁₅]), pyroxferroite ((Ca,Fe)(Fe,Mn)₆[Si₇O₂₁]), and pectolite (NaCa₂[Si₃O₈](OH)).

Double Chain Inosilicates

When two chains of SiO₄ tetrahedra connect on one side, they create a double chain. Amphiboles are examples of double chain inosilicates. The chemical composition of amphiboles is written 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 in amphiboles are mostly the same as in pyroxenes, except for the presence of (OH)⁻ as additional anions.

Amphibole nomenclature is extremely complicated. They are subdivided into:

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

Leake et al. (1997) have provided a modern list of amphibole end-member mineral species. The following table simplifies that list:

Mg-Fe-Mn-Li Amphiboles

Ca Amphiboles

Na-Ca Amphiboles

Na Amphiboles

Diagnostic Characteristics of Cyclosilicates and Inosilicates

Below, you'll find diagnostic characteristics to differentiate and identify some of the most common cyclosilicates and inosilicates. 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 properties that cyclosilicates, inosilicates, and other silicates share are their elevated hardness (generally more than 5), vitreous luster, high transparency, and white streak.

Cyclosilicates

Structural unit: [Si₆O₁₈]^12-

Beryl

Formula: Be₃Al₂[Si₆O₁₈]

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

Elbaite (Tourmaline)

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

All members of the tourmaline supergroup share the same crystal system and morphology but have slightly different chemical compositions. Consequently, tourmalines occur in a variety of colors. Most gem-quality tourmalines are of the elbaite and fluor-liddicoatite species.

Tourmalines typically have ditrigonal prism crystal forms. A cross-section of a tourmaline crystal usually looks like a rounded triangle. Another characteristic feature of tourmaline is color zoning. Both elbaites and fluor-liddicoatites commonly show zoning. Color zoning occurs either parallel to crystal elongation or from the crystal core to the rim. Tourmalines also show vertical striations on their crystal faces.

Tourmaline's moderate to strong pleochroism may also help with identification, especially if you are dealing with faceted gemstones or crystals lacking well-defined crystallographic features.

Inosilicates (Single Chain)

Structural unit: [Si₂O₆]^4-

Pyroxenes

Enstatite-Ferrosilite

Formula: Mg₂[Si₂O₆] - Fe²⁺₂[Si₂O₆]

Pyroxenes usually occur as short, prismatic crystals. Distinguishing between series end-members requires advanced mineralogical testing. Enstatite generally forms as brown, semi-transparent, short, prismatic crystals, while ferrosilite is usually black because of Fe prevalence.

Keep in mind that cleavage can help distinguish pyroxenes from similar-looking amphiboles. Pyroxene cleavage planes intersect 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 belongs to the pyroxene group. Diopside is more well-known than hedenbergite because it can occur in vibrant green colors and may be 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 cleavage planes intersect at 87° and 93° angles.

Jadeite

Formula: NaAl[Si₂O₆]

Jadeite is often found in massive aggregates rather than crystals. This aggregated form is the one most coveted by artisans and connoisseurs. In fact, jadeite crystals are typically too small to be seen with the naked eye. Jadeite occurs in various colors, from green to white and occasionally lavender. A diagnostic feature of jadeite is its splintery fracture, which sets it apart from most other silicates that have conchoidal fractures.

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

Spodumene

Formula: LiAl[Si₂O₆]

Spodumene can occur in gem quality. Its varieties are more commonly known under the trade names kunzite (pink-colored) and hiddenite (green-colored). Spodumenes usually occur as flattened and heavily striated crystals, show moderate to strong pleochroism, and luminesce under ultraviolet (UV) light.

Pyroxenoids

Rhodonite

Formula: (Mn²⁺,Fe²⁺,Mg,Ca)[SiO₃]

Rhodonite 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 a carbonate and far softer than silicate rhodochrosite.

Pectolite

Formula: NaCa₂[Si₃O₈](OH)

Pectolite is a type of pyroxenoid mineral characterized by its acicular and radiating fibrous aggregate structure. 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 trade name "Larimar," which is highly valued for its pleasing "blue lagoon" color and exciting patterns.

Inosilicates (Double Chain)

Structural unit: [Si₄O₁₁]^6-

Tremolite-Ferro-Actinolite

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

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

Hornblende

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

Hornblende is a term used to refer to all Ca-amphiboles. Ca-amphiboles share similar properties, making it impossible to distinguish them without a chemical analysis. The table below describes the features of magnesio-hornblende, a common Ca-amphibole mineral.

References for Cyclosilicates and Inosilicates

• 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/.
• 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.
• Hawthorne FC, Dirlam DM (2011) Tourmaline the indicator mineral: From atomic arrangement to Viking navigation. Elements 7:307-312 link
• 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
• 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
• Klein, C., & Dutrow, B. (2007). Manual of mineral science. John Wiley & Sons, 704 p.
• 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
• Morimoto, N. (1988). Nomenclature of Pyroxenes. Mineralogy and Petrology, 39(1), 55-76. 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
• 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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