Mineralogy of Phyllosilicates and Tectosilicates

Crystal Chemistry of Phyllosilicates and Tectosilicates

Phyllosilicates and tectosilicates are comprised of numerous mineral groups and important mineral species. The most well-known phyllosilicates are talc, micas, serpentines, and clays. Tectosilicates include quartz and numerous quartz modifications, as well as the extensive feldspar family, feldspathoids, and zeolites.

Introduction to Phyllosilicates

Most rock-forming phyllosilicates are made up of infinite two-dimensional layers. Phyllosilicates are sometimes called "sheet" or "layered silicates" in reference to their layered structure and their sheet or scale-like mineral forms. The structural unit of phyllosilicates is [Si₄O₁₀]^4- (sometimes written as [Si₂O₅]^2- — as the structural unit divided by 2).

Phyllosilicate sheets or layers are not identical within one mineral. Rather, they alternate between layers of octahedral and tetrahedral atomic structure. As a result, phyllosilicates are further subdivided into:

• 1-layer silicates, where all layers are similar (for example, apophyllite (KCa₄[Si₄O₁₀]₂(F,OH)₂⋅8H₂O))
• 2-layer silicates (for example, kaolinite (Al₄[Si₄O₁₀](OH)₈))
• 3-layer silicates (for example, talc (Mg₃[Si₄O₁₀](OH)₂))

Phyllosilicates and Polytypes

Many phyllosilicates have several polytypes for one mineral species. Polytypes are minerals of the same species with the same chemical composition but difference crystal structures. For example, you may see pyrophyllite written pyrophyllite-2M, which stands for a monoclinic (2M) polytype, or pyrophyllite-TC, which stands for a triclinic (TC) polytype. (Note how polytypes differ from polymorphs, which are different mineral species with the same formula but different crystal structures). X-ray diffraction testing is required to distinguish between polytypes.

Pyrophyllite-Talc Group

The pyrophyllite-talc group consists of two members: pyrophyllite (Al₂Si₄O₁₀(OH)₂) and talc (Mg₃Si₄O₁₀(OH)₂). Ground talc, also known as talcum, is a material widely used in cosmetic products, from baby powder to blush. Talc is also well-known as the softest mineral on the Mohs scale. It has a hardness of 1.

Mica Group

Per Rieder et al. (1998), the general formula of the mica group is

IM_2-3 ☐_1-0[T₄O₁₀]A₂, with

• I commonly K⁺, Na⁺, Ca²⁺ NH₄, Rb, Ba, Cs
• M commonly Al³⁺, Mg²⁺, Fe³⁺, Fe²⁺, Li, Ti, Mn, Cr, V, Zn
• ☐ is vacancy
• T commonly Si⁴⁺, AI³⁺, Fe³⁺, Ti, Be, B
• A commonly F, OH, O, Cl, S

(The most frequently encountered elements are set in boldface).

Micas are also divided into:

• true micas
• brittle micas, and
• interlayer-deficient micas (former hydromicas)

Some mica minerals are too rare and tiny to differentiate with the naked eye alone.

To understand the logic of mica nomenclature, review the following list of true micas with end-member formulas according to Rieder et al. (1998). You can see the most common cations and their distribution.

True Micas

Brittle Micas

The following table lists brittle micas with end-member formulas.

Mica Series Names

You may wonder why popular micas like biotite and lepidolite are absent from the previous tables. They are absent because they are not end-members. Rather, they are series names used to emphasize particular chemical compositions.

Biotite stands for tri-octahedral micas between or close to the annite-phlogopite and siderophyllite-eastonite series. The term refers to dark-colored micas without lithium.

Lepidolite is a tri-octahedral mica close to the trilithionite-polylithionite composition. The term refers to light-colored micas with substantial lithium.

Zinnwaldite is a tri-octahedral mica close to the siderophyllite-polylithionite series. The term refers to dark-colored micas containing lithium.

Serpentine Group

The most common serpentine group minerals are three structural varieties of Mg₆[Si₄O₁₀](OH)₈: lizardite, antigorite, and chrysotile. (Faust & Fahey, 1962)

The general serpentine formula can be written asA₃[X₂O₅](OH)₄. The formulas of other serpentine subgroup members are listed in the table below, simplified after White & Dixon (2002).

Clay Minerals

Clay has been used to make earthenware for millennia due to its malleability. However, it may surprise some people to learn that clay is composed of tiny, fine-grained minerals. These minerals and their unique properties have been studied and applied in various fields, such as construction, soil enhancement, geochemistry, pharmaceuticals, water purification, animal care, the oil industry, and many others. Clay mineralogy focuses on understanding and utilizing these properties.

The unique properties of clay minerals stem from their intricate crystal structure. Phyllosilicates or "sheet silicates" are composed of layers that differ from one another, which produce repetitive unit structures that resemble sandwiches. Water molecules can occupy the spaces between these layers or "inter-layers." Clay minerals have the ability to take in and release H₂O molecules, as well as adsorb and exchange ions. This ability allows soils to store water and temporarily adsorb and supply nutrients. Some clay minerals can expand and contract, which explains the plasticity of certain clays and their diverse applications. Kaolinite, montmorillonite, and vermiculite are the most widely used clay minerals in our daily life.

Because of the small grain sizes of clay minerals, determining their exact species requires X-ray diffraction testing.

Main Clay Mineral Groups

The following table lists the main clay mineral groups with their most common and important representatives, after Bailey (1980).

Introduction to Tectosilicates

In tectosilicates, [SiO₄]^4- tetrahedra connect at all their four corners, forming three-dimensional structures. As a result, tectosilicates are sometimes called "framework silicates" with a [SiO₂]⁰ structural unit.

Quartz and Its Polymorphs

Quartz is one of the most abundant minerals in the Earth's crust. Although common, quartz occurs in many colors and can make beautiful gemstones.

Quartz has many polymorphs. There is one called α-quartz, a stable SiO₂ polymorph at 1 bar pressure and up to 573 C. Other quartz polymorphs include low-pressure varieties (tridymite and cristobalite), high-pressure varieties (coesite, stishovite, and seifertite), and even amorphous varieties like opal that have no crystal structure.

α-quartz also has its own varieties. There are quartzes in well-known crystal forms and various colors, like classical amethyst — a purple quartz — and citrine — a yellow quartz. There are also micro to cryptocrystalline α-quartz varieties like chalcedony, carnelian, agate, and onyx, as well as granular varieties like aventurine, jasper, and chert. Please see our quartz gem listing for more information.

Feldspar Family

Feldspars are the most abundant mineral group in the Earth's crust (with a proportion of more than 50 vol%). Feldspar minerals are part of the makeup of granite. Feldspar consists of two solid solution branches:

• Alkali feldspar is a solid solution between orthoclase, (Or) KAlSi₃O₈, and albite, (Ab) NaAlSi₃O₈
• Plagioclase is a solid solution between albite, (Ab) NaAlSi₃O₈, and anorthite, (An) CaAl₂Si₂O₈

Alkali and Plagioclase Feldspars

Alkali feldspars are characterized by simply substituting K⁺ with Na⁺ from orthoclase KAlSi₃O₈ to anorthite NaAlSi₃O₈. They occur with two modifications: monoclinic sanidine ((K,Na)[AlSi₃O₈]) and triclinic microcline (K[AlSi₃O₈]). Sanidine is stable at the highest temperatures and microcline at the lowest. (Brown & Parsons, 1989)

In plagioclase, a substitution of Na¹⁺ by Ca²⁺ must be accompanied by a substitution of Si⁴⁺ by Al³⁺ to compensate for the charge balance (coupled substitution Na¹⁺ + Si⁴⁺ ⇋ Ca²⁺ + Al³⁺). Usually, plagioclase representation is given in amount (per cent) of anorthite (or Ca part) in a mineral. For example:

• Albite: An₀-An₁₀ pure and almost pure NaAlSi₃O_8. An₁₀ means up to 10% of anorthite (CaAl₂Si₂O₈)
• Oligoclase: An₁₀-An₃₀
• Andesine: An₃₀-An₅₀
• Labradorite: An₅₀-An₇₀
• Bytownite: An₇₀-An₉₀
• Anorthite: An₉₀-An₁₀₀

Exsolution, a process through which an initially homogeneous solid solution separates into at least two different crystalline minerals, results in submicroscopic lamellar (layer-like) inter-growth in plagioclases. Depending on the thickness of the lamellae, they may create a phenomenal effect between blue (An_48-52) and red (An_55-59) known as labradorescence.

Feldspathoid Minerals

Feldspathoids or foids are poor in SiO₂ and, therefore, cannot coexist with quartz. (In a SiO₂-saturated system, feldspar would form instead of feldspathoids). Thus, feldspathoids occur only in SiO₂-poor rocks, commonly of an alkaline composition.

The following table lists the most common feldspathoids, simplified after Merlino (1984) and Deer et al. (2013).

Do you know that gemstones like lapis lazuli, a combination of lazurite, sodalite, and haüyne, are classified as feldspathoids? Lazurite's formula indicates it forms in an environment rich in sulfur but deficient in oxygen, so the association with pyrite (FeS₂) becomes clear. Moreover, since feldspathoids cannot coexist with quartz, this makes another mineral association, lazurite-calcite (CaCO₃), possible.

Scapolite Group

Scapolite is widely used as a gemstone. However, similar to olivine (forsterite-fayalite series), scapolite is actually a solid solution series between marialite (Na₄[AlSi₃O₈]₃Cl) and meionite (Ca₄[Al₂Si₂O₈]₃CO₃).

Zeolite Family

Versatile industrial minerals, zeolites are tectosilicates with remarkably spacious crystal structures. These structures form large cavities or channels, which, due to their loose bindings, have a high ion exchange capacity. These channels also house H₂O molecules, adding to the unique properties of zeolites.

Thanks to their crystal structures, both natural and synthetic zeolites serve as efficient ion exchangers. For instance, zeolites can effectively extract Ca²⁺ ions from water of high hardness by exchanging them with their own Na⁺ ions. This process is also reversible. In modern times, synthetic zeolites are used for water treatment.

Zeolites can incorporate foreign atoms or molecules up to a specific particle size. This makes them useful as molecular sieves. They are often used for fractionated purification of gases or gas mixtures, particularly noble gases. One important application of zeolites is the efficient removal of fission products from nuclear waste. They are also used to produce detergents, asphalt concrete, Portland cement, and solar thermal collectors. Volcanic tuffs that contain zeolites are widely used in the building industry due to their favorable properties, such as high thermal insulation capacity.

Significant Zeolite Species

There are about 100 recognized species of zeolites. The following table includes the most significant species, according to Deer et al. (2013):

Diagnostic Characteristics of Phyllosilicates and Tectosilicates

Below, you'll find diagnostic characteristics to differentiate and identify some of the most common phyllosilicates and tectosilicates. 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 common properties of phyllosilicates are their low hardness (no more than 4; 2.5 is the most common), perfect cleavage, and scaly aggregate forms. On the other hand, tectosilicates are far harder (more than 5) and have prismatic crystal forms.

Tenacity is also an important characteristic for identifying micas. Therefore, we include it in the list of properties for phyllosilicates.

Phyllosilicates

Talc

Formula: Mg₃[Si₄O₁₀](OH)₂

Talc is easily recognized due to its low hardness. It is the reference mineral for a hardness of 1 on the Mohs scale. Talc commonly occurs in fibrous and fine-grained, compact aggregates. One of talc's most distinguishing features is its greasy feel. Thus, it is sometimes called soapstone.

Muscovite

Formula: KAl₂[AlSi₃O₁₀](OH)₂

Muscovite is one of the most important micas. It is widely distributed in many metamorphic and igneous rocks. Usually called "white mica," muscovite occurs mainly in white, gray, or yellow color with light saturation. Perfect cleavage is a characteristic feature of muscovite, as it is for other micas. Muscovite can form pseudohexagonal crystals. Very fine-grained muscovite is known as sericite and resembles glitter.

Biotite

Formula: K(Mg,Fe²⁺,Al,Fe³⁺)₃[Al(Si,Al)₃O₁₀](OH,F)₂

Since differentiating between mica members with the naked eye is impossible, the term "biotite" is used for dark-colored micas. Biotite's characteristic features (like those of other micas) are perfect cleavage and scaly aggregate forms. Dark brown colors indicate biotite.

Lepidolite

Formula: K(Li,Al)₃[(Si,Al)₄O₁₀](F,OH)₂

Lepidolite occurs in pink and purple colors that distinguish it readily from other micas. Perfect cleavage and fine scaly aggregate forms help distinguish it from many other light pink or purple minerals.

Chlorite Series

Formula: Mg₅Al[AlSi₃O₁₀](OH)₈ - Fe²⁺₅Al[AlSi₃O₁₀](OH)₈

Chlorite is a common name for a solid solution series of clinochlore-chamosite, which are indistinguishable to the naked eye. Chlorite usually shows green colors and forms as foliated, fibrous masses. Compared to micas, it is less likely to form as well-developed crystals. When chlorite crystals do form, they are flexible but less elastic than micas.

Lizardite, Antigorite, and Chrysotile

Formula: Mg₆[Si₄O₁₀](OH)₈

Lizardite, antigorite, and chrysotile are three important mineral representatives of the serpentine group. They share identical chemical compositions but have different crystal structures that can only be identified by X-ray diffraction testing. These minerals usually have white and pale green colors and occur as fibrous aggregates. They look like fiber. Like fiber, serpentine minerals can easily bend, a quality that distinguishes them from many other minerals.

Kaolinite

Formula: Al₄[Si₄O₁₀](OH)₈

Kaolinite mainly occurs in compact, earthy, claylike masses of white color. Its crystals are indistinguishable, so low hardness, earthy masses, and opaque white colors help distinguish kaolinite.

Tectosilicates

α-QUARTZ

Formula: SiO₂

Quartz can vary in color from colorless to yellow, pink, green, violet, brown, and black. Quartz can also appear zoned or mottled. It can occur in perfectly formed crystals up to several meters long as well as in microcrystalline aggregates. This wide variation has lead to the division of quartz in numerous varieties. The most popular varieties are amethyst, citrine, rose quartz, smoky quartz, agate, and carnelian. There are many more less well-known.

Because of this great variety, quartz can resemble many minerals. However, the properties most useful for distinguishing quartz are its hardness (7), prismatic crystal morphology, and perpendicular striations on its crystal faces.

Microcline

Formula: KAlSi₃O₈

Microcline occurs most commonly in pale yellow, orange, and pink colors. However, they can also occur in white, so it is hard to distinguish microcline from plagioclase in these cases. Green feldspar is almost 100% microcline. In rare cases, banded perthitic inter-growths from exsolution (cross-hatched or tartan twinning patterns) will help distinguish microcline from plagioclase.

Plagioclase Series

The plagioclase series is a solid solution between albite and anorthite and consists of six members: albite, oligoclase, andesine, labradorite, bytownite, and anorthite. Differentiating these members requires a chemical analysis. Below, we will describe the first and the last members of the plagioclase series, as the differences between albite and anorthite are most notable. Albite crystals are commonly tabular, while anorthite crystals are short and lamellar. Anorthite also has a higher density and less distinct cleavage than albite.

Albite

Formula: Na[AlSi₃O₈]

Anorthite

Formula: Ca[Al₂Si₂O₈]

Felspathoids

Nepheline

Formula: Na₃(Na,K)[AlSiO₄]₄

Nepheline is commonly confused with quartz. However, nepheline has a lower hardness and an often greasy luster.

Lazurite

Formula: Na₆Ca₂[(AlSiO₄)₆]·S

Lazurite has an extraordinary deep blue color, which makes it easily distinguishable from blue phyllosilicates. However, lazulite (a phosphate mineral) and azurite (a carbonate mineral) can be confused with lazurite. However, lazurite is harder than azurite. Mineral paragenesis can help distinguish lazurite, because it usually occurs with pyrite. Azurite often occurs with malachite.

Scapolite

Formula: Na₄[AlSi₃O₈]₃Cl - Ca₄[Al₂Si₂O₈]CO₃

Scapolites can be confused with a number of minerals, including quartz (amethyst and citrine), spodumene, and fluorite. However, scapolite is softer than quartz and spodumene. In scapolite, striation occurs along the direction of crystal elongation, while in quartz, it occurs perpendicular to the direction of elongation. Scapolite is also more reactive to ultraviolet (UV) light than quartz.

Natrolite

Formula: Na₂[Al₂Si₃O₁₀]·2H₂O

A member of the zeolite family, natrolite can be identified due to its combination of white color and long, prismatic, acicular crystal form. These acicular crystals commonly combine into radiating, fibrous aggregates and spherical clusters. Natrolite makes a brittle mineral specimen, and these thin crystals can be damaged easily.

References for Phyllosilicates and Tectosilicates

• 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/.
• Bailey, S. W. (1980). Summary of recommendations of AIPEA nomenclature committee. Clay Minerals, 15(1), 85-93. link
• Brown, W. L., & Parsons, I. (1989). Alkali feldspars: ordering rates, phase transformations and behaviour diagrams for igneous rocks. Mineralogical Magazine, 53(369), 25-42. link
• Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals. Mineralogical Society of Great Britain and Ireland.
• Drits, V. A., Guggenheim, S., Zviagina, B. B., & Kogure, T. (2012). Structures of the 2: 1 layers of pyrophyllite and talc. Clays and Clay Minerals, 60(6), 574-587. link
• Faust, G. T., & Fahey, J. J. (1962). The serpentine-group minerals (No. 384-A). US Govt. Print. Off.,. link
• Harrison, A. D., Whale, T. F., Carpenter, M. A., Holden, M. A., Neve, L., O'Sullivan, D., … & Murray, B. J. (2016). Not all feldspars are equal: a survey of ice nucleating properties across the feldspar group of minerals. Atmospheric Chemistry and Physics, 16(17), 10927-10940. link
• Klein, C., & Dutrow, B. (2007). Manual of mineral science. John Wiley & Sons, 704 p.
• Merlino, S. (1984). Feldspathoids: their average and real structures. Feldspars and Feldspathoids: Structures, Properties and Occurrences, 435-470. 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
• Rieder, M., Cavazzini, G., D'yakonov, Y. S., Frank-Kamenetskii, V. A., Gottardi, G., Guggenheim, S., … & Wones, D. R. (1998). Nomenclature of the micas. Clays and clay minerals, 46(5), 586-595. link
• Strunz, H., & Nickel, E. H .(2001). Strunz mineralogical tables. Schweizerbart, Stuttgart. 869 p.
• Tischendorf, G., Forster, H. J., Gottesmann, B., & Rieder, M. (2007). True and brittle micas: composition and solid-solution series. Mineralogical Magazine, 71(3), 285-320. link
• White, G. N., & Dixon, J. B. (2002). Kaolin-serpentine minerals. Soil mineralogy with environmental applications, 7, 389-414. link

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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