Mineralogy of Phyllosilicates and Tectosilicates

Crystal Chemistry of Phyllosilicates and Tectosilicates

Phyllosilicates and tectosilicates subclasses comprise numerous mineral groups and important mineral species. Phyllosilicates' most familiar representatives are talc, micas, serpentine, and clays. Tectosilicates include quartz and numerous quartz modifications, as well as the extensive feldspar family, feldspathoids, and zeolite family.

Introduction to Phyllosilicates

Most rock-forming sheet silicates are made up of infinite two-dimensional layers. You may imagine them as combining double chains of inosilicates into an infinite two-dimensional layer. The phyllosilicates unit is [Si₄O₁₀]^4-, sometimes written as [Si₂O₅]^2- — as divided by 2.

Sometimes, this subclass is called sheet or layered silicates to emphasise layered structure and sheet or scally form of mineral occurrence. However, mostly it is called phyllosilicates (from Greek phýllon' leaf').

Phyllosilicate sheets or layers are not identical within one mineral; they are mostly like cakes on atomic structure with alternating octahedra and tetrahedra layers and, therefore, are further subdivided into

• 1-layer silicates, where all layers are similar (with an example of 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.

Phyllosilicate Polytypes

Many phyllosilicates have several polytypes for one mineral species. Polytypes stand for differences in symmetry with the same chemical composition. For example, you may see pyrophyllite written pyrophyllite-2M, which stands for monoclinic (2M) polytype, or pyrophyllite-Tc, which stands for triclinic (Tc) polytype.

It is impossible to differentiate among numerous polytypes with the naked eye. Only X-ray diffraction may give a complete answer.

Pyrophyllite-Talc Group

The pyrophilite-talc group consists of two members: pyrophilite Al₂Si₄O₁₀(OH)₂ and talc Mg₃Si₄O₁₀(OH)₂. We would like to pay attention to this group as ground talc, known as talcum, is a common material widely used in households as cosmetic products, from baby powder to blush. Also, talc is a representative of the softest mineral that corresponds to 1 in the Mohs scale.

Mica Group

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

IM_2-3 ☐_1-0[T₄O₁₀]A₂, with

I is commonly K⁺, Na⁺, Ca²⁺ NH₄, Rb, Ba, Cs,

M is commonly Al³⁺, Mg²⁺, Fe³⁺, Fe²⁺, Li, Ti, Mn, Cr, V, Zn

☐ is vacancy

T is commonly Si⁴⁺, AI³⁺, Fe³⁺, Ti, Be, B

A is 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)

There is no need to panic and try to memorise all the names and formulas. Some minerals are too rare and tiny to identify, and micas are impossible to differentiate with the naked eye alone. So you can understand the nomenclature logic, we are giving a complete list in a table so you can see the most common cations and how they are distributed among the positions.

Here is a list of true micas with end-member formulas according to Rieder et al. (1998).

True Micas

Brittle Micas

Here is a list of brittle micas with end-member formulas:

Mica Series Names

After carefully checking the tables, you may wonder why popular micas like biotite and lepidolite are absent. The reason for these terms' absence is that they are not end-members; rather, they are series names or terms used for more convenient communication to emphasise a particular chemical composition.

Biotite stands for trioctahedral micas between or close to the annite-phlogopite and siderophyllite-eastonite joins, which is used to call dark micas without lithium.

Lepidolite is a trioctahedral mica close to the trilithionite-polylithionite composition. The term is used for light micas with substantial lithium.

Zinnwaldite is a trioctahedral mica close to the siderophyllite-polylithionite composition. Used to name dark micas containing lithium.

Serpentine Group

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

The general formula can be written the following A₃[X₂O₅](OH)₄ with other serpentine subgroup members listed in the table below simplified after White & Dixon (2002):

Clay Minerals

Clay has traditionally been thought of as a material used for making earthenware due to its malleability. However, it may surprise people that clay is composed of tiny, fine-grained minerals. These minerals have unique properties that are studied and applied in various fields, such as construction, soil enhancement, geochemical barriers for absorbing toxic heavy metals, pharmaceuticals, water purification, animal care, carrier material for insecticides and pesticides, flush fluid for rotary drilling in the oil industry, sound-proofing, thermal insulation, and more. Clay mineralogy is a separate discipline focusing on understanding and utilizing these properties.

Clay minerals have unique properties that stem from their intricate crystal structure. Sheet silicates are composed of layers that differ from one another, producing repetitive unit structures that resemble sandwiches or packages. These layers have interlayers, which are spaces that water molecules can occupy. Moreover, 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, the exact determination of mineral species requires X-ray diffraction methods.

Main Clay Mineral Groups

Here is a list of the main clay minerals groups with the most common and important representatives after Bailey (1980).

Introduction to Tectosilicates

Tectosilicates, the last and most complex structure where [SiO₄]^4- tetrahedra connect over all their four corners, form a three-dimensional frameworks. Therefore, tectosilicates are sometimes called framework silicates with unit [SiO₂]⁰.

Quartz and SiO 2 Polymorphs

Quartz is one of the most abundant minerals in the Earth's crust. We used to think of quartz as a common mineral, but it is definitely beautiful and can occur in many colors. Quartz is exciting! First, we deal with quartz mineralogically called α-quartz, a stable SiO₂ polymorph at 1 bar pressure and up to 573 C.

There are numerous other quartz polymorphs that are low-pressure (tridymite and cristobalite), high-pressure (coesite, stishovite, and seifertite), and even amorphous like opal that have no crystal structure.

At the same time, α-quartz has its own numerous varieties. The first group is physical varieties of typical quartz in excellent crystal form, like classical amethyst - a purple variety of quartz, citrine - a yellow. However, there are also micro- to cryptocrystalline α-quartz varieties like chalcedony, carnelian, agate, onyx, and 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%). They are a rock-forming mineral of granite rocks and are the backbone of rock classification. Feldspars are two branches of solid solutions:

• 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. Alkali feldspars occur in 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, substituting Na¹⁺ by Ca²⁺ must be accompanied by substituting Si⁴⁺ by Al³⁺ to compensate for the charge balance (coupled substitution Na¹⁺ + Si⁴⁺ ⇋ Ca²⁺ + Al³⁺). Usually, plagioclase representative is given in amount (per cent) of anorthite (Ca) part in a mineral. They range in the following ways:

• 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 intergrowth in plagioclases. Depending on the thickness of the lamellae, they may give rise to a phenomenal effect between blue (An_48-52) and red (An_55-59) known as labradorescence.

Feldspathoid Minerals (Foids)

Feldspathoids are quite extravagant minerals. They are poor in SiO₂ and, therefore, cannot coexist with quartz, because feldspar would form instead in a SiO₂-saturated system. Thus, feldspathoids occur only in SiO₂-poor rocks, commonly of an alkaline composition.

We list 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? The formula of lazurite explains how it is formed in an environment rich in sulfur and deficient in oxygen, so the association with pyrite (FeS₂) becomes clear. Moreover, feldspathoids cannot coexist with quartz. This makes another mineral association, lazurite-calcite (CaCO₃), possible.

Scapolite Group

We are including the scapolite group of framework silicates, as scapolite is widely used as a gemstone. However, similar to olivine (forsterite-fayalite series), scapolite is a solid solution series between marialite Na₄[AlSi₃O₈]₃Cl and meionite Ca₄[Al₂Si₂O₈]₃CO₃.

Zeolite Family

Zeolites, versatile industrial minerals, are framework silicates with remarkably spacious crystal structures. These structures form large cavities or channels, which, due to their loose binding, exhibit 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 equally reversible. In modern times, exclusively synthetic zeolites are employed for water treatment.

Zeolites can incorporate foreign atoms or molecules up to a specific particle size, making 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 in detergent production, 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. We are listing the most significant species according to Deer et al. (2013):

Diagnostic Characteristics of Phyllosilicates and Tectosilicates

Here, we would like to provide diagnostic characteristics to differentiate and identify some of the most common phyllosilicates and tectosilicates. We are giving the most ubiquitous and economically significant ones like talc, muscovite, biotite, lepidolite, chlorite series (clinochlore-chamosite), serpentine minerals, kaolinite, quartz, microcline, plagioclase series, nepheline, lazurite, scapolite, natrolite, emphasizing how to differentiate them from the most similarly looking minerals. The best diagnostic characteristics are highlighted in bold.

Some similar properties of phyllosilicates are their low hardness (no more than 4, 2,5 is the most common), perfect cleavage, and scaly aggregates form of occurrence. On the other hand, tectosilicates are far harder (more than 5) and prismatic.

Phyllosilicates

Tenacity is also an important characteristic that allows the identification of micas; therefore, we include it in the list of characteristics.

Talc

Formula: Mg₃[Si₄O₁₀](OH)₂

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

Muscovite

Formula: KAl₂[AlSi₃O₁₀](OH)₂

Muscovite is one of the most essential minerals representative of mica. It is widely distributed in many metamorphic and igneous rocks. Muscovite is usually referred to as "white mica." It occurs mainly in white, gray, or yellow color but is of light saturation. For muscovite, as for others, micas perfect cleavage is a prominent distinguishing feature. Muscovite can form crystals of pseudohexagonal shape. 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

As it is impossible to differentiate between members macroscopically, the biotite term is used for dark-colored micas. Biotite's distinguishing features are the same as those of other mica representatives: perfect cleavage, scaly aggregates, and dark brown colors, specifically for biotite.

Lepidolite

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

Lepidolite is a favorite mica among students as it occurs in very specific pink and purple colors so that it can be easily differentiated from other micas. Perfect cleavage and fine scaly aggregates make it completely different from many other light-pink or purple minerals.

Chlorite Series (Clinochlore-Chamosite)

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

Chlorite is a common name for a solid solution series of clinochlore and chamosite, which are indistinguishable macroscopically. Chlorite is typically identified due to its green color and its shades and form of occurrence of foliated, fibrous masses. Chlorite is less common in well-developed crystals compared to mica and biotite. Also, chlorite crystals are flexible but less elastic than micas.

Serpentine Group (Lizardite, Antigorite, and Chrysotile)

Formula: Mg₆[Si₄O₁₀](OH)₈

Lizardite, antigorite, and chrysotile are three important mineral representatives of the serpentine mineral group. They share identical chemical compositions but different crystal structures that can only be identified by XRD (X-ray diffraction). Other physical properties are similar. These minerals are typical with their white and pale green fibrous aggregates. They look like fiber. Like fiber, serpentine minerals can be easily bent, differentiating them from other minerals.

Clay Minerals (Kaolinite)

Formula: Al₄[Si₄O₁₀](OH)₈

Kaolinite mainly occurs in compact, earthy, claylike masses of white color. The crystals are indistinguishable, so low harness, earthy claylike masses, and opaque white colors are helpful in identifying kaolinite.

Tectosilicates

α-QUARTZ

Formula: SiO₂

Quartz is an extraordinary mineral that can occur in a variety of colors and forms. Colors vary from colorless through yellow, pink, and green to violet, brown, and black. Quartz can be zoned or mottled. It can occur in perfectly crystallized crystals up to several meters long and in microcrystalline aggregates. This possible variation gives rise to numerous quartz varieties, with the most popular being amethyst, citrine, rose quartz, smoky quartz, agate, carnelian, and many others. So, quartz looks like a vast amount of minerals. The most valuable properties that can help identify quartz are its hardness (7), crystal morphology of prismatic crystals, and striation perpendicular to elongation.

Microcline

Formula: KAlSi₃O₈

Most commonly, microcline occurs in pale yellow, orange, and pink colors. However, they can also be white, so it is hard to tell microcline apart from plagioclases in these cases. Green feldspar is almost 100% microcline. In rare cases, banded perthitic intergrowths from the exsolution process (cross-hatched or tartan twinning patterns) will help in microcline differentiation from plagioclases.

Plagioclase Series (Feldspar)

The plagioclase series is a solid solution between albite and anorthite and consists of six members: albite, oligoclase, andesine, labradorite, bytownite, and anorthite. It is only possible to differentiate between them with chemical analysis. We describe the first and the last members of the plagioclase series, as the differences are the most contrasting in albite and anorthite. Albite crystals are commonly tabular, while anorthite crystals are short and lamellar. Also, anorthite has a higher density than albite and less distinct cleavage.

Albite

Formula: Na[AlSi₃O₈]

Anorthite

Formula: Ca[Al₂Si₂O₈]

Felspathoids

Nepheline

Formula: Na₃(Na,K)[AlSiO₄]₄

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

Lazurite

Formula: Na₆Ca₂[(AlSiO₄)₆]·S

Lazurite is an extraordinary mineral because of its deep blue color. It is easily distinguishable from other blue phyllosilicates. However, phosphate lazulite and carbonate azurite can create some issues in identification. Lazurite is harder than carbonate azurite. Mineral paragenesis can also give some hints in identification. Lazurite usually occurs with pyrite, while azurite co-occurs with malachite.

Scapolite (Marialite-Meionite)

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 spodumenes. Also, striation will help you a lot. In scapolite, striation is along crystal elongation, while in quartz, it is perpendicular. Additionally, scapolite is more reactive to UV light.

Natrolite (Zeolite)

Formula: Na₂[Al₂Si₃O₁₀]·2H₂O

Natrolite can be identified due to its combination of white color and long prismatic, acicular crystal habit. Natrolite acicular crystals commonly combine into radiating fibrous aggregates and spherical clusters. The mineral specimen is brittle, and thin crystals can be easily damaged.

References for Phyllosilicates and Tectosilicates

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2. Bailey, S. W. (1980). Summary of recommendations of AIPEA nomenclature committee. Clay Minerals, 15(1), 85-93. link
3. 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
4. Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals. Mineralogical Society of Great Britain and Ireland.
5. 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
6. Faust, G. T., & Fahey, J. J. (1962). The serpentine-group minerals (No. 384-A). US Govt. Print. Off.,. link
7. 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
8. Klein, C., & Dutrow, B. (2007). Manual of mineral science. John Wiley & Sons, 704 p.
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10. 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
11. 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
12. Strunz, H., & Nickel, E. H .(2001). Strunz mineralogical tables. Schweizerbart, Stuttgart. 869 p.
13. 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
14. 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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