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Geology is the Way

Cordierite

Orthorhombic (pseudo-hexagonal)

(Mg,Fe)2Si5Al4O18•nH2O

Cordierite is a water-bearing magnesium aluminium silicate that is a fundamental constituents of many metapelitic metamorphic rocks and some Al-rich granites and rhyolites. Cordierite was named in honor of the French geologist and mining engineer Pierre Louis Antoine Cordier (1777 – 1861), who first described this mineral. 

Structure and chemistry
Cordierite is the Mg-rich end member of the cordierite [(Mg,Fe)2Si5Al4O18] – sekaninaite [(Fe,Mg)2Si5Al4O18] series and the low-temperature polymorph of indialite, an hexagonal mineral stable above 1450 °C that is very rare in nature. The structure of cordierite consists of Al, and Si tetrahedrons organized in Al2Si4O18 rings, oriented parallel to the basal plane (001). In theory, this structure is similar to that of cyclosilicates (like tourmaline). However, in cordierite, the tetrahedral rings are linked laterally and vertically by additional Si and Al tetrahedrons, which make cordierite a tectosilicate. In cordierite (orthorhombic), Al and Si have a preference for distinct sites, while in indialite (hexagonal), Si and Al may substitute each other in all tetrahedral sites. The hexagonal structure of indialite is isostructural with that of beryl [Al2Be3Si6O18]. In any case, the deviation of cordierite from the hexagonal structure is minimal and cordierite is considered a pseudo-hexagonal mineral. In the cordierite structure, each tetrahedral ‘ring’ is surrounded by 6 octahedral sites that contain Mg and Fe2+. This structure (shown below), repeats along the c-axis, producing large channels centered on the tetrahedral rings, which contain structural water (H2O) and commonly also alkali elements (K, Na, Ca) and CO2. The substitution of Mg by Fe2+ is continuous in the octahedral site but natural cordierites are commonly rich in Mg. In addition Fe2+ (and even Fe3+) can in part substitute Si and Al in the tetrahedral sites. Mn is also commonly present in the octahedral sites.

cordierite crystal structure

Sketch showing the crystal structure of cordierite, as seen on its basal plane (001). Modified after Deer et al. (1992).

cordierite

Cordierite crystal elongated parallel to the foliation in a schist and surrounded by muscovite and biotite. CPL. Width: 3 mm. Calamita Schists, Isola d’Elba, Italy.

Properties
Habit: tabular, prismatic
Hardness: 7
Density: 2.5 – 2.8 g/cm3
Cleavage: {100} moderate, {001} and {010} poor
Twinning: {110}, {310} simple, lamellar and cyclic, common
Color: greenish blue to dark blue, violet, lilac
Luster: greasy, vitreous
Streak: white
Alteration: phyllosilicate mixtures (pinite)
In thin section…
α(//c): 1.527-1.560
β(//b): 1.532-1.574
γ(//a): 1.537-1.578
2Vα: 35-106°
Color: colorless to very pale blue
Pleochroism: visible only in thick sections, α
pale yellow/green, β violet/violet blue, γ pale blue
Birefringence (δ): 0.008-0.018 (first-order grey/yellow)
Relief: low, similar to quartz
Optic sign: + or –
[Mindat]

cordierite crystal habit

Sketch of a cordierite crystal with the orientation of crystal axes, refractive indices, and cleavage planes. Modified after Optical Mineralogy: Principles and Practice.

Field features
Cordierite is a mineral indicative of low-pressure/high-temperature metamorphism that is commonly found in metapelites and metasandstones of contact aureoles and regional metamorphic rocks of the amphibolite- and granulite-facies. In second order, cordierite is also a product of peraluminous magmas that can be found in granitic/rhyolitic rocks and associated products (e.g. pegmatites). Cordierite is ideally a transparent mineral with blue to lilac/violet color, but it is generally altered and rich in inclusions (especially in metamorphic rocks). Hence, it commonly appears opaque with dull white, grey, yellow, or black color. For the same reason, even if cordierite is a ‘hard’ mineral (hardness: 7), it is difficult to test it for hardness because altered crystals scratch easily. Cordierite exhibit a ‘stocky’ tabular habit that is generally not much elongated, which may resemble feldspars in the field. However, contrarily to feldspars, cordierite shows only one moderately developed set of cleavage planes. Only in exceptional cases, it is possible to recognize in the field the typical sector twinning of cordierite, characterized by hexagonal- or six-pointed star-shaped associations of 3 or 6 crystals. In most cases, a thin section is required for the correct identification of cordierite.

cordierite gem

Transparent crystalline cordierite with lilac blue color. These colors are rarely visible in most cordierites due to the ease of its alteration to phyllosilicates (pinite). Adachi, Kawasaki, Miyagi Prefecture, Japan. Photo © Nishio Hamane.

twinned cordierite

Cordierite with sector-twinning, commercially referred to as ‘cherry blossom stone’, in a hornfels. Cordierite was altered to fine-grained sericite (pinite) and now appears of dull white/grey color. Mikata, Honshu, Japan. Photo © James St. John.

cordierite schist

White crystals of altered (pinitized) cordierite in a contact-metamorphosed schist. Width: about 3 cm. Monte Arco, Island of Elba, Italy.

cordierite spotted schist

Spotted schists are common metamorphic rocks in contact aureoles. The ‘spots’ commonly consist of cordierite crystals that were replaced by fine-grained phyllosilicates (pinite) during retrograde metamorphism. Norsi beach, Island of Elba, Italy.

Cordierite in thin section
Cordierite is transparent at PPL and shows first-order grey to yellow interference colors at CPL, similar to quartz and feldspars. Cordierite crystals are often nearly equidimensional and not much elongated along the c-axis, and commonly occur as anhedral rounded grains, especially in metamorphic rocks. Cordierite crystals may display simple twinning, polysynthetic twinning, or cyclical twinning (combinations of 3 or 6 crystals twinned at 60° or 120° angles). Untwinned and unaltered cordierite resembles quartz, since it shows nearly identical relief and interference colors: the two minerals can be distinguished based on the sign of elongation (quartz: length slow; cordierite: length fast), in the rare cases in which euhedral grains are visible. More commonly, cordierite appears often altered to sericite, chlorite, and other phyllosilicates, it contains tiny opaque inclusions, and appears often twinned. Hence, it is more often mistook for albite, when characterized by simple twins, or plagioclase, when it displays polysynthetic twinning. The presence of cyclical twins intersecting at 60° or 120° is diagnostic (plagioclase can eventually be oriented at 90°). However, these twins are not always present and it might be necessary to analyze the mineral or use thick sections to display cordierite pleochroism, in order to distinguish it from plagioclase. Other features of cordierite are (1) the typical dusting by opaque minerals, (2) the presence of pleochroic haloes around zircon inclusions (as in biotite), and (3) the presence of a single set of cleavage planes (plagioclase has 2). As a ferromagnesian mineral, cordierite is commonly found associated with chlorite in metamorphic rocks. Cordierite alteration is unique and diagnostic, as it alters to pinite, a fine-grained mixture of sericite, chlorite, and serpentine with very characteristic yellowish, greenish, even bluish colors.

CPL
CPL
CPL
PPL
PPL

Group of cordierite crystals associated with biotite and muscovite in a schist from the contact aureole of the Porto Azzurro Pluton. Width: 3 mm. Calamita Schists, Island of Elba, Italy.

CPL + λ
CPL + λ
CPL + λ
CPL
CPL
PPL
PPL

Elongated cordierite crystal with inclusions of opaque oxides and surrounded by muscovite and biotite. The auxiliary plate (λ) highlights the negative sign of elongation of cordierite. Note the presence of a crack filled with deep Berlin blue chlorite. Width: 3 mm. Calamita Schists, Island of Elba, Italy.

Cordierite twins

CPL
CPL
CPL
PPL
PPL

Cordierite with simple twinning. In this case, cordierite can be misidentified for albite. Width: 1 mm. Calamita Schists, Island of Elba, Italy.

Cordierite

Polysynthetic twinning may lead to confuse cordierite with plagioclase. However, note the association with biotite and chlorite (Fe-Mg-bearing minerals) and the alteration to chlorite. CPL. Width: 1 mm. Calamita Schists, Island of Elba, Italy.

cyclical twinning in cordierite

Cordierite showing well-developed cyclical twinning. CPL. Width: 1 mm. San Vincenzo Rhyolite, Tuscany, Italy. Photo © Alessandro Da Mommio/Alexstrekeisen.

cyclical twinning in cordierite

Cordierite showing well-developed cyclical twinning. CPL + λ. Width: 1 mm. San Vincenzo Rhyolite, Tuscany, Italy. Photo © Alessandro Da Mommio/Alexstrekeisen.

cordierite hornfels

Cordierite blast with cyclical twinning in a contact metamorphosed hornfels. CPL. Width: 2 mm. Monte Linas, Sardinia, Italy. Photo © Alessandro Da Mommio/Alexstrekeisen.

Cordierite alteration

CPL
CPL
CPL
PPL
PPL

Cordierite altering to chlorite along cracks. Alteration to chlorite occurs because both cordierite and chlorite are ferromagnesian minerals. This type of alteration is not possible around albite and plagioclase. Width: 1 mm. Calamita Schists, Island of Elba, Italy.

CPL
CPL
CPL
PPL
PPL

The typical alteration of cordierite which produces the distinctive yellowish/greenish pinite. Alteration is very common in cordierite because of the presence of large channels in its structure, which allow the easy movement of cations and water and facilitates retrograde and alteration reactions. Mylonitic schist. Width: 1 mm. Calamita Schists, Island of Elba, Italy.

Gallery 1 – Cordierite schist
This cordierite schist is from the Calamita Schists, part of the contact aureole of the Porto Azzurro Pluton (Island of Elba, Italy). Metamorphic temperatures are estimated to be in the range of 500-700 °C at pressure around 2 kbars. In this rock, cordierite occurs as blasts rich with inclusions of opaque minerals, associated with quartz, muscovite, and biotite.

Gallery 2 – Cordierite-chlorite schist
Chlorite is a common retrograde (alteration) product of cordierite. The cordierite grains in this schist are partly replaced by chlorite along their rims and in fractures. Other minerals present are muscovite, biotite, quartz, and opaque minerals. Calamita Schists, contact aureole of the Porto Azzurro Pluton (Island of Elba, Italy).

Gallery 3 – Cordierite grains
Detail of cordierite porphyroblasts from the cordierite schists of the Calamita Schists, shown in the previous galleries.

Gallery 4 – Mylonite with pinitized cordierite
Cordierite is very susceptible to deformation in the presence of fluids. In this mylonite, cordierite altered to pinite and deformed to elongated cordierite-fish objects. Calamita Schists, Island of Elba, Italy.

Occurrence
Cordierite occurs in nature in two primary settings: (1) in high-temperature/low- to medium-pressure metapelitic/metapsammitic rocks and (2) in peraluminous granitic magmas. In both contact aureoles and regional metamorphic rocks, cordierite can form at amphibolite-facies due to the destabilization of chlorite, and may coexist with andalusite, kyanite, garnet, muscovite, biotite, K-feldspar, plagioclase, sillimanite, and pyroxene. Garnet coexists with cordierite mostly in medium-pressure assemblages. Cordierite is stable to very high-temperatures in migmatites and granulite-facies rocks, where it coexists with orthopyroxene, corundum, spinel, sapphirine, and osumilite. Cordierite is progressively destabilized towards higher-pressures, where staurolite– and garnet-bearing parageneses are more common.
Cordierite occurs also in peraluminous igneous rocks and even in pegmatites. For example, the igneous cordierite crystals shown in this page are from the peraluminous San Vincenzo rhyolite (Tuscany, Italy). In many granites, cordierite is present both as an igneous mineral, as a product of contact metamorphism in metapelitic xenoliths, and, hence, as xenocrystals. The incorporation of metapelitic material during magma ascent is commonly invoked to explain the presence of cordierite in some norites.

References
Gibbs, G. V. (1966). The polymorphism of cordierite I: The crystal structure of low cordierite. American Mineralogist: Journal of Earth and Planetary Materials, 51(7), 1068-1087.
Holdaway, M. J., & Lee, S. M. (1977). Fe-Mg cordierite stability in high-grade pelitic rocks based on experimental, theoretical, and natural observations. Contributions to Mineralogy and Petrology63(2), 175-198.
Miyashiro, A. (1957). Cordierite-indialite relations. American Journal of Science255(1), 43-62.
Ogiermann, J. C. (2002). Cordierite and its retrograde breakdown products as monitors of fluid-rock interaction during retrograde path metamorphism: case studies in the Schwarzwald and the Bayerische Wald (Variscan belt, Germany) (Doctoral dissertation).

        

The information displayed on this page is after Introduction to the Rock-Forming Minerals and Optical Mineralogy: Principles and Practice.

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