Cordierite
Orthorhombic (pseudo-hexagonal)
(Mg,Fe)2Si5Al4O18•nH2O
Cordierite is a water-bearing magnesium aluminium silicate, which occurs primarily in high-temperature metapelitic metamorphic rocks and it is found also in 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 de facto 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.
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), sericite, chlorite
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]
[HoM]
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 crystallization 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 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. They also commonly occur as anhedral rounded grains, especially in metamorphic rocks. Cordierite crystals may display simple, polysynthetic, 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 case euhedral and elongated crystals are available. More commonly, cordierite presents some degree of alteration to sericite, chlorite, and other phyllosilicates, contains tiny opaque inclusions, and shows twinning. Consequently, it can be 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 (albite – pericline twins in plagioclase are oriented at about 90° in respect with one another). However, these twins are not always present and it might be necessary to analyze the mineral or use thick sections to display its pleochroism. 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). Cordierite is a ferromagnesian mineral and, as such, it is often found associated with or altered to chlorite in metamorphic rocks. The alteration of cordierite is unique and diagnostic: pinite, a fine-grained mixture of sericite, chlorite, and serpentine, shows very distinctive yellowish, greenish, and even bluish colors.
CPL
PPL
⇔ slider. Group of cordierite crystals associated with biotite and muscovite in a contact metamorphosed schist. Width: 3 mm. Calamita Schists, Island of Elba, Italy.
CPL + λ
CPL
PPL
⇔ slider. 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
PPL
⇔ slider. Cordierite with simple twinning. In this case, cordierite can be misidentified for albite. Width: 1.2 mm. Calamita Schists, Island of Elba, Italy.
Cordierite alteration
CPL
PPL
⇔ slider. Cordierite altered to chlorite along cracks. Alteration to chlorite may occur because both cordierite and chlorite are ferromagnesian minerals. This type of alteration is not possible around albite and plagioclase. Width: 1.2 mm. Calamita Schists, Island of Elba, Italy.
CPL
PPL
⇔ slider. The typical alteration of cordierite producing 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: 1.2 mm. Calamita Schists, Island of Elba, Italy.
Examples of cordierite-bearing rocks
Cordierite schist
This cordierite schist formed due to contact metamorphism of schistose rocks in the contact aureole of a monzogranite intrusion. Estimated temperatures are in the 500-700 °C range.
Sample: cordierite schist
Assemblage: cordierite, biotite, muscovite, quartz, chlorite (retrograde), ilmenite, rutile, apatite, zircon, monazite, tourmaline
Locality: Spiaggia del Remaiolo, Calamita, Isola d’Elba, Italy
Cordierite-chlorite schist
Chlorite is a common product of alteration and retrograde metamorphism of cordierite. The cordierite grains in this schist are partly replaced by chlorite along their rims and in fractures.
Sample: cordierite-chlorite schist
Assemblage: cordierite, chlorite (retrograde), biotite, muscovite, quartz, ilmenite, rutile, apatite, zircon, monazite, tourmaline
Locality: Spiaggia del Remaiolo, Calamita, Isola d’Elba, Italy
Cordierite grains
Detail of the cordierite porphyroblasts present in the schists shown in the previous galleries.
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.
Sample: cordierite mylonite
Assemblage: cordierite (pinite), biotite, muscovite (sericite), chlorite, quartz, ilmenite, rutile, zircon, apatite, monazite
Locality: Capanne di Gustavo, Calamita, Isola d’Elba, Italy
Occurrence
Cordierite occurs in nature in two primary settings: (1) in high-temperature/low- to medium-pressure metapelitic/metapsammitic metamorphic rocks and (2) in peraluminous granitic igneous rocks. In contact aureoles and regional metamorphic rocks, cordierite can form at amphibolite-facies conditions 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 can be present both as a product of direct crystallization from the magma and of contact metamorphism of metapelitic xenoliths within the magma. The incorporation of metapelitic material during magma ascent is, indeed, a commonly cited mechanism to explain the presence of cordierite in some norites.
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 Petrology, 63(2), 175-198.
Miyashiro, A. (1957). Cordierite-indialite relations. American Journal of Science, 255(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).
Resources
An introduction to the Rock-Forming Minerals. Deer, Howie & Zussmann.
Optical Mineralogy: Principles & Practice. Gribble & Hall.
Transmitted Light Microscopy of Rock-Forming Minerals: An Introduction to Optical Mineralogy (Springer Textbooks in Earth Sciences, Geography and Environment). Schmidt.
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