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Chlorite

Monoclinic

(Mg, Fe2+, Fe3+, Mn, Al)5-6[Si2-4Al2-0O10](OH)8

Chlorites are a group of sheet silicates that are important constituents of many low-grade metamorphic rocks and occur as alteration minerals in igneous rocks or in hydrothermal systems. In the field and in thin section, chlorite commonly appears green, hence its name (from the Greek chloros, χλωρός, ‘green’).

Structure and chemistry
The structure of chlorite is related to that of trioctahedral micas, as it consists of a layered structure with ‘sheets’ of (Si,Al)O4 tetrahedrons interconnected as six-sided rings extending infinitely in the 2D plane and trioctahedral ‘brucite’ layers, , characterized by (Mg, Fe, Al) cations surrounded by 3 O2- or OH. The OH ion lies at the center of the rings in the tetrahedral sheets. Each trioctahedral sheet (O) is sandwiched between 2 tetrahedral layers (T), forming T-O-T sandwiches or talc-like layers. The T-O-T layers are separated by an additional octahedral layer (‘brucite’ layer) where Mg, Fe, Al cations are coordinated between (OH) anions. The structure of chlorite lacks the large cation sites (interlayer sites) that occur in micas.

chlorite crystal structure

Sketch of the crystal structure of chlorite, as seen on a section perpendicular to the sheet (or parallel to the c-axis), with a detail of the plain view of the tetrahedral sheets. Based on Deer et al. (1992).

Chlorites are characterized by several cation substitutions, producing an extensive chemical variability [see below]. The Tschermak substitution, i.e. the substitution of (Mg,Fe)2+ for Al3+ in the octahedral sites balanced by the substitution of Si4+ for Al3+ in the tetrahedral site, steers the miscibility between the Al-rich corundophilite [(Mg)4Al2Si2Al2O10(OH)8] and the Al-free chlorite [(Mg)6Si4O10(OH)8] end-members. The latter is a theoretical end-member that has never been found in nature and that is chemically identical to serpentine, although serpentine group minerals differ from chlorites as they have a T-O layering. Wiewiora & Weiss (1990) defined three intermediate compositions along the corundophilite – Al-free chlorite series: sheridanite [Mg4.5Al1.5Si2.5Al1.5O10(OH)8], clinochlore [Mg4Al1Si3Al1O10(OH)8], and pennine [Mg5.5Al0.5Si3.5Al0.5O10(OH)8].
Another important chlorite end-member is Sudoite, a mineral characterized by only five cations out of six available octahedral sites with formula [(Mg,Fe)2□Al3Si3AlO10(OH)8]. The miscibility between the corundophilite – Al-free chlorite series and sudoite is governed by the di/trioctahedral substitution, the replacement of (Fe,Mg)2+ by Al3+ and a empty site or vacancy (□) in the octahedral site. The further incorporation of vacancies in the structure can lead to even more extreme compositions, such as Donbassite [(Mg,Fe)0.51.5Al4Si3AlO10(OH)8].
Finally, all chlorites are subject to the ferromagnesian substitution (Fe → Mg) and the substitution of Al3+ by Fe3+. Additional cations that commonly enter the structure are Mn2+ and Cr3+. Some Fe2+-bearing varieties are given different names in the scheme of Wiewiora & Weiss (1990), shown below. 

Note: there is a discrepancy in the nomenclature of chlorites between mineralogists and petrologists (e.g. chlorite group – Mindat). For example, some workers [e.g. Vidal & Parra, 2000; Cathelineau et al., 2013; Bourdelle et al., 2015] refer to corundophilite as amesite (which technically is, however, a serpentine group mineral), and to chamosite, the Fe2+-bearing variety of clinochlore, as daphnite.

Chlorite replacing biotite

Chlorite replacing biotite in a granite, showing its typical deep Berlin blue interference colors. CPL. Width: 2 mm. Photo © Alessandro Da Mommio.

Properties
Habit: platy, lamellar
Hardness: 2 – 3
Cleavage: {001} perfect (basal cleavage)
Twinning: {001} twin plane; {001} composition plane – [310] twin axis
Color: mostly green with white, yellow, red, and brown varieties
Luster: vitreous to dull
Streak: pale green to grey
Alteration: clay minerals
In thin section…
α(α^c / α^a): 1.57-1.67
β(//b): 1.57-1.69
γ(γ^a / γ^c): 1.57-1.69
2V: 20°(-) – 60°(+)
Color: colorless to green
Pleochroism: weak to moderate from pale to dark green or yellow (α < β = γ or α = β > γ)
Birefringence (δ): 0.00-0.02 (low to moderate; anomalous interference colors: deep Berlin blue, brown, violet)
Relief: moderate
Optic sign: + or –
[Mindat]

Fe chlorites
Fe chlorites
Fe chlorites
Mg chlorites
Mg chlorites

Diagram showing the chemical variability of natural chlorites (grey triangle), considering Al-, Mg-, and □ (= vacancy)-bearing end-members. Slide to see Fe2+-bearing chlorite end-members. Further complexity arises from the substitution of Al for Fe3+, which produces Fe3+ end-members (not shown). Based on Wiewiora & Weiss (1990).

Field features

chlorite crystal habit

Sketch of a chlorite crystal. Based on Deer et al. (1992).

Chlorite, like other phyllosilicates, forms platy crystals and flakes with a characteristic basal cleavage, commonly appearing greenish in color (although yellow, grey, or red chlorites do exist). It is less shiny than micas and tends to show a vitreous to dull luster. Its hardness is low (2 – 3) and can be scratched by hand and split in small scales. It is rare to find chlorite grains that are visible with the unaided eye in the field, as its common occurrence are fine-grained metamorphic rocks like phyllites or metabasites. In this case, the presence of chlorite may be suspected because of green hues in the color of the rock (warning: many other minerals are green, especially in metabasalts).

chlorite in quartzite

Veins filled by green chlorite in a quartzite. Praticciolo, Calamita, Elba, Italy.

chlorite in quartzite

Masses of chlorite in a metasomatized quartzite. Praticciolo, Calamita, Elba, Italy.

chlorite in quartzite

Patinas of chlorite within cracks in a quartzite. Note the greenish color and moderate reflectivity. The high reflectivity grains consist of muscovite. Calamita, Island of Elba, Italy.

chlorite garnet schist

Greenish chlorite schist with red garnet crystals. Width: 9.6 cm. Lake Martin, Alabama, USA. Photo © James St. John.

chlorite slickenlines on fault plane

Slickenfibres on a fault mirror, colored in green hues thanks to the presence of chlorite. Brno, Žabovřesky, Czech Republic. Photo © Petr Hykš.

Chlorite in thin section
Chlorites show very distinctive features in thin section. At PPL they appear colorless to green with a typical weak to moderate pleochroism on green hues. Iron-rich varieties generally show deeper colors. Very commonly, chlorite shows anomalous deep Berlin blue colors at CPL and, less commonly, brown to violet or grey interference colors, lower than those displayed by micas. In addition, chlorite shows typical platy crystals or flaky aggregates with perfect basal cleavage.

CPL
CPL
CPL
PPL
PPL

Biotite grain (brownish), substituted by chlorite in a schist. Brownish relics of biotite are visible, almost entirely replaced by greenish chlorite. Width: 1 mm. Posada Valley, Sardinia, Italy.

chlorite

Chlorite crystals showing a platy habit, perfect basal cleavage and anomalous deep Berlin blue interference colors. CPL. Width: 1 mm. Posada Valley, Sardinia, Italy.

Chlorite muscovite schist

Chlorite crystals (Berlin blue colors) surrounded by muscovite (high interference colors) in a schist. CPL. Width: 4 mm. Posada Valley, Sardinia.

Occurrence
Chlorite is a typical metamorphic mineral that is stable in diagenetic environment up to the low-grade. In this range, it is typically found in metasediments (metasandstones and metapelites) and metabasic rocks. Chlorites form from the replacement of Fe,Mg,Al-bearing clays and igneous minerals and, as such, it can also be found in sedimentary rocks both as a detrital or authigenic mineral. Chlorite is common in the zeolite- and prehnite-pumpellyite facies, together with zeolites, prehnite, pumpellyite, epidote, and albite. Chlorite is stable at greenschist-facies in association with mica, amphiboles, epidote, and feldspars. In metasedimentary rocks with increasing metamorphic grade, chlorite reacts with muscovite producing biotite and other minerals (e.g. kyanite/andalusite, cordierite, garnet). In metabasic rocks, the formation of Al-rich amphiboles at amphibolite-facies (e.g. hornblende) generally coincides with the elimination of chlorite from the mineral assemblage. Chlorite also occurs in blueschist-facies rocks together with epidote, lawsonite and sodic amphibole but it is replaced by garnet and omphacite at eclogite-facies. During the metamorphism of ultramafic rocks, chlorite appears at low- to medium-grade from the destabilization of serpentine and may survive as a stable mineral up to high-grade conditions, where it coexists with pyroxenes and olivine.
Chlorite represents a common alteration phase of many igneous and metamorphic minerals. Typical examples are chlorite pseudomorphs after biotite and garnet. Chlorite is also a product of low-temperature hydrothermal circulation and can be found as veins, Airedales, and in hydrothermally-altered rocks.

References
Cathelineau, M. (1988). Cation site occupancy in chlorites and illites as a function of temperature. Clay minerals23(4), 471-485.
Eggleton, R. A., & Banfield, J. F. (1985). The alteration of granitic biotite to chlorite. American Mineralogist70(9-10), 902-910.
Vidal, O., & Parra, T. (2000). Exhumation paths of high‐pressure metapelites obtained from local equilibria for chlorite–phengite assemblages. Geological Journal35(3‐4), 139-161.
Walshe, J. L. (1986). A six-component chlorite solid solution model and the conditions of chlorite formation in hydrothermal and geothermal systems. Economic Geology81(3), 681-703.
Wiewióra, A., & Weiss, Z. (1990). Crystallochemical classifications of phyllosilicates based on the unified system of projection of chemical composition: II. The chlorite group. Clay Minerals25(1), 83-92.

        

Mineral Properties
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