Clorite
Monoclino
(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’).
Struttura e chimica
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.
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+ per Al3+ in the octahedral sites balanced by the substitution of Si4+ per 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)0.5□1.5Al4Si3AlO10(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.
Proprietà
Abito: platy, lamellar
Durezza: 2 – 3
Sfaldatura: {001} perfect (basal cleavage)
Geminazione: {001} twin plane; {001} composition plane – [310] twin axis
Colore: mostly green with white, yellow, red, and brown varieties
Lucentezza: vitreous to dull
Struscio: pale green to grey
Alterazione: clay minerals
In sezione sottile...
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°(+)
Colore: colorless to green
Pleocroismo: weak to moderate from pale to dark green or yellow (α < β = γ or α = β > γ)
Birifrangenza (δ): 0.00-0.02 (low to moderate; anomalous interference colors: deep Berlin blue, brown, violet)
Rilievo: moderato
Segno ottico: + o -
[Mindat]
[HoM – clinochlore]
[HoM – chamosite]
⇔ slider. 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).
Caratteristiche di terreno
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 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.
⇔ slider. Fibrous chlorite crystals in a vein. Note the faint pale green color of chlorite at PPL and its deep Berlin blue colors at CPL. Width: 1.2 mm. Palombini shales, Norsi, Island of Elba, Italy.
Video. The pleochroism of chlorite, varying in color from pale green to very pale yellowish green/transparent. PPL. Width: 1.2 mm. Chlorite – quartz vein, Massa Unit. La Rocchetta, Alpi Apuane, Massa-Carrara, Italy.
Examples of chlorite-bearing rocks
Chlorite-quartz vein
Fibrous chlorite crystals in a vein produced by hydrothermal fluids in a shale.
Campione: veined shale
Associazione mineralogica: chlorite, quartz, titanite, sericite, clay minerals
Località: Capo Norsi, Norsi, Island of Elba, Italy
Vermiform chlorite
Vermiform (or vermicular) chlorite is characterized by an unusual elongation along the c-axis, producing worm-like chlorite aggregates. It is related to preferred growth along the c-axis during precipitation from a hydrothermal fluid.
Campione: quartz – chlorite vein in phyllite
Associazione mineralogica: chlorite, quartz, muscovite, hematite, epidote
Località: La Rocchetta, Alpi Apuane, Massa-Carrara, Italy
⇔ slider. Vermiform chlorite surrounded by quartz. Note the triplets of chlorite basal sections at the center with the characteristic hexagonal outline. Width: 1.2 mm.
Ritrovamento
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.
Cathelineau, M. (1988). Cation site occupancy in chlorites and illites as a function of temperature. Clay minerals, 23(4), 471-485.
Eggleton, R. A., & Banfield, J. F. (1985). The alteration of granitic biotite to chlorite. American Mineralogist, 70(9-10), 902-910.
Kameda, J., Okamoto, A., Mikouchi, T., Kitagawa, R., & Kogure, T. (2010). The occurrence and structure of vermiform chlorite. Clay Science, 14(4), 155-161.
Vidal, O., & Parra, T. (2000). Exhumation paths of high‐pressure metapelites obtained from local equilibria for chlorite–phengite assemblages. Geological Journal, 35(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 Geology, 81(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 Minerals, 25(1), 83-92.
Risorse
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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