Chloritoid
Monoclinic
(Fe,Mg,Mn)2(Al,Fe3+)Al3[SiO4]2(OH)4
Chloritoid is a mineral produced mostly by metamorphism in schists and mafic rocks. The mineral was originally named chlorispath, in reference to its similarity to chlorite and its hardness (‘spath’ means ‘brittle stone’ in German) and was renamed ‘chloritoid’ in 1835 by August Breithaupt.
Structure and chemistry
The monoclinic structure of chloritoid can be visualized as a series of octahedral layers linked by individual [SiO4] tetrahedrons that point alternatively up and down along the c-axis in the crystal structure. There are two types of octahedral layers: brucite type (L1) and corundum-type (L2). Brucite-type layers have (Fe2+, Mn, Mg)4Al2O4(OH)8 formula, whereas corundum-type layers have Al6O16 formula. The L1 layer have two distinct octahedral sites: a slightly larger one, which represents 2/3 of the octahedral sites, is occupied by Fe2+, Mn, and Mg, and a smaller one, which is filled by Al and vacancies. Similarly, The L2 layer contains only Al and 1/4 of its octahedral sites is not occupied. Some types of chloritoid exhibit a triclinic or trigonal crystal structure, related to changes on the piling of the layers or variations in the spacing of the L1 and L2 layers.
There are two main substitutions in chloritoid: (Mg, Mn) substituted by Fe2+ and Al by Fe3+. In general, chloritoid exhibits a predominantly Fe2+-rich composition with limited Mg and Mn substitutions, but Mg-chloritoid has been found in high pressure metamorphic rocks, and Mn-rich chloritoid (ottrelite) occurs in Mn-rich rocks. Trivalent Al and Fe may be substituted also by Cr3+.
Field features
CPL
PPL
⇔ slider. Rosette-like aggregate of twinned chloritoid crystals with their typical green colors, surrounded by quartz in a schist. Width: 1.2 mm. Lower Phyllites (Paleozoic), Massa Unit, Alpi Apuane, Italy.
Properties
Habit: prismatic
Hardness: 6.5
Density: 3.46-3.80 g/cm3
Cleavage: {001} perfect, {110} moderate, {010} parting
Twinning: {001} very common simple and lamellar
Color: dark green, black
Luster: vitreous
Streak: white, grayish, slightly greenish
Alteration: chlorite
In thin section…
α(//b): 1.705-1.730
β(^a or //b): 1.708-1.734
γ(γ^c = 2-30°): 1.712-1.740
2Vγ: 37-124°
Color: green to colorless
Pleochroism: α pale green/green, β slaty blue/indigo, γ colorless/pale yellow
Birefringence (δ): 0.005-0.022 (low but masked by the greenish color and pleochroism of the mineral, anomalous dark green and blue colors are common)
Relief: high
Optic sign: + or –
[Mindat]
[HoM]
Chloritoid forms tabular crystals resembling ‘coffins’ with a distinctive very dark green to black color. It can be found as individual crystals or as rosette-like aggregates. It is mostly found in metapelitic or metapsammitic rocks where it can be difficult to distinguish from phyllosilicates, especially chlorite. However, the hardness of chloritoid (6.5) is greater than most phyllosilicates and it generally stands out as resistant grains in schistose rocks. Amphiboles may show similar dark green colors and coexist with chloritoid but can be distinguished from the latter for their distinctly different prismatic habit.
Chloritoid in thin section
Chloritoid can be easily identified in thin section for the typical dark green/green to indigo color and pleochroism, the pervasive lamellar twinning, and the high relief. The green hues of chloritoid are darker than chlorite and the relief of chloritoid is higher. Tourmaline may show similar colors but lacks the characteristic lamellar twinning of chloritoid. The low interference colors of chloritoid are generally not visible, as they are masked by the dark green colors of the mineral, except for magnesian varieties of chloritoid which are characterized by paler greenish color and may show first-order grey to yellow interference colors.
Video. Chloritoid shows variable pleochroic color from pale yellow/green to green/blue/indigo. The colors appear paler when the shorter c-axis is oriented parallel to the polarizer. PPL. Width: 1.2 mm. Chloritoid schist, Massa Unit. Monte Brugiana, Alpi Apuane, Italy.
Video. The anomalous green interference colors of chloritoid at CPL. Also note the common lamellar twins parallel to the long side of the crystal. CPL. Width: 1.2 mm. Chloritoid schist, Massa Unit. Monte Brugiana, Alpi Apuane, Italy.
CPL
PPL
⇔ slider. Chloritoid grain with lamellar twinning in association with white mica and quartz in a schist. Width: 1.2 mm. Chloritoid schist, Massa Unit. Monte Brugiana, Alpi Apuane, Italy.
CPL
PPL
⇔ slider. Rosettes (aggregates) of chloritoid, surrounded by white mica and quartz. The dark inclusions of graphite outline the orientation of the metamorphic foliation within chloritoid grains. Width: 3 mm. Chloritoid schist, Massa Unit. Monte Brugiana, Alpi Apuane, Italy.
CPL
PPL
⇔ slider. Close up of a chloritoid rosette from the previous sample. Chloritoid aggregates like this one are common at low metamorphic grade. Width: 1.2 mm. Chloritoid schist, Massa Unit. Monte Brugiana, Alpi Apuane, Italy.
Examples of chloritoid-bearing rocks
Chloritoid schists from the Massa Unit
Chloritoid is a primary constituent of many schists of the Massa Unit, which formed due to the metamorphism of Permian – Triassic pelites, sandstones, and conglomerates at blueschist/greenschist-facies (T ~ 400 – 500 °C; P = 0.7 – 1.3 GPa). The rocks of this metasedimentary succession are marked by variable contents of Al, Mg, Fe, and Fe3+– and show various chloritoid-bearing assemblages and chloritoid composition. The most common assemblage containing chloritoid is chloritoid + quartz + white mica (phengite ± paragonite), but some rocks contain also chlorite and the very rare chloritoid + kyanite assemblage. The composition of chloritoid is also very variable, with Fe-chloritoid more common in graphite-bearing schists and more Mg-rich chloritoid compositions occurring in hematite-bearing schists.
Samples: chloritoid schists
Assemblage: chloritoid, white mica (muscovite ± paragonite), quartz, ± chlorite, ± kyanite, graphite/hematite, pyrite, rutile, titanite
Locality: Monte Brugiana area, Massa Unit, Alpi Apuane, Italy
Papeschi, S., Rossetti, F., & Walters, J. B. (2023). Growth of kyanite and Fe‐Mg chloritoid in Fe2O3‐rich high‐pressure–low‐temperature metapelites and metapsammites: A case study from the Massa Unit (Alpi Apuane, Italy). Journal of Metamorphic Geology, 41(8), 1049-1079.
Occurrence
Chloritoid is probably the first ‘true’ metamorphic mineral (other than phyllosilicates) to form during prograde metamorphism of metapelites and metapsammites containing white mica, chlorite, and quartz. Its presence at greenschist- and blueschist-facies is, however restricted to Al- and Fe-rich metapelites where it may coexist with carpholite, chlorite, and kyanite. At amphibolite-facies conditions, chloritoid is progressively replaced by staurolite and garnet and becomes rarer with increasing metamorphic grade, although chloritoid-bearing rocks from the sillimanite zone are known. Chloritoid has been also found in metabasic blueschists and retrogressed eclogites associated with glaucophane, garnet, stilpnomelane or pumpellyite.
Ganguly, J. (1969). Chloritoid stability and related paragenesis; theory, experiments, and applications. American Journal of Science, 267(8), 910-944.
Halferdahl, L. B. (1961). Chloritoid: its composition, X-ray and optical properties, stability, and occurrence. Journal of Petrology, 2(1), 49-135.
Harrison, F. W., & Brindley, G. W. (1957). The crystal structure of chloritoid. Acta Crystallographica, 10(1), 77-82.
Hoschek, G. (1969). The stability of staurolite and chloritoid and their significance in metamorphism of pelitic rocks. Contributions to Mineralogy and Petrology, 22(3), 208-232.
Okay, A. I. (2002). Jadeite–chloritoid–glaucophane–lawsonite blueschists in north‐west Turkey: unusually high P/T ratios in continental crust. Journal of Metamorphic Geology, 20(8), 757-768.
Pourteau, A., Bousquet, R., Vidal, O., Plunder, A., Duesterhoeft, E., Candan, O., & Oberhänsli, R. (2014). Multistage growth of Fe–Mg–carpholite and Fe–Mg–chloritoid, from field evidence to thermodynamic modelling. Contributions to Mineralogy and Petrology, 168, 1-25.
Smye, A. J., Greenwood, L. V., & Holland, T. J. B. (2010). Garnet–chloritoid–kyanite assemblages: eclogite facies indicators of subduction constraints in orogenic belts. Journal of Metamorphic Geology, 28(7), 753-768.
Vidal, O., Theye, T., & Chopin, C. (1994). Experimental study of chloritoid stability at high pressure and various f O2 conditions. Contributions to Mineralogy and Petrology, 118(3), 256-270.
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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