CHLORITOID

Chloritoid – (Fe2+,Mg,Mn)2(Al,Fe3+)Al3O2(SiO4)2(OH)4 – is a relatively rare nesosilicate, characteristic of some specific types of metamorphic rocks. It has no economic importance.

The mineral is part of the Chloritoid Group and forms a series with carboirite, an extremely rare triclinic nesosilicate that occurs in only one location in the world (France). 

“Sismondine” is a synonym for chloritoid. “Masonite” is a variety formed by an impure chloritoid. “Ottrélite” is a name originally given to a Mn rich variety of chloritoid, but today it designates another mineral of the Chloritoid Group, which is also typical of low to medium grade metamorphic rocks.

1. Characteristics

Crystal system:  Triclinic pinacoidal but, through twinning, acquires monoclinic prismatic symmetry. It can form pseudo-hexagonal crystals.         

Color: Dark green to gray green, can be almost black.     

Habit: Tabular thin to thick, massive aggregates. Rarely pseudohexagonal.       

Cleavage: {001} perfect, {110} good.      

Tenacity: Brittle.        

Twinning: {001} common, polysynthetic.       

Fracture: Irregular.       

Mohs Hardness: 6.5

Parting: On {010}.         

Streak:  White, grayish, slightly greenish.        

Lustre: Vitreous, pearly on cleavages.          

Diaphaneity: Transparent. 

Density (g/cm³): 3.4 – 3.8

 

2. Geology and Deposits

Chloritoid typically occurs in pelitic metamorphic rocks of low to medium grade regional metamorphism, such as phyllites and schists, from greenschist facies to the lower staurolite zone.

Chloritoid can occur in quartz veins with carbonates.

Also in glaucophane schists and as an essential mineral in metabauxites (diasporites).

 

3. Mineral Associations

Chloritoid occurs with quartz, albite, chlorite, biotite, garnet (almandine), rutile and glaucophane.

It can occur with staurolite, kyanite, yttrocrasite(Y), pyrophyllite, tourmaline (dravite), piemontite and paragonite.

It never occurs together with stilpnomelane.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.713 – 1.730    nβ: 1.719 – 1.734    nγ: 1.723 – 1.740

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Rarely colorless. Usually with moderate to strong pleochroism:

X = colorless, grey-green, olive green, blue-green,

Y = blue-gray, indigo blue, blue-green and

Z = colorless to yellow-green to brown-green.  

Relief: High.           

Cleavage: {001} perfect and {110} good, normally not visible in thin sections.           

Habits: Tabular, foliate aggregates, hourglass zonation may occur, many (quartz) inclusions are typical. Euhedral, porphyroblastic, lamellar, radial, mica-like habits. Rarely pseudohexagonal crystals.

It is very like chlorite, but the relief is much higher.

May be zoned with darker cores and lighter edges.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence up to 0.010: 1st order interference colors up to a maximum of straw yellow.

Anomalous 1st order (gray) interference colors are common. Color zoning is common. Basal sections are approximately isotropic.

Extinction: Tends to oblique. In the triclinic form, oblique extinction of 20º. In the monoclinic form, parallel extinction along the b axis.           

Elongation sign: ES(-).            

Twins:  Simple or lamellar or polysynthetic parallel to the cleavage, common according to {001}. Twins can be triples (“triplets” or “trillings”) generating a pseudohexagonal symmetry.        

Zoning: Often zoned with darker colors at the core and lighter colors at the edges. May show hourglass zoning.             

CONVERGENT LIGHT

Character:  normally B(+), can be B(-).         

2V angle: 36-70º, with some outliers.         

Alterations: alters to chlorite, sericite and iron oxides and hydroxides. Under progressive regional metamorphism it is replaced by staurolite, almandine (garnet) and hercynite (spinel). May show retrograde chloritization and/or change to limonite and/or kaolinite.          

May be confused with: few other minerals, as pleochroism in shades of green or in strong green and blue is very diagnostic.

Biotite and stilpnomelane have higher birefringence, an always visible perfect cleavage and parallel extinction (mottled in biotite and unmottled in stilpnomelane).

Chlorite has lower relief, lower birefringence and nearly parallel extinction.         

In PPL, chloritoid with hourglass zonation.
In XPL, chloritoid with lamellar twins, simple to polysynthetic (reminiscent of plagioclase twins).
Chloritoid 01: In PPL, idiomorphic (porphyroblastic) crystals of chloritoid in muscovite-quartz-schist. They show slight pleochroism in green, there may be hourglass zonation.
Chloritoid 01: The same section as of the previous image, now in XPL. Chloritoid presents anomalous interference colors, somewhat masked by the green color of the mineral. In addition, well-defined simple and polysynthetic twins may occur.
In XPL, chloritoid with simple twin. In PPL this twin is not visible.
In XPL, two intergrown crystals showing polysynthetic twins. In PPL these twins are not visible.
Chloritoid 02: In PPL, fibrous aggregates of chloritoid with a clear pleochroism between almost colorless and bluish gray.
Chloritoid 02: The same section as of the previous image, now in XPL. The interference color is gray, the extinction oblique and the ES(-). In Convergent Light these chloritoids are B(+).
Chloritoid 03: In PPL, large chloritoid aggregate with a definite green color and a slight pleochroism.
Chloritoid 03: The same section as of the previous image, now in XPL. The strong interference colors, still heavily impacted by the mineral's own color, are diagnostic. Anomalous colors may occur.
In XPL, large chloritoid aggregate with anomalous interference colors, bluish gray to light gray.
In XPL, corrugation twins in chloritoid.
Chloritoid 04: In PPL, chloritoid aggregate with slightly greenish colors, medium to high relief and weak pleochroism.
Chloritoid 04: The same section as of the previous image, now in XPL. Chloritoid shows greenish and anomalous interference colors, very characteristic.
Chloritoid 05: Radial cluster of twinned chloritoid crystals. Chloritoid can form aggregates with a few paralell crystals, or fan-shaped aggregates or, as here, radial aggregates.
Chloritoid 05: The same section as of the previous image, now in XPL. The green colors of chloritoid mask the interference colors.
In PPL, two chloritoid crystals in the center of the image, showing two colors of pleochroism that can occur: on the left with a gray-green cast and on the right with a definite cast to blue.
Chloritoid 06: Chloritoid with exceptional development in metamorphic rock, with sillimanite, cordierite, muscovite, magnetite and others. In PPL, it shows strong pleochroism between blue-green and light yellowish green.
Chloritoid 06: The same section as of the previous image, now in XPL. In XPL these chloritoids present colors between light bluish gray and deep dark blue.
Chloritoid 07: In PPL, two chloritoid grains with pleochroism in different colors. The next image shows the same two grains rotated 90 degrees.
Chloritoid 07: The same section as of the previous image, now rotated 90 degrees. Sometimes the color stays pretty much the same, sometimes it changes a little, and sometimes it changes a lot.
Chloritoid 07: The same section as of the two previous images, now in XPL. Some grains show an intense deep blue interference color, others just a bluish gray.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of chloritoid. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with chloritoid.

Sample preparation: the chloritoid acquires a good polish without difficulty.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Light gray, approximately the same color as quartz and feldspar.       

Pleochroism: No.      

Reflectivity: Low (<<10%).        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.        

Internal reflections: Generalized in dark green tones, directly proportional to the thickness of the grain at the considered point: the finer, the lighter. Thick grains have reflections so dark they appear black. Intense light is required to perceive green tones.      

May be confused with: thick amphiboles and pyroxenes with dark colors and tinge of green. The shape of chloritoid porphyroblasts is diagnostic, the twins and hourglass zonation as well. In the absence of these features, it is very difficult to identify the chloritoid.       

General Characteristics: 

Cleavage is not visible.

Twinning is not visible.

Hourglass zonation it is easy to recognize when it occurs.

Chloritoid 01: Chloritoid porphyroblast in a mica-schist in PPL. The excellent polishing of the chloritoid is visible, in contrast to the poor polishing of the rock, which is very fine-grained. The excellent polishing of the chloritoid is visible, in contrast to the poor polishing of the rock, which is very fine-grained.
Chloritoid 01: The same section as of the previous image, now in XPL. The green tones are easily visible and so is the hourglass zonation.