CHLORITE

Chlorite – (Mg,Al,Fe)3(Si,Al)4O10(OH)2.(Mg,Al,Fe)3(OH)6 – is a very common phyllosilicate that can occur in many types of igneous, metamorphic and sedimentary rocks. It has no economic importance.              

“Chlorite” is actually not a mineral, but just a generic term applied to the phyllosilicates that make up the 12-member Chlorite Group. The most common are clinochlore (of Mg) and chamosite (of Fe). Its differentiation is not possible under the petrographic microscope.              

Chlorites are orthorhombic, monoclinic and triclinic. All of them have varieties: clinochlore has 11 and chamosite has 6. All chlorites have impurities: clinochlore can contain Ca and Cr, chamosite can contain Ca, Na, K and Mn. Other impurities are Ni and Zn. They are weakly magnetic.

1. Characteristics

Crystal system: Monoclinic, pseudohexagonal.

Color: Green in various shades, yellowish, white, pink, reddish pink.

Habit: Scaly, foliate, fibrous, massive, earthy, granular aggregates.

Cleavage: {001} perfect.

Tenacity: Flexible.      

Twinning: On {001}

Fracture: No information available.

Mohs Hardness: 2 – 2.5

Parting: No.

Streak: Colorless.

Lustre: Silky, pearly, dull.

Diaphaneity: Transparent.

Density (g/cm³): 2.6 – 3.02

 

2. Geology and Deposits

Chlorite is very common. In igneous rocks it occurs due to the alteration of mafic minerals such as amphiboles, pyroxenes, biotite, staurolite, cordierite, garnet and chloritoid. In altered volcanic rocks it is found in cavities and fractures.              

In metamorphic rocks it is characteristic of the greenschist facies of rocks formed in contact and regional metamorphism, such as chlorite-schists, serpentinites, marbles, chalcosilicate rocks, phyllites, quartzites, amphibolites. It can occur in ultramafic rocks.              

Chlorite also occurs in hydrothermal vein deposits, in sediments and in soils. It is formed by the hydrothermal alteration of any type of rock, through the recrystallization of clay minerals or the alteration of mafic minerals. Chlorite occurs in banded iron formations and forms under reducing conditions in the presence of decomposed organic material (chamosite).

 

3. Mineral Associations

It is associated with many different minerals, any listing will always be incomplete.              

Chamosite is commonly associated with quartz (rock crystal and smoky crystal), kaolinite, pyrite, titanite, fluorite, plagioclases (albite), hematite, garnet (almandine), carbonates (calcite, siderite), magnetite, olivine and epidote. 

Clinochlore is associated with quartz, garnets (grossular, uvarovite and andradite), plagioclase, talc, olivine, diopside, hessonite, titanite, chromite, vesuvianite, magnetite, serpentine, epidote and carbonates (calcite, dolomite)              

Also with staurolite, andalusite, chloritoid, cordierite, actinolite, etc.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.562 – 1.594        nβ: 1.565 – 1.594         nγ: 1.565 – 1.565 (clinochlore)

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless or with weak to moderate pleochroism in various shades of green. The green tones become more intense with higher Fe contents.

Chlorite-Cr is pinkish violet, blue. Chlorite-Mg is colorless.

Relief: Low to moderate. If with high iron content, it has high relief.

Cleavage: {001} perfect, controls the orientation of the fragments.

Often not visible because the crystals are too small.

Habits: Pseudohexagonal tabular crystals, usually micaceous or foliate, forming radiating aggregates. Also fibrous, granular, earthy or massive.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Zero to 0.02 birefringence, very low: colors rarely go beyond first-order white or yellow. It often presents anomalous interference colors, which can be of four main types: deep blue (“Berlin blue”), bluish-gray, violet-yellow or leather-brown; more tenuous or more intense and defined.

Extinction: Oblique, angle of 0 – 9º by the cleavage. It can simulate parallel extinction but is not mottled (“birds-eye”), which is an important diagnostic feature.

Elongation sign: Clinochlore has ES(-) and chamosite has ES(+).

Twins: On {001}, common but often difficult to recognize.

Zoning: Often zoned, may exhibit pleochroic halos.

CONVERGENT LIGHT

Character: B(+) or B(-), depending on composition and mineral, can simulate being uniaxial.

2V angle: B(+): 0o-60º, B(-): 0o-40º

Alterations: chlorites are quite resistant to alteration. They can alter to clay minerals, to Fe oxides or to Mg and Fe carbonates. With progressive metamorphism, with the availability of K, they change to biotite and/or amphibole.

May be confused with: the slight green color and pleochroism are very diagnostic. When they do not show color or pleochroism, usually the shape and the interference colors (bluish/gray/dark brown) allow them to be recognized. They are similar to micas, but have no mottled extinction.

Serpentine has lower birefringence (gray to white colors).

Clintonite and chloritoid have higher birefringence.

In sediments it is practically impossible to differentiate chlorites from clay minerals.

 

In XPL, biotite (greenish-yellow interference colors) changing to chlorite (blue to lilac colors), a very common situation. The blue interference colors of chlorite are anomalous. The deep blue color is called "Berlin blue". Around it, quartz (gray).
Clorite 01: Volcanic rock with an idiomorphic phenocryst (mica?) completely altered to an aggregate with hundreds of submicroscopic chlorite crystals. In PPL, chlorite has the typical pale green color and a slight pleochroism.
Clorite 01: The same section as of the previous image, now in XPL. Chlorite shows one of its anomalous interference colors, a deep blue. The jackets of Prussian soldiers, until World War I, had this color, so the color is called “Berlin blue” to this day.
Chlorite 02: Chlorite in large aggregates in metamorphic rock. There are several aggregates. In PPL, they exhibit the typical pale green colors. Pleochroism is not so simple to see because the size of the crystal is very small.
Chlorite 02: The same section as of the previous image, now in XPL. Chlorite exhibits here its second anomalous interference color, a gray with more or less bluish tones.
Chlorite 03: Chlorite in a serpentinite. Here, the chlorite is almost colorless, probably to high Mg-content.
Chlorite 03: The same section as of the previous image, now in XPL. Chlorite shows the third possible anomalous interference figure, a color called "leather-brown". Around it, serpentine.
Chlorite 04: Chlorite aggregate which, in PPL shows a slight pleochroism between almost colorless and pale green.
Chlorite 04: The same section as of the previous image, now in XPL. This is the fourth possible anomalous interference color of chlorite, in violet and orange. This habit of aggregates that tend to be spherical, with a radial structure (“booklets”), is quite common.
Chlorite 05: In a volcanic rock, a vesicle (gas bubble) filled by radial aggregates of chlorite from the cavity walls. In PPL, habit, discrete green colors and pleochroism are diagnostic.
Chlorite 05: The same section as of the previous image, now in XPL. Such anomalous blue and purple interference colors are common in chlorites.
Chlorite 06: Serpentinite formed by opaque minerals (black, probably magnetite), serpentine and chlorite. In PPL, serpentine and chlorite are almost colorless, but serpentine is slightly darker than chlorite.
Chlorite 06: The same section as of the previous image, now in XPL. Chlorite forms lenticular bodies (mica-fish type) (arrow indicates one of them), subparallel, with pseudo-parallel and non-mottled extinction. It is probably a magnesian chlorite, which therefore has a gray interference color like the serpentine around it as well.
In intensely altered metagabbro, accompanied by epidote, chlorite occurs in the common (lamellar) habit and, at some points, these globular chlorite aggregates. As the visualization of these aggregates is integral, their diameter must be much smaller than the thickness of the thin section (30 microns). Image in PPLe, 63x objective + zoom. In XPL, this chlorite has very dark colors.
In XPL, in metamorphic rock, with talc, especially well developed aggregates of chlorite with gray interference colors. Typical are habit, near-parallel extinction, and unmottled extinction.
From the same section of the previous image, now in detail. XPL.
Chlorite 07: In PPL, siderite (high relief), chlorite (medium relief) and talc (low relief), all colorless.
Chlorite 07: The same section as of the previous image, now in XPL. Chlorite (gray, diagonal of the image), talc (colored) and siderite (brown).
Chlorite 08: In the center of the images, chlorite aggregates with a well-developed vermiform habit. Around it, biotite (brown), amphibole (green) and quartz (colorless).
Chlorite 08: The same section as of the previous image, now in XPL. Chlorite show dark gray interference colors. It is a rarer habit, but possible in the case of phyllosilicates.
Chlorite 09: In the center of the images, thick tabular chlorite crystal, surrounded by other ones, in serpentinite. The chlorite crystals show well developed cleavage, parallel extinction (but not mottled!) and ES(+). Around the crystals, serpentine, talc and magnetite.
Chlorite 09: The same section as of the previous image, now in XPL. Talc is colorful, the other minerals are in gray.

5. Reflected Light Microscopy

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

Sample preparation: chlorite acquires a good polish without difficulty, but there will always be some persistent polishing grooves due to the low hardness.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, a shade more pure gray than serpentine.

Pleochroism: No.

Reflectivity: Very low (4%?)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  It shows no anisotropy, just a slight variation in luminosity.

Internal reflections: Generalized in very dark colors. At the edges of the grains, it is possible to observe lighter areas (thinner portions of the grains) where the color tone becomes better defined.

May be confused with: biotite and phlogopite, which have similar habits, occur in the same paragenesis and also have dark internal reflections, but do not have them with bluish, lilac and leathery brown tones.

Chlorite 01: Chlorite aggregate in serpentinite. It has a slightly lighter color than the serpentine around it. The white dots are magnetite, a product of alteration.
Chlorite 01: The same section as of the previous image, now in XPL. Chlorite shows internal reflections in bluish brown. With Transmitted Light, these chlorites show the anomalous interference color called “leather brown”.
In the image, in PPL, on the left, in pure gray, a chlorite aggregate. On the right, in more yellowish grey, serpentine. In dark brown, unidentified material. The rock is a serpentinite. In the cleavages of the chlorite and along the edges of the aggregate, magnetite (very light, almost white) develops as an alteration product.
<