TALC

Talc – Mg3Si4O10(OH)2 – is an uncommon phyllosilicate, important as a rock former and very important as an industrial mineral.

It has 5 varieties and can contain Fe, F, Ni, Al, Ca, Na and H2O. Sometimes talc is contaminated with fibrous amphiboles (“asbestos”) and, in these cases, the inhalation of dust poses a health risk (asbestosis).

1. Characteristics

Crystal system: Triclinic or monoclinic prismatic. 

Color: Usually light to dark green, can be colorless, white, beige, pink, brown, gray to black.

Habit: Tabular, micaceous, fibrous, fine granular, massive, radial, pyramidal crystals extremely rare.

Cleavage: {001} perfect.

Tenacity: Sectile, flexible.

Twinning: No.

Fracture: No information available.

Mohs Hardness: 1

Parting: No.

Streak: White.

Lustre: Pearly, waxy.

Diaphaneity: Transparent.

Density (g/cm³): 2.58 – 2.83

 

2. Geology and Deposits

Talc is an uncommon mineral that occurs in mafic and ultramafic rocks rich in Mg and hydrothermally altered in conditions of metamorphism of greenschist facies, such as serpentinites and steatites, where brucite is associated. In these occurrences it occurs with chlorite or magnesite in the reaction zones around the serpentinite bodies.

It also occurs with calcite and tremolite in metamorphosed siliceous marbles and in talc-tremolite-schists. Occurs in magnesian skarns.

“Steatite” designates a compact metamorphic rock, popularly known as “soapstone”, which is mainly composed of talc, with very subordinate amounts of other minerals (chlorite, tremolite, quartz, magnesite, etc.). The granulation is very fine and even under the microscope it is very difficult to distinguish, for example, between tremolite and talc. The rock has a greasy feel due to the large amount of talc and has some very interesting properties: it is impermeable, it is not attacked by alkaline or acidic substances, it resists temperatures from negative to 1,000oC and it resists weathering. This rock has many different uses and has the potential to create a regional hub for the production of decorative artifacts and utilities, for example.

 

3. Mineral Associations

It is associated with quartz, pyrite, kyanite and

– carbonates (calcite, dolomite, magnesite), generally abundant,

– other phyllosilicates (chlorite, vermiculite, clay minerals),

– typical low-grade metamorphic minerals such as serpentine, actinolite, tremolite, anthophyllite, brucite, “asbestos” and

– minerals from source rocks such as olivine (forsterite), pyroxenes (diopside), hematite, magnetite, anthophyllite and chromite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.539 – 1.550     nβ: 1.589 – 1.594      nγ: 1.589 – 1.600

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. Only in the dark varieties can it present color and a discreet pleochroism.

Relief: Low to moderate.

Cleavage: {001} perfect (like the micas!)

Habits: Very fine granular, lamellar, fibrous, foliate, radiated, massive. Flakes and aggregates, often with cleavage in one direction. “Steatite” is a massive variety of very fine-grained talc with a greasy feel that makes up “soapstone”.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence of up to 0.051: colors of up to 3rd order, giving it a white (“pearlaceous”) appearance with various colors mixed together. In the basal sections it presents gray colors of 1st order.

Extinction: Parallel to the direction of cleavage and mottled (like micas!)

Elongation sign: ES(+) (like the micas!)

Twins: No.

Zoning: No.

CONVERGENT LIGHT

Character: B(-), can simulate being uniaxial (2V = 0). The high interference colors and the generally tiny size of the crystals make obtaining the interference figure usually very difficult, almost impossible.

2V angle: 0 – 30º  

Alterations: talc is very resistant, but can alter to chlorite in some cases.

May be confused with: other flake-like minerais with strong interference colors. When in small grains, it cannot be distinguished from muscovite, pyrophyllite and sericite by optical means. In this case, the association of minerals is used for identification.

Muscovite is very similar and easy to confuse with talc, especially if the grains are very small. Compared with muscovite, talc has a higher birefringence and “zoned” or banded appearance of colors. In addition, talc has a 2V smaller angle than muscovite (2V = 35-50º).

Paragonite has 2V = 40-50º, an information of less importance due to the difficulty to obtain good interference figures in minerals which such high interference colors.

Pyrophyllite is virtually identical with talc, but has 2V = 53 – 62º (usually difficult to measure).

Brucite is similar, but has lower interference colors and is U(+) (difficult to see, grains are very small).

Gibbsite is B(+), but the same problem as with brucite: flakes too small for a decent interference figure.

 

Talc 01: In PPL, talc in large crystals. Cleavage is well visible, the same as for muscovite, for examplo.
Talc 01: The same section as of the previous image, now in XPL. The interference colors are higher than the ones of mucovite.
Talc in large crystals in XPL. The colors are as colorful as those of muscovite, but more luminous and “pearlaceous”, a perception that only develops with some practice.
In XPL, talc (colored) and calcite (brownish colors) in marble. The association of talc with calcite and magnesite is very common.
Talc in large crystals in XPL with its typical habit, very similar to the muscovite habit.
In XPL, talc in small crystals (colored) and chlorite (dark colors) in talc-chlorite-schist.
In XPL, basal section of talc with typical gray interference colors. Around it, longitudinal sections with their strong colors.
Talc and other minerals, all extremely fine-grained, in soapstone (steatite). Image obtained with the lowest magnification objective in XPL. Even with the maximum magnification objective, it is very difficult to individualize the minerals.
Talc 02: Rock formed by dolomite porphyroblasts (confirmed by colorimetric test), extremely fine-grained talc and equally fine chlorite dispersed in talc. Here, in PPL, dolomite and talc are colorless. Dolomite has higher relief. Chlorite is difficult to perceive, but exhibits a discreet green color with faint pleochroism.
Talc 02: The same section as of the previous image, now in XPL. Dolomite exhibits the typical interference color of carbonates, talc is very colorful (difficult to see due to the tiny size of the crystals) and chlorite has dark anomalous colors.
Talc 02: In XPL, detail of the same rock pictured before. Dolomite is in extinction position (dark), talc is very colorful and chlorite, difficult to see in the image, is dark, tending to bluish gray.
In XPL, talc-chlorite-schist of very uniform granulation, with lamellae of the same size of talc and chlorite. Talc has intense colors and chlorite has gray, somewhat bluish colors. Image with the lowest magnification objective (2.5x). In PPL, talc and chlorite are colorless.
Talc 03: In XPL, skarn formed by chlorite and talc in parallel bands, seen with the 2.5x objective (ocular 10x). Talc is remarkably similar to muscovite.
Talc 03: The same rock as of the previous image, now observed with the objective of 10x magnification (ocular 10x).

5. Reflected Light Microscopy

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

Sample preparation: polishing talc, due to its very low hardness and micaceous habit, is almost impossible. Even when grinding very carefully, for a short time, with a lot of water and little force, a polished surface is not achieved. Other associated minerals, usually of much greater hardness, end up with a high and uncomfortable relief. A property of talc is to make the blade “stick” to glass or porcelain during abrasive grinding. It is almost impossible to do the thinning with water, being necessary to use mineral oil. 

PLANE POLARIZED LIGHT – PPL

Reflection color: Light gray. As talc does not provide a uniform polished surface, it is possible to observe the talc lamellae in their different observations.

Pleochroism: No.

Reflectivity: Low (<<10%).

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  There appears to be a slight grayscale anisotropy, largely masked by internal reflections, the orientation of the individual lamellae and the associated carbonates (which have strong gray anisotropy).

Internal reflections: Generalized colorless, clear, white, yellowish to slightly greenish. Spots with multicolored reflections may occur.

May be confused with: other phyllosilicates and amphiboles of similar habit and light colors that occur in the same paragenesis. It is not possible to identify talc with Reflected Light.

Talc 01: Talc in very pure talc ore. A uniform polished surface is not formed due to the low hardness and the talc habit.
Talc 01: The same section as of the previous image, now in XPL. Flakes show clear, white to greenish reflections.
Talc 02: Talc-schist in PPL. The very likely existence of at least three or four different minerals, all transparent and light in color, makes it difficult to recognize talc in the rock mass. Associated minerals, which are harder and with defined colors, are likely to be easy to recognize.
Talc 02: The same section as of the previous image, now in XPL. Yellox internal reflections dominate.
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