ANTHOPHYLLITE

Anthophyllite – ☐{Mg2}{Mg5}(Si8O22)(OH) – is an inosilicate of the Amphibole Supergroup. It is a rarer amphibole, typical of medium to high grade metamorphic rocks. Anthophyllite with a fibrous habit is one of the six minerals known as “asbestos” and was mined in some locations until the 1970s, when mines were closed due to problems caused by asbestos dust.

Anthophyllite forms a series with cummingtonite, another series with magnesium-anthophyllite, and one more with iron-anthophyllite. It is easy to confuse with proto-anthophyllite. It may contain Ca, Na, Al, Mn and Ti. When with Cr, it constitutes the “kupfferite” variety. When handling the mineral, the production of dust must be avoided, because the anthophyllite fibers are highly carcinogenic when aspirated.

1. Characteristics

Crystal system: Ortorhombic, bipiramidal.

Color: Gray to green, can be brown to beige.

Habit: It usually occurs as lamellar or fibrous, asbestiform crystals. Long prismatic, up to 25 cm.

Cleavage: {210} perfect, {010} imperfect, {100} imperfect.

Tenacity: Brittle. Fibers are elastic.

Twinning: No.

Fracture: Conchoidal.

Mohs Hardness: 5.5 – 6

Parting: No.

Streak: White to gray.

Lustre: Vitreous, pearly on cleavages.

Diaphaneity: Transparent.

Density (g/cm³): 2.95 – 3.2

 

2. Geology and Deposits

Anthophyllite is found only in medium to high grade Mg-rich metamorphic rocks. The reaction is:

olivine + tremolite + talc = olivine + tremolite + anthophyllite.

It is also typical of ultrabasic igneous rocks, by the reaction

olivine + tremolite + talc = olivine + tremolite + anthophyllite.

In impure dolomitic shales, the reaction is

talc + tremolite + magnesite = tremolite + anthophyllite + magnesite.

It occurs in amphibolites, anthophyllite-cordierite-gneisses, metaquartzites, iron formations (Banded Iron Formations – BIF), granulites, anthophyllite-talc-schists derived from clayey sediments, hornfels (cornubianite) and mafic to ultramafic igneous rocks.

It also forms as a product of retrometamorphic reaction in ultramafic rocks, associated with serpentinites. In this case it forms small, poorly developed crystals. It is a frequent process in shear zones, where the fractures open space for the passage of fluids.

 

3. Mineral Associations

Anthophyllite occurs with quartz, feldspar (plagioclase), garnet (almandine), other amphiboles (actinolite, gedrite, tremolite), rutile, micas (phlogopite, vermiculite, Chlorite Group), cordierite, sillimanite, olivine and staurolite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.598 – 1.674    nβ: 1.605 – 1.685     nγ: 1.615 – 1.697

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Usually colorless, more rarely pale green or pale yellow. If Fe-rich, moderate pleochroism in brown, yellowish-brown, gray-brown, gray-blue to green and lilac colors.

Relief: Moderate.

Cleavage: Perfect {210}, generating two cleavages in the basal sections with angles of 54.5º and 125.5º to each other. In the prismatic sections only one cleavage. Other cleavages at {010} and {100}, both distinct.

Habits: Usually lamellar or fibrous, asbestiform (hence the name “gray asbestos”). Also as aggregates of prismatic crystals without well-developed endings.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Medium birefringence, ranging from 0.017 to 0.023, resulting in colors up to the end of 1st order and beginning of second order: yellow, orange, intense red, purple to blue.

Extinction: Parallel in longitudinal sections (diagnostic!), symmetrical in basal sections.

Elongation sign: ES(+)

Twins: No.

Zoning: No.

CONVERGENT LIGHT

Character: B(+) or B(-)

2V angle: 57 – 80o

Alterations: no information available.

May be confused with: actinolite and tremolite may have identical habits, but have oblique extinction.

 

Anthophyllite 01: in PPL, anthophyllite shows pale colors, acicular habit and moderate relief.
Anthophyllite 01: now in XPL, the same crystals as in the previous image. First order end colors, paralell extinction and ES(+).
Anthophyllite 02: On the diagonal of the image, a bundle of anthophyllite acicular crystals in PPL, with biotite (brown in PPL), quartz, zircon (very small) and chlorite (not in this image).
Anthophyllite 02: the same crystals of the previous image, this time in XPL. Typical 1st order colors and paralell extinction - very diagnostic!
Anthophyllite 03: in PPL, a very weak pleochroism is seen. This is the first color, the next image will show the second color.
Anthophyllite 03: also in PPL, now the same group of crystals of the previous image, rotated 90 degrees and with the second color of pleochroism.

5. Reflected Light Microscopy

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

Sample preparation: the polishing of anthophyllite is like the polishing of other silicates, despite the excellent cleavage and fibrous habit. The chipping problems of kyanite and wollastonite do not occur and a final good polishing quality is achieved without problems.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray to light gray, depending on the orientation of the cleavages. Sometimes the crystals appear much darker than common silicates (quartz) and sometimes have the same hue as common silicates.

Pleochroism: No.

Reflectivity: Very low (<10%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anistropy was not observed, but paralell extinction is clear.

Internal reflections: There are many internal, generalized, very bright, white and strong reflections, mixed with some multicolored reflections.

May be confused with: actinolite, whose habit is similar, but the internal reflections are different. Other amphiboles with a fibrous habit may be similar, such as gedrite.

General Characteristics: 

Grain shape: a well-developed fibrous habit is easy to observe and quite diagnostic, with radiated aggregates of acicular crystals forming bundles among the other minerals in the rock.

Cleavage parallel to length, well developed, is very sharp in PPL and in XPL.

Parallel extinction can be observed in XPL.

Anthophyllite 01: in PPL, anthophyllite (light gray, diagonally across the image), other silicates (dark gray) and magnetite (light pinkish gray, in grains on the right).
Anthophyllite 01: same crystals of the previous image, now in XPL. Magnetites are black (isotropic), the other silicates have varied reflections and the anthophyllite beams have very bright white reflections, with some multicolored portions.
<