ANDALUSITE

Andalusite – Al2SiO5 – is a relatively common nesosilicate, characteristic of some types of metamorphic rocks. It could be used as a raw material for silico-aluminous refractories, but seldom occurs in economically viable volumes.

It is one of the polymorphs of the Al2SiO5 Group: kyanite, sillimanite and andalusite. Forms a series with kanonaite (Mn3+Al(SiO4)O). There is a variety with Ti and a green variety with Mn (“viridine”).

In the chiastolite variety, andalusite presents, inside the crystals, carbon in very fine grains, forming two alignments that cross each other in the basal sections, simulating a cross. That’s why the mineral is marketed as “Cross stone” (like twinned staurolite too!) and the basal sections of large crystals end up being used to make pendants.

1. Characteristics

Crystal system: Orthorhombic bipiramidal.

Color: Reddish brown, yellowish, can be pink, yellow, green, gray and white. Rarely violet.

Habit: Idiomorphic prismatic crystals or in columnar aggregates. Fibrous, compact, massive.

Cleavage: {110} good, {100} poor.

Tenacity: Brittle

Twinning: On {101}, rare.

Fracture: Irregular

Mohs Hardness: 6.5 – 7.5

Parting: No.

Streak: White.

Lustre: Vitreous to sub-vitreous.

Diaphaneity: Transparente.

Density (g/cm³): 3.1 – 3.2

2. Geology and Deposits

Andalusite is characteristic of Al-rich metamorphic rocks generated by contact metamorphism and by low to medium grade regional metamorphism such as shales, phyllites and slates, formed under low pressures and medium temperatures.

More rarely, it occurs in granites and associated rocks, such as pegmatites and greisens.

Due to its high hardness and resistance to alteration, it can be found as a detritic mineral in some sands and sandstones.

3. Mineral Associations

Andalusite occurs with quartz, feldspar (plagioclase), micas (biotite, muscovite (sericite), chlorite, “zinnwaldite”), pyrophillite, other members of the Al2SiO5 Group (kyanite, sillimanite), cordierite, staurolite, garnet, corundum, tourmaline, minerals of the Spinel Subgroup and diaspore.

4. Transmitted Light Microscopy

Refraction indices: nα: 1,632     nβ: 1,636      nγ: 1,643

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Usually colorless. More rarely faintly colored, in this case sligthly pleochroic:

X = pale pink in bands or zones,

Y,Z = colorless or pale green.

Mn contents result in greenish colors, Fe contents in pinkish colors and much higher refractive and birefringence indices.

Relief: Moderate to high.

Cleavage: {110} perfect: longitudinal (prismatic) sections have only one direction, in the basal sections there are two cleavages almost perpendicular to each other (angle of 89º), but generally these cleavages are not visible, they are only visible in very large grains. 

Habits: Euhedral granular crystals, short prisms with almost square section with diagonal extinction (very characteristic!).

Also fibrous, compact, massive or porphyroblastic.

In the chiastolite variety there are black carbon inclusions along two perpendicular directions, forming a cross.

May show pleochroic (black) halos around radioactive mineral inclusions. 

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence is low, from 0.007 to 0.013, resulting in 1st order gray colors, at most first order straw yellow.

Extinction: Parallel in the longitudinal sections (prismatic, parallel to the cleavage). Symmetrical in basal sections (which show two cleavage directions when the grains are very large).

Elongation sign: ES(-) in the longitudinal sections. Diagnostic!

Twins: Rarely show cross-shaped interpenetration twins.

Zoning: No.

CONVERGENT LIGHT

Character: B(-). The green variety of andalusite, with Mn, called viridine, is B(+).

2V angle: 71º – 86º

Alterations: andalusite alters to damourite and other phyllosilicates, generating complete pseudomorphs of pinite (= sericite and clay minerals). With increased pressure and temperature, it has relationships with cordierite, staurolite, garnet, sillimanite and kyanite.

May be confused with: several other minerals.

Sillimanite has much higher birefringence (it is more colorful), ES(+) and a smaller 2V angle.

Kyanite has oblique extinction.

Dumortierite (which can be pink!) and tourmaline (which can be colorless!) have ES(-) as well, but their interference colors are much more colorful (higher birefringence).

Orthopyroxenes have ES(+).

 

Andalusite 01: in PPL, andalusite crystals in schist, showing the typical rhombic basal sections, square to rectangular, somewhat altered to sericite. In the center of the grains, black carbon inclusions can form a cross.
Andalusite 01: now in XPL, the same situation of the previous image. Low interference colors and some alteration are typical.
Andalusite 02: in PPL, one longitudinal section and two basal sections of andalusite. Absence of cleavage is very diagnostic.
Andalusite 02: now in XPL, the same crystals of the previous image. The grains are greatly altered, but the parallel extinction and ES(-) are still present.
Andalusite 03: in PPL, basal section of andalusite showing partial alteration. You can clearly see the unaltered portions with higher relief and the alteration with lower relief.
Andalusite 03: now in XPL the same section of the previous figure. Unaltered portions show gray interference colors, typical of andalusite.
In PPL, in garnet-staurolite-mica schist, andalusite in the center of the images, diagonally. It forms aggregates of parallel prismatic crystals whose relief is higher than quartz but lower than staurolite (high relief grains). Parallel extinction, ES(+), gray interference colors and a B(-) figure are diagnostic. In XPL, everybody here is gray.

5. Reflected Light Microscopy

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

Sample preparation: polishing andalusite is relatively simple, despite its high hardness.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray like many other silicates. Lighter than biotite, pyroxene and amphibole, approximately the same shade as feldspar.

Pleochroism: No.

Reflectivity: Low (<10%).

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.

Internal reflections: Generalized in light, yellow colors and tending to milky, resembling the reflections of some potassic feldspars.

May be confused with: feldspars, but the oriented inclusions of carbonaceous material, when they occur, are quite diagnostic.

Andalusite 01: in PPL, section of a large andalusite crystal, of the chiastolite variety, with carbonaceous inclusions in the left half of the image and the crystal without inclusions on the right.
Andalusite 01: now in XPL, the same section of the previous figure. Internal reflections are yellowish to brown.
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