KYANITE

Kyanite – Al2SiO5 – is a common nesosilicate, typical of metamorphic rocks, constituting an important industrial mineral used in the production of refractories and ceramics. An ancient name for kyanite, still widely used by some authors, is “Disthen”. In Portuguese, the name of the mineral is “Cianita”.

Kyanite is one of the three polymorphs of the Al2SiO5 Group, with sillimanite and andalusite. Can form crystals up to 50 cm long. A white to gray-white variety is called “rhätizite”. It rarely shows pink-red fluorescence under long UV waves. May incorporate small amounts (<1%) of Cr and Fe.

Aggregates tending to radial, forming bundles of long prismatic black crystals, are sold in stone shops as “witches’ broom”. These kyanites come from the locality of Ribeirão da Folha, in the municipality of Minas Novas (state of Minas Gerais, Brazil).

1. Characteristics

Crystal system: Triclinic pinacoidal.          

Color: Light gray, white, blue, green, rarely pink, yellow, orange, red to black. Zonation is possible.

Habit: Tabular or laminar crystals, often deformed. Rarely fibrous or porphyroblastic with “sieve texture”       

Cleavage: {100} perfect, {010} good.       

Tenacity: Brittle.        

Twinning: Lamellar on {100}, common.       

Fracture: Splintery.       

Mohs Hardness: 5,5 parallel to [001], 7 parallel to [100].

Parting: On {001}.         

Streak: Colorless.         

Lustre:  Vitreous, silky, resinous.         

Diaphaneity: Transparent.           

Density (g/cm³): 3.53 – 3.67

 

2. Geology and Deposits

Kyanite is a common medium to high pressure metamorphic mineral that occurs mainly in Al-rich metasedimentary rocks of pelitic composition, such as schists and gneisses.

It can occur in granitic pegmatites, granulites, eclogites and kimberlites.

Detrital it occurs in sediments and sedimentary rocks, being frequently mined from placers.

It does not occur in igneous rocks.

 

3. Mineral Associations

It occurs with common minerals like quartz, feldspars (anorthite), micas (muscovite, biotite, phengite, paragonite), amphiboles (“hornblendes” and “omfacites”), and clinopyroxenes.

With other typical metamorphic minerals like staurolite, sillimanite, andalusite, chloritoid, garnet and corundum.

Also with gedrite, mullite, rutile, pyrite, talc and hematite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.712 – 1.718    nβ: 1.720 – 1.725   nγ: 1.727 – 1.734

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Normally colorless. May have bluish spots. Rarely pale blue with weak pleochroism: X = colorless, Y = pale violet blue, Z = pale cobalt blue.

Relief: High.           

Cleavage:  {100} perfect and {010} good. In the longitudinal sections, only one perfect cleavage is observed. In the basal sections, the two cleavages are observed, which intersect at 79º (diagnostic!).          

Habits: Columnar to elongated crystals, tabular parallel to {100}.

The tabular habit, with a cleavage direction parallel to the elongation, is a very diagnostic feature.

Due to the excellent cleavage and type of tenacity of kyanite, which shatters very easily during the process of making the thin blade, holes often occur in the blade associated with kyanite crystals. 

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.012 to 0.016 (low to medium), resulting in 1st order interference colors: gray to faint yellow (straw yellow) and orange. These low interference colors are very typical and diagnostic.

Due to the difficulty of making the thin sheet with kyanite, it is possible that some crystals are thicker than 30 microns and present orange to red and blue colors.          

Extinction: Oblique from 0 to 32º in tabular crystals. It can simulate being parallel!           

Elongation sign: ES(+).            

Twins:  Simple and lamellar, very frequent.        

Zoning: No.             

CONVERGENT LIGHT

Character:  B(-).         

2V angle: 78 – 82º         

Alterations: alters to sericite or other phyllosilicates. With more intense metamorphism, it is replaced by sillimanite.          

May be confused with: some other minerals, including some from the same paragenesis.

Pyroxenes may be similar, but in the basal section their cleavages intersect at an angle of 87º and 93º and their interference colors are higher than those of kyanite.

Andalusite and sillimanite, its polymorphs, have smaller extinction angles and different interference colors.

Clinozoisite has typically lower birefringence, often shows anomalous interference colors, does not form tabular crystals with a cleavage parallel to the elongation, and has a smaller 2V angle.

Orthopyroxenes have parallel extinction and pleochroism.

As kyanite shatters easily, it makes thin section making very problematic, losing fragments during the grinding process and creating holes in the thin section. These holes, often associated with kyanite remains, are nonetheless diagnostic.         

 

Kyanite 01: These are several basal sections with their two cleavages intersecting at 79º angles. In PPL, high relief and the angle between the two cleavages are characteristic.
Kyanite 01: The same section as of the previous image, now in XPL. The correct interference colors of kyanite are less vivid, more grayish to yellowish, but this thin section is slightly thicker than 30 microns.
Kyanite 02: In PPL, longitudinal sections of kyanite, colorless and with high relief.
Kyanite 02: The same section as of the previous image, now in XPL. These are the correct interference colors of kyanite, something between gray and yellowish.
Kyanite 03: Aggregate of kyanite crystals. Due to the splintery tenacity of kyanite, holes and missing parts are common in thin sections with kyanite.
Kyanite 03: The same section as of the previous image, now in XPL. This image is very representative of kyanite appearance in XPL.
In XPL, radial grouping of long prismatic kyanite crystals. The thin section is thicker than 30 microns and the interference colors of the kyanites are too colorful.
In XPL, two radial clusters of kyanite side by side, from the same thin section as the previous image.
In XPL, clustering kyanite crystals. Their interference colors are incorrect due to the thickness above normal of the thin section. As said, kyanite is very splintery and show very anisotropic hardness, which turns thin section making very difficult, like wollastonite.

5. Reflected Light Microscopy

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

Sample preparation:  the polishing of kyanite, due to the excellent cleavage, the anisotropic hardness and the mineral’s property of shattering, is very difficult, almost impossible. The surface is left with many holes, terminations of cleavage planes and even cleavages raised, loose, one on top of the other. It’s one of the worst minerals to polish, as bad as wollastonite. This poor quality of the polish, allied to the generally tabular shape of the crystals, is very diagnostic, obviously considering the paragenesis.      

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, somewhat lighter than quartz and feldspar. Colored internal reflections are usually visible even in PPL.

Pleochroism: No.     

Reflectivity: Low (<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: It is not possible to distinguish isotropy or anisotropy due to the reflections.        

Internal reflections: Abundant, very colorful and bright. In some cases only milky, depending on the orientation of the cleavages in relation to the plane of the polished section. When the kyanite has an intense blue color, the internal reflections appear in various shades of blue, depending on the thickness of the kyanite crystal present in the polished section.      

May be confused with: some amphiboles with a similar habit (actinolite) and other light colored silicates which, however, will present a much better quality polish.       

General Characteristics: 

Grain shape: the tabular shape of the crystals is very typical.

Polishing scratches are more abundant than in other common rock-forming silicates.

Cleavage can be very clearly visible, but it depends on the angle between the section surface and the cleavage. There will be sections with no visible cleavage and others with well-developed cleavage.

Polishing defects are often very abundant, represented by holes in the section. In this case, the reflections can be of different colors. In PPL the holes appear black.

On the diagonal of the image, in XPL and with the 4x objective, kyanite prismatic crystal with a strong blue color, presenting internal reflections in various shades of blue and well-marked cleavages. Kyanite can present several colors (colorless, black, greenish, blue) in various intensities, making the internal reflections very dependent on the color, the color intensity and the mineral thickness present in the polished section.
Kyanite 01: Vertical tabular crystal of kyanite, with two acicular crystals of sillimanite to its right, hosted in quartz. In PPL, there are some colored internal reflections, lots of grooves and polishing problems.
Kyanite 01: The same section as in the previous image, now in XPL. Multicolored internal reflections are very typical.
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