PYROPHYLLITE

Pyrophyllite – Al2Si4O10(OH)2 – is a rarer phyllosilicate, specific to some types of metamorphic rocks. It is an important industrial mineral, an ore with many different applications whose price depends on the alumina content, other associated minerals and the Fe content, considered an impurity. It is desirable and necessary to remove Fe from the ore until the content is less than 1%.

It is classified in the Pyrophyllite-Talc Group and is the Al analogue of ferrypyrophyllite. A monoclinic and a triclinic polytype are known. There are three varieties (with Fe oxides, with Cr, in gel).

The identification of pyrophyllite by microscopy is problematic, and it is necessary to combine the use of stereomicroscopy, Transmitted Light microscopy and Reflected Light microscopy with other analytical techniques such as Scanning Electron Microscopy, X-Ray Diffractometry, Fourier Transform Infrared Spectroscopy, Thermogravimetry and wet chemical analysis.

1. Characteristics

Crystal system:  Triclinic or monoclinic.         

Color:  White, gray, pale colors (green, yellow, blue), pink, brownish green.    

Habit: Lamellar, micaceous, forming fan-like aggregates, tending to spherulitic. massive. Crystals up to 8 cm. 

Cleavage: {001} perfect, like the micas.      

Tenacity: Flexible, but not elastic, like talc.        

Twinning: No.       

Fracture: No information available.      

Mohs Hardness: 1 – 2

Parting: No.         

Streak: No information available.         

Lustre: Pearly, dull.          

Diaphaneity: Transparent.           

Density (g/cm³): 2.65 – 2.9

          

2. Geology and Deposits

Pyrophyllite occurs in aluminous metamorphic rocks such as metapelites (phyllites and schists), metabauxites and quartzites up to greenschist facies. It can also occur in blueschist facies rocks, formed under high pressure and low temperature metamorphism conditions. It always originates from the alteration of Al-rich minerals such as kyanite and corundum. May occupy irregular veins that traverse altered rocks.

In volcanic rocks with moderate to high silica content, it originates from the hydrothermal alteration of feldspars, presenting a very fine grain and associating with quartz, clay minerals, sericite, zoisite and other aluminous minerals. Pyrophyllite is one of the minerals that form the rock known as “agalmatolite” or “pagodite”, a metamorphic rock formed by the hydrothermal alteration of rhyolites and aluminous metamorphites.

It also occurs in quartz veins and can be formed diagenetically in sandstones.

 

3. Mineral Associations

It occurs with some very common minerals such as quartz, calcite and pyrite.

In its specific paragenesis, it is associated with talc, kyanite, chloritoid, andalusite, zoisite, corundum, topaz, mica (sericite) and hematite;

Pyrophyllite ore may contain goethite, quartz, orthoclase, kaolinite, muscovite, rutile, chloritoid, tourmaline, gorceixite and zircon.

It also occurs with some rare minerals such as svanbergite, yttrocrasite-(Y), lazulite, variscite, karpolitha-Mg and gormanite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.534 – 1.556    nβ: 1.586 – 1,589       nγ: 1.596 – 1.601

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, never shows color or pleochroism.  

Relief: Low to moderate.           

Cleavage: {001} perfect, like the micas.           

Habits: Subhedral tabular flakes, compact spherulitic aggregates of radiated acicular crystals, fine-grained, granular, massive foliate laminae.

Individual crystals may be heavily twisted.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence of 0.044 to 0.062, very high, resulting in intense colors of up to 3rd to 4th orders.           

Extinction: Parallel to the cleavage. Always mottled like micas! 

Elongation sign: ES(-) or ES(+), it is not diagnostic.  

Twins: No.        

Zoning:  No.            

CONVERGENT LIGHT

Character: B(-)          

2V angle: 53 – 62º         

Alterations: pyrophyllite hardly alteres.          

May be confused with: talc and muscovite, mainly. When in very small grains, identification under the microscope is impossible and it is necessary to use other analytical techniques, such as X-Ray Diffractometry.

Talc and muscovite are very similar, but pyrophyllite has a smaller 2V angle than these. As the determination of the interference figure is very difficult due to the intense interference colors, chemical analyzes or X-Ray Diffractometry are necessary to obtain a conclusive identification of the pyrophyllite.

Gibbsite and brucite are B(+) and have lower birefringences.

Clay minerals have lower birefringence.         

 

Pyrophyllite 01: Pyrophyllite in a sandstone matrix, whose thin section was prepared after impregnation with resin (blue colors in PPL). Pyrophyllite, among the quartz grains, is colorless.
Pyrophyllite 01: The same section as of the previous image, now in XPL. Pyrophyllite appears as very fine grains with high interference colors.
Pyrophyllite 01: From the same thin section of the two previous images, pyrophyllite among quartz grains in a sandstone. XPL.
Pyrophyllite, muscovite, quartz, rutile and others in agalmatolite (metamorphic rock formed by the hydrothermal alteration of rhyolites and aluminous metamorphites). Image obtained with the lowest magnification objective (2.5x) and 10x ocular. The granulation of the rock is so fine that even at higher magnifications it is not possible to differentiate pyrophyllite from muscovite, both with very similar optical properties. In this case, it is necessary to use other analytical methods (X-Ray Diffractometry) for the mineralogical characterization of the rock.
Pyrophyllite 02: In PPL, large pyrophyllite crystals, very similar to muscovite.
Pyrophyllite 02: The same section as of the previous image, now in XPL. Pyrophyllite is extremely similar to muscovite, but the interference colors are of a higher order, which you can recognize with some experience.

5. Reflected Light Microscopy

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

Sample preparation:  the polishing of pyrophyllite, even when in large crystals, is always of poor quality due to the very low hardness of the mineral. Any harder associated mineral eventually loosens during the polishing process and generates deep polishing scratches in the pyrophyllite. Difficulties increase in small-grained pyrophyllites, which flake off during polishing. Even so, it is possible to reasonably polish both pyrophyllite into large crystals and pyrophyllite into agalmatolite to recognize the associated opaque minerals.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray.       

Pleochroism: No.      

Reflectivity: Very low (<<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy is not perceived due to internal reflections.        

Internal reflections: Abundant, generalized in the color of the mineral in hand specimen. 

May be confused with: many other light colored transparent minerals, especially with talc and other low hardness minerals. It is impossible to identify pyrophyllite in Reflected Light.       

Pyrophyllite 01: In PPL, pyrophyllite with dark gray reflection colors. The polishing scratches are very evident, as well as tiny grains of other minerals, all with a good polish (almost white colors) and high relief (black border around them) due to their higher hardness (probably magnetite). Holes in black.
Pyrophyllite 01: The same section as of the previous image, now in XPL. Internal reflections, in this case orange (same as the color of the sample in hand specimen), dominate, but there are colorless reflections and, in some points, multicolored reflections depending on the orientation of the cleavage planes. Some of the small grains of other minerals are isotropic (magnetite?) while others have internal reflections as well.