Pyrophyllite

Chemical Formula

Al₂Si₄O₁₀(OH)₂

Topics Covered

Background

Pyrophyllite belongs to the montmorillonite group of minerals. It is remarkably similar to talc, which belongs to the same mineral family, except that magnesium is substituted for aluminium. In fact, chemical; tests are often requires to tell the two apart.

Its name comes from the Greek words “pyr” and “fullon” meaning fire and leaf respectively. The derivation stems from the fact that pyrophyllite breaks or exfoliates into leaves when fired to temperatures in excess of 800°C as a result of dehydration of its structure.

Occurrence

Pyrophyllite is quite abundant but not particularly common. It is found most commonly found in slate, phyllite and some schists.

The main deposits are found in China and Carolina in the USA. Other notable occurrences are located in Belgium, Switzerland, Mexico; Brazil, Sweden, Russia, and Japan.

Structure

Pyrophyllite has a sheet-like structure consisting of two silicate layers being sandwiched between gibbsite (Al(OH)₃) layers.

Key Properties

· Pyrophyllite is quite soft with a Mohs hardness of 1 to 1.5 and can be scratched with a finger nail

· It can be machined to tight tolerances using conventional maching techniques and tools

· It is thermally stable up to about 800°C when it will begin to exfoliate

Applications

Historically pyrophyllite has been used for carving statues and ornaments owing to its relative ease of machinability. More recently, this property has seen it used as a machinable ceramic material used for electrical resistors, transducer cores, high vacuum gaskets and insulators in electron microscopes.

Other uses for pyrophyllite are in:

· Ceramic formulations e.g. tile and refractory compositions

· Plastics and rubber as a filler

· Paint

· Insecticides