Perlite

For the two-phased structure in steel see pearlite.

Perlite is an amorphous volcanic glass that has a relatively high water content, typically formed by the hydration of obsidian. It occurs naturally and has the unusual property of greatly expanding when heated sufficiently.

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Properties and uses

When it reaches temperatures of 850–900 °C, perlite softens (since it is a glass). Water trapped in the structure of the material vapourises and escapes and this causes the expansion of the material to 7–16 times its original volume. The expanded material is a brilliant white, due to the reflectivity of the trapped bubbles.
Unexpanded ("raw") perlite bulk density: around 1100 kg/m³ (1.1 g/cm³).
Typical expanded perlite bulk density: 30–150 kg/m³

Due to its low density and relatively low price, many commercial applications for perlite have developed. In the construction and manufacturing fields, it is used in lightweight plasters and mortars, insulation, ceiling tiles and filter aids. In horticulture it makes composts more open to air, while still having good water-retention properties; it makes a good medium for hydroponics. Perlite is also used in foundries, cryogenic insulations, as a lightweight aggregate in mortar (firestop) and in ceramics as a clay additive.

Typical analysis of perlite

• 70-75% silicon dioxide: SiO₂
• 12-15% aluminium oxide: Al₂O₃
• 3-4% sodium oxide: Na₂O
• 3-5% potassium oxide: K₂O
• 0.5-2% iron oxide: Fe₂O₃
• 0.2-0.7% magnesium oxide: MgO
• 0.5-1.5% calcium oxide: CaO
• 3-5% loss on ignition (chemical / combined water)

Production trends

  In 2005, Greece was the top producer of perlite with at least one-third world share followed by China, USA, Japan and Turkey, reports the British Geological Survey.

In 2001 the cost of perlite was about US$36.31 per metric ton. The yearend price for mined perlite in the US has increased since then^[1]:

2001.....$36.3 per metric ton

2002.....$36.5 per metric ton

2003.....$38.2 per metric ton

2004.....$40.6 per metric ton

2005.....$42.5 per metric ton

See also

• Vermiculite (Many expanders of perlite are also exfoliating vermiculite and belong to both trade associations)
• Diatomite (used for filter-aids)
• Industrial minerals
• Mortar (firestop)

References

1. ^ (January 2006) "Perlite". U.S. Geological Survey Mineral Commodity Summaries,: 122-123. [1].