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Asbestos in the United States: Occurrences, Use and Control
(Released April 2008)

 
  by Andreas Saldivar & Vicki Soto  

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Chemistry and Crystal Structure

All of the regulated asbestos minerals can crystallize in a massive habit (non-fibrous) or in the asbestiform habit (fibrous). When crystallizing in the asbestiform habit the mineral grows rapidly in thin, hair-like crystals. This structure is very strong and flexible, with tensile strength an order of magnitude higher than the massive habit. The surface structures are more resistant to acids than the same minerals in other habits.

All the asbestos minerals are silicates, minerals with a silica tetrahedron structure that consists of one oxygen atom surrounded by four silicon atoms in a pyramid. The asbestos minerals are split into two groups: Serpentine and Amphibole. The Serpentine group has one member, chrysotile.

Picture 5: Chemical structure of Amphibole and Serpentine Asbestos
triangular/hexagonal structure

Chrysotile

The chemistry of chrysotile is Mg,3Si2O5(OH)4. It belongs to the phyllosilicates (sheet or layer silicates), a mineral group that includes clays and micas. Phyllosilicates all have two basic layers. The first layer,or tetrahedral layer, is made up of a continuous sheet of silica tetrahedra. The second layer, or octahedral layer, consists of a cation surrounded by six oxygen atoms. This layer resembles two pyramids connected base to base.

The unit cell or basic structure of chrysotile is one tetrahedral layer linked with a single octahedral layer. The layers combine and share oxygen atoms. The spacing of the oxygen atoms in the tetrahedral layer is 0.305 nm. The spacing in the octahedral layer is 0.342 nm. The structure accommodates this mismatch in oxygen distances by curving and scrolling. The result are sheets that roll up into long scrolls. An easy visualization of this is to imagine rolling up a poster. When laid out flat the poster resembles a sheet; when rolled up it resembles a fiber.

Picture 6: Chemical Structure of Chrysotile (Serpentine) Asbestos
3D model

Regulated Amphibole Asbestos

Amphiboles asbestos minerals are double chained inosilicates. The United States currently regulates five fibrous amphiboles: Amosite (Fe,Mg) 7Si8O22 (OH) 2, Crocidolite Na2(Fe+2,Mg) 7Fe+32Si8O22 (OH) 2, Anthophyllite (Mg,Fe) 7Si8O22 (OH) 2, Tremolite Ca2Mg5Si8O22 (OH) 2, and Actinolite Ca2(Mg,Fe) 5Si8O22 (OH) 2. There are four groups of amphiboles: iron-magnesium-manganese, calcic, sodic-calcic, and alkali. Common to all the regulated amphiboles is Si8O22 (OH) 2. The amount of magnesium, iron, sodium, and calcium determines which group the mineral belongs to.

Amphiboles also participate in solid solutions, minerals where the ratio of one element to another can vary without the crystal structure of the mineral changing. For example, Tremolite is in solid solution with Actinolite. The two possess the same crystal structure, but the ratio of magnesium to iron is different. Tremolite is all magnesium, while Actinolite is a mixture of magnesium and iron. And, finally, the non-regulated amphibole Ferroactinolite -- Fe2Mg5Si8O22 (OH) 2 -- is all iron.

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