Mica

Mica is a group of sheet-silicate minerals, such as muscovite, biotite and phlogopite, that split into thin, flexible, shiny flakes. Ground mica was used as a filler in joint compound, paint, roofing and other products, and vermiculite forms when biotite or phlogopite is altered. Mica is not asbestos, but products containing it sometimes also contained asbestos.

Origin

The word comes from Latin, probably meaning a crumb or to glitter. Mica has been mined for centuries; sheet mica was prized as an electrical insulator, while ground or scrap mica went into fillers and coatings.

Significance in Asbestos Context

Mica matters in asbestos cases in several ways. First, it is a common ingredient in joint compounds, textured coatings and roofing products, some of which also contained chrysotile into the 1970s, so a bulk-sample report may list mica alongside asbestos. Second, vermiculite, including the Libby, Montana vermiculite contaminated with amphibole asbestos, is an altered mica, so the geology of mica deposits bears on that contamination. Third, mica can complicate analysis: in a polarized light microscope, flakes seen on edge can look elongated, and analysts must distinguish them from fibers. Mica dust itself can cause a pneumoconiosis with heavy exposure, but it is not known to cause mesothelioma. In a claim, the presence of mica in a product is mainly useful for identifying the product and its formula.

Example Sentence

The lab report listed the joint compound as 10 percent mica, 40 percent calcite and 5 percent chrysotile.

Related Terms

vermiculite, Libby Vermiculite, Phyllosilicates, Joint Compound, Mineral Filler, Bulk Sample Analysis, Polarized Light Microscopy (PLM)

Notes

• A product that lists mica can still contain asbestos; only testing can tell.
• Do not sand or disturb old textured walls or ceilings to get a sample; ask a licensed professional.