Amphibole refers to a group of dark-colored silicate minerals that is typically composed of iron and magnesium ions. Tremolite is one form of amphibole mineral, and it is believed to be the deadliest.
What Does Tremolite Look Like?
Tremolite’s magnesium and iron content varies significantly among different samples, causing it to be different colors. Tremolite that is pure magnesium is creamy white. However, when tremolite contains other minerals in addition to magnesium, it can be gray, lavender, green, or yellow in color. Its fibers can be either elongated or flattened crystals, and are sharply pointed.
Where is Tremolite Found?
Outside of the United States, tremolite was primarily mined in South Africa and India. In South Africa, it was extensively mined in the beginning of the 20th century, but it became secondary to the mining of amosite and crocidolite. In India, it was being commercially mined as recently as the mid1990s in the South Rajasthan region. There were also some commercial tremolite mines in South Korea.
Within the U.S., there was a limited amount of tremolite mining in Powhatan and Pylesville, Maryland. However, tremolite occurs naturally in California in areas where there is ultramafic rock and serpentinite. Ultramafic rocks are igneous rocks that have extremely low silica content but are rich in iron and magnesium. Serpentinite is a metamorphic rock that contains one or more minerals from the serpentine group. It is formed when an ultramafic rock is changed by water. According to the Division of Mines and Geology of the California Department of Conservation, there are areas of ultramafic rock and serpentinite where tremolite asbestos might occur in El Dorado County, California.
The Libby, Montana Mine and Tremolite
From 1963 to 1990, W.R. Grace & Co. operated a vermiculite mine in Libby, Montana. It was eventually discovered that this vermiculite was contaminated with tremolite. The health risk associated with the Libby mine was disclosed by the Seattle Post-Intelligencer in a series of articles that began in its November 18, 1999 issue. The newspaper broke the story of the huge number of deaths over the previous decade resulting from both occupational and non-occupational exposure to tremolite asbestos dust from the Libby vermiculite mine.
The newspaper initiated its investigation of the health crisis in Libby in the summer and fall of 1999 because of a 1996 civil court deposition by former mine business manager Earl Lovick. His testimony proved that W. R. Grace & Co operated the mine even though Company officials were aware of the hazards of asbestos exposure, and that Grace made no efforts to warn either its employees or the town’s residents of the danger.
After the Seattle Post Intelligencer began publishing its articles, local community and business leaders denied the allegations. However, as the facts of the Environmental Protection Agency (EPA) investigations have become public, the validity of Libby’s health crisis has become undeniable.
How Much of a Risk do Vermiculite-Containing Garden Products Pose Today?
Although the Libby vermiculite mine was closed in 1990, there was still concern that vermiculite mined from other sources may also be contaminated with tremolite. To determine how much tremolite a consumer might inhale when using vermiculite-containing garden products, the EPA conducted tests in 2000 to simulate various scenarios in which these products would be used. The agency concluded that consumers, “face only a minimal health risk from occasionally using vermiculite products at home or in their gardens.”