Contact Metamorphism

Contact metamorphism is the change in a rock's minerals caused by heat from a nearby body of molten rock (magma) that pushes into it. When the surrounding rock is a silica-bearing dolomite or limestone, the heat and fluids can produce minerals such as tremolite, diopside, talc and wollastonite, sometimes including fibrous forms.

Origin

Geologists have distinguished contact metamorphism, which works over a limited zone around an intrusion, from regional metamorphism, which affects large areas during mountain building, since the nineteenth century. The zone of altered rock around an intrusion is called a contact aureole.

Significance in Asbestos Context

Contact metamorphism helps explain why some deposits of talc, wollastonite, vermiculite host rock and crushed stone contain amphibole fibers. Tremolite in particular is a classic product of heating silica-bearing dolomite, and in some settings it grows as asbestiform fibers while in others it forms ordinary prismatic crystals. Geology reports on a mine often describe these processes, and experts on both sides use them: plaintiffs to show that the conditions for asbestos formation existed in a deposit, defendants to argue that the amphiboles there grew in a non-asbestiform habit. Because the same deposit can contain both habits, geology alone rarely settles the question; testing of the actual product or ore is what counts.

Example Sentence

The expert explained that contact metamorphism of the dolomite had produced tremolite in the talc ore.

Related Terms

Metamorphism, Regional Metamorphism, Skarn, Tremolite, Talc, Dolomitic Limestone, Metamorphic Process

Notes

• Geology can suggest what a deposit might contain, but testing of the actual product is stronger evidence.
• Ask experts to explain in plain terms how a mine's geology relates to the fibers found.