SERPENTINE
Serpentine is a magnesium hydroxysilicate (Mg3Si2O5(OH)4).
It's a common mineral that occurs in three main forms (polymorphs): antigorite,
chrysotile, and lizardite. Serpentine is usually a metamorphic mineral -
it forms from metamorphic alteration of olivine in the presence of water.
Olivine-rich igneous rocks, such as peridotites, are often expected to be
partially or completely converted to serpentine-bearing rocks (serpentinites)
in mountain belts.
Serpentine is typically mottled green, ranging from
very pale green to black. It has a nonmetallic luster, a hardness of 3 or
4 or 5, and often has a waxy luster & feel. Chrysotile serpentine is
often found in fibrous "veins" (asbestos) having a silky luster.
Antigorite serpentine (3.6 cm across) from northern Michigan, USA.
Geology:
metamorphosed Deer Lake Peridotite, Ishpeming Greenstone Belt, late Neoarchean
Locality:
Ropes Gold Mine, Marquette County, western Upper Peninsula of Michigan, USA
Many serpentine-rich rocks have asbestos
associated with them. Asbestos is not a mineral - the term refers to
fibrous forms of several different minerals. “White asbestos” is
chrysotile serpentine. Other varieties include “blue asbestos” (=
crocidolite amphibole) and “brown asbestos” (= amosite amphibole).
White asbestos was formerly used as a heat insulator,
but the American government has since considered it to be a hazardous material.
So, asbestos removal has occurred throughout America in recent years. Of
the different varieties of asbestos, white asbestos (chrysotile) is the least
hazardous (it slowly dissolves in lung fluids), but this was the most commonly
used form in America. The American government does not appear to acknowledge
that different varieties of asbestos have different health risks, so all kinds
are considered equally "bad".
Chrysotile serpentine (white asbestos) in the form of compacted, extremely
thin, flexible fibers (3.0 cm across at its widest). The whitish areas
are the asbestos “veins”. The dark matrix is serpentinite rock.
Prismatic chrysotile serpentine (16.3 cm across at its widest). Not all
occurrences of chrysotile serpentine are veins of thin, flexible fibers.
The sample shown above is prismatic, having moderately thick sticks of
chrysotile.
Geology:
metamorphosed Deer Lake Peridotite, Ishpeming Greenstone Belt, late Neoarchean
Locality:
Ropes Gold Mine, Marquette County, western Upper Peninsula of Michigan, USA
Fibrous chrysotile serpentine (white asbestos) from a quarry in Northampton County,
eastern Pennsylvania, USA (CM public display, Carnegie Museum of Natural
History, Pittsburgh, Pennsylvania, USA).
Serpentinite (above & below) with “vein” of greenish chrysotile asbestos fibers
from Asbestos, Canada.
