Serpentine is a major rock forming mineral and is found
as a constituent in many metamorphic and weather igneous
rocks. It often colors many of these rocks to a green
color and most rocks that have a green color probably
have serpentine in some amount.Serpentine is actually
a general name applied to several members of a polymorphic
group. These minerals have essentially the same chemistry
but different structures. The following is a list of
these minerals, their formulas and symmetry class:
Antigorite; (Mg,Fe)3Si2O5(OH)4; monoclinic.
Clinochrysotile; Mg3Si2O5(OH)4; monoclinic.
Lizardite; Mg3Si2O5(OH)4; trigonal and hexagonal.
Orthochrysotile; Mg3Si2O5(OH)4; orthorhombic.
Parachrysotile; (Mg,Fe)3Si2O5(OH)4; orthorhombic.
Their differences are minor and almost indistinguishable
in hand samples. However, the chrysotile minerals are
more likely to form serpentine asbestos, while antigorite
and lizardite form cryptocrystalline masses sometimes
with a lamellar or micaceous character. Asbestos had
been used for years as a fire retarding cloth and in
brake linings. Its links to cancer however has led to
the development of alternative materials for these purposes.
Serpentine's structure is composed of layers of silicate
tetrahedrons linked into sheets. Between the silicate
layers are layers of Mg(OH)2. These Mg(OH)2 layers are
found in the mineral brucite and are called brucite
layers. How the brucite layers stack with the silicate
layers is the main reason for the multiple polymorphs.
The stacking is not perfect and has the effect of bending
the layers. In most serpentines, the silicate layers
and brucite layers are more mixed and produced convoluted
sheets. In the asbestos varieties the brucite layers
and silicate layers bend into tubes that produce the
fibers. Serpentine can be an attractive green stone
that takes a nice polish and is suitable for carving.
It has been used as a substitute for jade and is sometimes
difficult to distinguish from jade, a testament to the
beauty of finer serpentine material.Non-fiberous serpentine
is not a cancer concern. Asbestos serpentines should
be kept in closed clear containers, but makes an attractive
specimen. Sometimes with a golden color as the name
chrysotile in greek means golden fibers.