SERPENTINITE
Serpentinite forms by metamorphism of olivine-rich
peridotites (dunites - ultramafic, intrusive igneous rocks).
Metamorphism of olivine in the presence of water results in the formation of
the mineral serpentine (Mg3Si2O5(OH)4).
A metamorphic rock composed principally of serpentine is thus a serpentinite.
Serpentinite has a mottled greenish color, has the
look & feel of hard candle wax, and ranges in texture from crystalline to
"foliated". Many serpentinites have a foliated look to them,
but it's really not due to an alignment of crystals. The appearance of
"foliated" serpentinites is really the result of extensive
development of slickenlined surfaces.
Many serpentinites also have a small component of magnetite,
but it's usually significant enough to feel a slight tug when a magnet is
placed next to the rock.
Many Precambrian greenstone belts have significant
occurrences of serpentinites. Slices of dunitic mantle caught up in
orogenic belts by obduction (ophiolites) are often serpentinized. Many
mantle peridotite masses that have been caught up in rising magmas have been
serpentinized (for example, in kimberlites & lamproites).
Serpentinite (10.0 cm across) ("foliated") from Quebec's Thetford Mines
Ophiolite Complex, representing ancient oceanic lithospheric peridotite that
was metamorphosed during the Taconic Orogeny (Ordovician).
Locality:
Thetford Mines area, southeastern Mégantic County, southern Quebec, Chaudiere
Appalachians, south of the St. Lawrence River, southeastern Canada.
Serpentinite (3.6 cm across) ("foliated"), composed of picrolite
antigorite serpentine, from a picrolite vein in metamorphosed Deer Lake
Peridotite (late Neoarchean) in northern Michigan, USA.
Locality:
Ropes Gold Mine, Ishpeming Greenstone Belt, Marquette County, western Upper
Peninsula of Michigan, USA.
Serpentinite (9.6 cm across) (crystalline) from a metamorphosed body of oceanic
lithospheric dunite (mantle peridotite) hosted in schists amd quartzites of the
Ottauquechee Formation (Lower Cambrian). Metamorphism occurred during the
Taconic Orogeny and Acadian Orogeny (Early Paleozoic & Middle Paleozoic).
Locality:
J.A. Vermont Verde Antique International Quarry, eastern side of Quarry Hill
Road, northeast of Rochester, northwestern Windsor County, Green Mountains
central Vermont, USA (43° 54' 55" N, 72° 48' 26" W).