SCHIST
Schist
is an intermediate- to high-grade, foliated metamorphic rock. It is
highly variable in appearance, depending on the mineral content, which is a
function of the precursor rock and specific temperature-pressure conditions.
Garden-variety schists form by metamorphism of phyllites. Schists
typically have medium- to large-sized crystals, unlike the microcrystalline
nature of slate & phyllite.
The foliation of the various types of schist shown
below is often only seen when the specimens are viewed on edge.
Muscovite schist (3.3 cm across) - a common variety of mica schist.
Biotite schist (3.6 cm across) - another common variety of mica
schist, dominated by biotite mica. Biotite schists often contain a
significant muscovite mica component.
Garnet
schist (5.5 cm across) has conspicuous, large or small garnet crystals. Garnets in schists are
typically very deep red-colored. The
surrounding matrix is often a grayish and mica-rich.
The garnet-muscovite schist shown above is early Late
Cretaceous in age (89 to 92 million years).
It comes from Garnet Ledge, southeastern Alaska, USA. Rocks
from this locality contain near-gem-quality almandine garnets.
Chlorite schist (8.2 cm across) with black, octahedral crystals of
magnetite (Fe3O4).
Chlorite schist is a moderately common metamorphic rock. It is principally
composed principally of chlorite, a silicate mineral with a greenish to
grayish-green color. The sample of chlorite schist (metapelite) shown
above is from the Wissahickon Schist (aka Wissahickon Formation,
Glenarm Series/Glenarm Supergroup) of Neoproterozoic to Cambrian age in
Maryland, USA. Metamorphism took place during the Taconic Orogeny (Late
Ordovician to Silurian), which affected much of eastern North America.
Locality:
Jarrettsville, western Harford County, northeastern Maryland, USA.
Talc
schist (above & below; above: 9.0 cm across; below:
centimeter scale). Talc schist is a metamorphic rock dominated
by the mineral talc. Many samples have a mottled creamy white color, with
pearly luster, and a slick & soapy feel.
Talc schist, like talc and soapstone and steatite, is very soft (H = 1).
Unlike soapstone and steatite (both talcose, crystalline-textured metamorphic
rocks), talc schist has a foliated texture (in the sample shown below, the
foliation is only seen when viewing the rock on edge).
Some talc schists formed by hydrothermal metamorphism
of dolomitic marbles. Other talc schists formed by hydrothermal
metamorphism of serpentinized peridotites.
Some info. provided by the U.S. Geological Survey
& David Von Bargen.
Staurolite
schist (3.8 cm across).
Staurolite schist has prominent
dark brown staurolite crystals in a light-colored muscovite schist
matrix. It is an intermediate- to high-grade metamorphic rock. The
mineral staurolite (Fe,Mg,Zn)2Al9(Si,Al)4O22(OH)2
- iron magnesium zinc hydroxy-aluminosilicate) often forms cruciform twins -
two intergrown crystals forming a cross or X-shaped form.
This sample comes from the Keivy Schist at Mt.
Ploskaya in the western part of the Keivy Terrane, east-central Kola Peninsula,
Murmansk Region, far-northwestern Russia.
The original, pre-metamorphism sedimentary rock is late Neoarchean in
age (~2.5 to 2.7 billion years).
Tremolite schist (7.5 cm across) is a ~monomineralic, foliated
metamorphic rock composed of tightly interlocking tremolite crystals.
Tremolite is a whitish amphibole (Ca2Mg5Si8O22(OH)2)
that forms small, acicular (needle-shaped) crystals. This sample comes from near Balmat, New York
State, USA. At this locality, tremolite
schist is closely associated with talc schists and talc-tremolite schists.
Stratigraphy: Upper Marble, Grenville Series, Mesoproterozoic (1150-1300 m.y.).
Locality:
Balmat, Balmat-Edwards Mining District, southwestern St. Lawrence County,
Adirondack Lowlands, northern New York State, USA.
Actinolite schist (~12 cm across) is an intermediate- to high-grade,
foliated metamorphic rock dominated by the mineral actinolite. Actinolite
is a dark greenish-colored amphibole (Ca2(Mg,Fe)5Si8O22(OH)2
- calcium ferromagnesian hydroxy-silicate) that forms long blades or
needle-like crystals.
Blueschist (glaucophane schist; glaucophanite) (3.4 cm across) - this scarce,
glaucophane-rich rock is a classic example of a low-temperature, high-pressure
metamorphic rock. The bluish material is the mineral glaucophane
(Na2Mg3Al2Si8O22(OH)2
- sodium magnesium hydroxy-aluminosilicate). Calling this particular rock
sample a "schist" is misleading, because it lacks schistose texture.
This sample comes from a blueschist knocker (as
they're called) in serpentinitic melange of the Franciscan Complex
(mid-Cretaceous) at Jenner, western coastal California, USA. The Jenner,
California area is famous for its outcrops of blueschist and eclogite.
Blueschists form by deep burial metamorphism of basaltic oceanic crust in
subduction zones.
Fuchsite
schist (5.0 cm across) - this uncommon variety of mica
schist is dominated by the greenish mineral fuchsite (chromiferous
muscovite mica), (K(Al,Cr)2AlSi3O10(OH,F)2
- potassium chromium hydroxy-fluoro-aluminosilicate. The rock has
decent-sized crystals, which results in a sparkly, glittery appearance in
strong light.
Graphite
schist (4.2 cm across) - intermediate- or higher-grade
metamorphism of coal results in a foliated rock dominated by the mineral
graphite (C). Graphite schist has a
metallic luster, silvery-gray color, a slick and greasy feel, is very soft (H =
1), and easily marks paper.