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 (a.k.a. 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.

 


 

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