Phyllite

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Phyllite Slabs

Not a trade name
Photo (c) 2003 Andrew Alden, licensed to About.com (fair use policy)

Phyllite is between slate and schist in the spectrum of metamorphic rocks. Geologists tell them apart by their surfaces: slate has flat cleavage faces and dull colors, phyllite has flat or crinkled cleavage faces and shiny colors, and schist has intricately wavy cleavage (schistosity) and glittering colors. Phyllite is "leaf-stone" in scientific Latin; the name may refer as much to phyllite's color, which is often greenish, as to its ability to cleave into thin sheets. Phyllite generally is in the pelitic series—rocks that are derived from clay sediments—but sometimes other rock types can take on the characteristics of phyllite too. That is, phyllite is a textural rock type, not a compositional one. The sheen of phyllite is from microscopic grains of mica, graphite, chlorite and similar minerals that form under moderate pressure.

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Phyllite is a geologic name. Stone dealers call it slate because it's useful for flagstones and tiles. These specimens are stacked in a stone yard.

02
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Phyllite Outcrop

Roadside rock stand
Photo (c) 2008 Andrew Alden, licensed to About.com

In outcrop, phyllite looks like slate or schist. You have to inspect it close up to classify phyllite correctly.

This outcrop of phyllite is by a roadside parking area on route I-91 southbound, north of exit 6 between Springfield and Rockingham, Vermont. It is a pelitic phyllite of the Gile Mountain Formation, of late Early Devonian age (approx. 400 million years old). Gile Mountain, the type locality, is farther north in Vermont just across the Connecticut River from Hanover, New Hampshire.

03
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Slaty Cleavage in Phyllite

Slaty cleavage
Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)

The thin cleavage planes of phyllite face to the left in this view of a Vermont outcrop. Other flat faces that cross this slaty cleavage are fractures.

04
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Phyllite Sheen

Makes a pretty face
Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)

Phyllite owes its silky sheen to microscopic crystals of white mica—the variety called sericite, which is used in cosmetics for a similar effect.

05
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Phyllite Hand Specimen

Note crinkly cleavage
Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)

Phyllite is generally dark gray or green due to its content of black graphite or green chlorite. Note the crinkly cleavage faces typical of phyllite.

06
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Phyllite with Pyrite

Telltale brassy cubes
Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)

Like slate, phyllite can contain cubic crystals of pyrite, plus other low-grade metamorphic minerals.

07
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Chloritic Phyllite

The true green stuff
Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)

Phyllite of the right composition and metamorphic grade can be quite green from the presence of chlorite. These specimens have flat cleavage.

These phyllite specimens are from a roadcut about a kilometer east of Tyson, Vermont. The rock is a pelitic phyllite of the Pinney Hollow Formation, in the Camels Hump Group, and has recently been determined to be of Late Proterozoic age, about 570 million years old. These rocks appear to be the more strongly metamorphosed counterpart to the basal slates of the Taconic klippe farther east. They are described as silvery-green chlorite-quartz-sericite phyllite.

08
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Accessory Minerals in Phyllite

Mystery minerals
Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)

This green phyllite contains orange-red acicular crystals of a secondary mineral, possibly hematite or actinolite. Other light-green grains resemble prehnite.