File:Chlorite schist with magnetite (Wissahickon Formation; Jarrettsville, Maryland, USA) 1 - 53317552301.jpg

English: (~8.9 centimeters across at its widest)

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Chlorite schist is a foliated metamorphic rock dominated by chlorite, a greenish to grayish-green silicate mineral. This example is from the Wissahickon Schist of Maryland, USA. The unit consists principally of schists and quartzites, which are metamorphosed shales and sandstones originally deposited in a continental slope environment. Published depositional ages range from the Neoproterozoic to the Cambrian to the Ordovician. These slope deposits were subsequently metamorphosed by multiple tectonic collision events between Laurentia (= the ancient North American paleocontinent) and various landmasses. The Taconic Orogeny was a mountain building event in the Late Ordovician to Silurian that involved a colliding volcanic island arc. The Silurian to Devonian Acadian Orogeny involved a colliding microcontinent. The Allegheny Orogeny involved Africa colliding with Laurentia during the Pennsylvanian. Seven separate deformation events have been identified in Wissahickon Formation rocks.

Chlorite schist can have a component of magnetite (Fe3O4 - iron oxide), as in this sample. The magnetite is usually in the form of relatively small, blackish, octahedral crystals.

Stratigraphy: Wissahickon Schist (a.k.a. Wissahickon Formation), Glenarm Series (Glenarm Supergroup), Neoproterozoic to Ordovician; multiply metamorphosed during Paleozoic orogenies (Late Ordovician to Pennsylvanian)

Locality: unrecorded / undisclosed site at or near the town of Jarrettsville, western Harford County, northeastern Maryland, USA