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Green Rocks in the Green Mountain State - Greenstones, green schists, green phyllites, green slate, greenschist facies metamorphism, and the Green Mountains

AMPHIBOLITE METADIABASE METAVOLCANIC VERDE ANTIQUE TALC

1. What are the types of green rocks in Vermont? Vermont has a variety of green to gray-green colored metamorphic rocks. It is home to green schists, phyllites, slates and metamorphosed igneous rocks such as metadiabase, metabasalt, and serpentinite (see also the Belvidere Mtn page). The green color in all these rocks comes from green minerals such as chlorite, amphiboles, and epidote. The green minerals are sometimes referred to as mafic minerals or ferromagnesian minerals because they are comparatively high in iron and magnesium. Dark colored rocks with abundant ferromagnesian minerals are also referred to as mafic and even ultramafic rocks; light colored rocks higher in light-colored mineral such as quartz, feldspar and muscovite (eg. granite) are referred to as felsic rocks.

2. What is green schist? Green schist in Vermont is a metamorphic rock which has a foliation ( very fine layers) due to parallelism of platy minerals such as chlorite. The foliation may or may not be parallel to original sedimentary layering (bedding). In general, grain size decreases from schist to phyllite to slate. Green slate occurs mainly in southwestern Vermont whereas green phyllite and schist occurs closer to the Green Mountains.

3.What are greenstones? Greenstones are altered igneous rocks (ex. ancient volcanic rocks) which get their green color from chlorite +/- amphiboles and epidote. In Vermont these rocks may also be referred to as metamorphosed volcanics, pillow volcanics, pillow lavas, metaigneous rocks, mafic volcanics, metadiabase, metagabbro, amphibolite, and mafic schist. Yet all are metamorphosed igneous rocks of one type or another and the original parent rock type is not always known. The AGI Glossary of Geology defines greenstone (meta) : A field term applied to any compact dark-green altered or metamorphosed basic igneous rock (eg. spilite, basalt, gabbro, diabase) that owes its color to the presence of chlorite, actinolite, or epidote. In Vermont, the term greenstone has also been used to refer to metamorphosed sedimentary rocks derived from an igneous source.

4. What is greenschist facies? Metamorphism is a change of an original parent sedimentary, igneous, or metamorphic rock due to a change in temperature and pressure. Different minerals are stable at different temperatures and pressures; different mineral assemblages are associated with each facies. A facies diagram shows the different temperature and pressure fields for different metamorphic facies which from lower temperature and pressure to higher temperature and pressure are: zeolite, prehnite/pumpellyite, greenschist, amphibolite, and granulite. Greenschist facies temperatures are generally 350-500 degrees centigrade.

5. The Green Mountain Giant - a glacial erratic.

 

Generalized Geologic 
	Map of Vermont - 1970 - click for larger map image

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