The collection below includes both common and unusual rocks found in Vermont, three State Rocks ( granite, marble and slate), and talc, the State Mineral. For more on rocks and minerals visit the photo galleries and the Green Rocks in Vermont page.
1. Granite (State Rock) 2. Limestone 3. Garnet Schist 4. Kaolinite 5. Monkton Quartzite 6. Marble (State Rock) 7. Greenstone 8. Gneiss 9. Amphibolite 10. Shale 11. Magnetite Schist 12. Phyllite with Pyrite 13. Concretions 14. Talc 15. Cheshire Quartzite 16. Dolomite 17. Graphitic Schist 18. Serpentinite 19. Micaceous Marble 20. Slate (State Rock)
1. Granite, Barre, VT
Granite is a medium to coarse grained, light-colored, intrusive igneous rocks that is composed of feldspar, quartz, and mica. The feldspar, usually white or pink in color, is a potassium feldspar (orthoclase and/or microline). Albite (Na rich plagioclase feldspar) or oligoclase, a plagioclase feldspar with more sodium (Na) than calcium (Ca), is commonly present in small amounts. Quartz is grey or glassy in appearance. Small amounts of black mica (biotite) and/or white mica (muscovite) are present as platy minerals. Hornblende needles (an amphibole), and rarely pyroxene, may make up the mafic (dark-colored) parts of the rock. Accessory minerals (less than 2% of the mineral content) may be present in the granite. Zircon, a mineral that contains uranium, is normally present in granite and is used by the geochronologist in Uranium/ Lead (U/Pb) analysis to determine the age of crystallization for the rock.
Granite formed from the cooling of molten material (magma) deep underground. The name "granite" is derived from the fact that the rock appears to be composed of numerous "grains" or crystals that are intergrown to form a solid, durable rock.
Granite is a commercially valuable rock in Vermont. The granite industry, centered in Barre, has been in operation since the 1800's. Commercial uses of granite include monument stone, building stone, street curbing, and crushed rock aggregate.
Granite, one of the State Rocks of Vermont, is abundant in the northeastern section of Vermont. This sample was collected from the Rock of Ages quarry in Barre. The granite is Devonian in age (around 380 million years old).
2. Fossiliferous Limestone, Glens Falls Formation, South Hero, VT
This rock is a medium gray, fine- to medium- grained, limestone with fossil fragments. The rock is mainly composed of calcite. Fossil fragments, most less than 3mm across, are visible. The sample of fossiliferous limestone was collected from a highway aggregate quarry on South Hero Island. The rock is from the Ordovician ( 460 million years old) Glens Falls Formation.
Most limestone develops as a result of the accumulation of the shells or hard skeletal remains of marine organisms. As these accumulations are buried and compacted by younger sedimentary deposits, the voids are gradually filled by finer particles, some solution and recrystallization occurs, and finally the entire mass hardens into a solid rock. Ancient limestones, as represented by this sample, have been buried for millions of years and the rock is very dense and hard.
Limestones of this type are often quarried and sold as "marble" and when polished make remarkably beautiful building stone. The waste rock can be crushed and used for highway aggregate and other commercial products such as agricultural lime. There are numerous commercial limestone quarries in western Vermont.
3. Muscovite Schist with garnet, Cavendish Formation, Chester, VT
This sample is a medium- grained muscovite schist composed primarily of muscovite and quartz with garnet porphyroblasts. The sample of the Gassetts Schist is from the Cavendish Formation and is Cambrian in age. The sample was collected near Gassetts (Chester, Vermont) on the west flank of the Chester dome.
In this sample, garnet is the dark red mineral that appears as rounded eight-sided crystals. Muscovite is the silvery white, flaky mineral that makes up the masses surrounding the garnets. Quartz is the grey-colored mineral with the glassy appearance that occurs in clusters, layers, or lenses.
The mineral content of a metamorphic rock such as schist is dependent on both the chemical composition of the original rocks and the temperature and pressure of the metamorphic environment. Different minerals (or combinations of minerals) will crystallize under different temperature and pressure conditions.
4. Kaolinite, Monkton, VT
Kaolin is composed almost exclusively of the clay mineral kaolinite, a hydrous aluminum silicate [Al2(Si2O5)(OH)4]. Pure kaolin is solid white in color and has a soft, silky feeling. The gritty feeling in this sample is due to quartz impurities in the kaolinite.
"Kaolin rock" was mined sporadically at Monkton from the early 1800's until the late 1960's. The deposit was not pure and contained quartz impurities. These impurities were costly to remove and were detrimental to the products that were manufactured using kaolinite. Kaolinite was used as fillers in paints and plastics and in the manufacture of glossy paper. The cost and difficulty in removing the impurities lead to the abandonment of the Monkton site.
Kaolinite was also mined in Bennington and Shaftsbury in the late 1800's and early 1900's. This clay was used in the early pottery industry of that area.
Kaolinite deposits are rare in Vermont. Their occurrence at the base of the Cambrian Cheshire Quartzite is unique and restricts their location to a linear, north-south belt along the western side of the State where the Cheshire Formation is present. The Cheshire Formation is approximately 570 million years old.
5. Monkton Quartzite, Monkton Formation, Winooski, VT
The sample is a reddish- buff quartzite composed of round to angular quartz sand grains cemented by quartz (SiO2) with some iron oxide (FeO2, producing the red color) and calcite (CaCO3). This rock is a sedimentary orthoquartzite that has not been subjected to sufficient heat and pressure to recrystallize the rock. Cementation and compaction have created a very durable rock
This orthoquartzite is from the Cambrian Monkton Formation and was collected near Winooski. Quarries near Burlington supplied blocks of Monkton Quartzite used to build the Redstone Campus at the University of Vermont.
6. Marble, Shelburne Formation, Danby, VT
This rock is a white, massive, medium-grained marble composed principally of calcite. Some green or dark streaks are due to the presence of pyrite, chalcopyrite, muscovite, or chlorite. This sample was collected from the OMYA, Inc. quarry in Danby. The rock belongs to the Columbian Marble Member of the Shelburne Formation (Lower Ordovician age).
Marble, a metamorphic rock composed principally of calcite, is found in western Vermont. The marble in Vermont was formed by the metamorphism of Cambrian to Ordovician age limestones. Marble from Vermont's Danby quarry has been used in famous buildings such as the Thomas Jefferson Memorial in Washington, D.C., the United Nations Building in New York, and the Chiang Kai-Shek Memorial in Taiwan. Most marble currently quarried in the state is crushed and used as filler in paint, paper and plastic.
Marble is an important natural resource and has been a long-standing industry in Vermont. Marble is one of the State Rocks of Vermont.
7. Greenstone, Stowe Formation, Middlesex, VT
The rock is a dark green, fine-to-medium-grained, massive greenstone (mafic schist) composed of chlorite, albite, epidote, actinolite, quartz and calcite. This greenstone, from the Cambrian/Ordovician Stowe Formation, may also have distinct light-colored compositional layers of albite, quartz and epidote. Greenstones are common in many Vermont rock formations. They are metamorphosed volcanically derived sediments, igneous lava flows, pillow lavas or dikes that were originally of basaltic composition. They are interlayered with metamorphosed sedimentary rocks. Detailed chemical information on the Vermont greenstones has helped to reveal the nature of the environment (island arc, mid-ocean ridge, rift, etc) where the greenstones and associated rocks were originally formed.
8. Gneiss, Mt Holly Complex, Chester, VT
Gneiss (pronounced "nice") is a metamorphic rock. Some of the oldest rocks on Earth are gneiss. The foliation, or layering, in gneiss has distinguishing characteristics that set it apart from other metamorphic rocks. Layers or lenses (lenticules) of granular (rounded) minerals alternate with layers of flaky or elongated minerals. The granular minerals are usually the light-colored calcium, sodium, and potassium rich minerals such as quartz and the various types of feldspar. The alternating layers are commonly composed of dark-colored iron and magnesium rich minerals including biotite, chlorite, and hornblende.
Since the mineralogy of many gneisses (quartz, feldspar, and mica) is similar to that of granite (see #1), many gneisses are referred to as granite gneiss. A granite gneiss may be derived from the metamorphism of a granite (an igneous rock) or may simply be a product of the metamorphism of a sedimentary rock that happens to have a composition similar to granite.
Gneiss is an abundant rock type in the Precambrian age "core" of the Green Mountains and areas of the southern Green Mountains. Areas where Precambrian rock (older than 570 million years before the present) is exposed are known as massifs. This sample was collected from an area of Precambrian rock located north of Chester. The gneiss exposures in this area are part of the Precambrian-age Mount Holly Complex.
9. Amphibolite, Standing Pond Amphibolite, Sharon, VT
This dark gray to black, medium- grained amphibolite consists of amphibole (prominant needle-like crystals of hornblende)and plagioclase with little quartz. Hornblende is the most common mineral in the amphibole group; actinolite is also present in many amphibolites.
The amphibolite has been interpreted as the metamorphic equivalent of an igneous basalt. This rock originated as a basaltic lava flow during the Late Silurian-Early Devonian Period 400 million years ago.
This rock was collected in Dummerston Center from the Standing Pond Volcanics. The type locality for the rock is at Standing Pond in Sharon.
10. Black Shale, Iberville Formation, South Hero, VT
Shale is a very fine-textured sedimentary rock that is relatively dense. Shale is composed of very fine(clay and silt size) particles. The particles are minerals that were weathered and eroded from rocks in a source area that was at some distance from the area where the sediment was finally deposited. Shales represent deposition in a quiet environment.
Although shale may occur in a variety of colors, shale in Vermont is usually dark grey or black. Where it occurs as thinly laminated strata, shale is often mistaken for slate (a metamorphic relative described below).
This black shale is both carbonaceous (contains carbon, C) and calcareous (contains calcium carbonate, CaCO3). Shales of this type may be found at many locations in the Champlain Valley, especially on the shores of Lake Champlain and throughout the Champlain Islands. This sample was collected from a quarry on South Hero Island. It is from the Ordovician Stony Point Formation (around 460 million years old).
11. Magnetite Schist, Underhill Formation, Jonesville, VT
In this schist, the mineral magnetite occurs as black "knots" or porphyroblasts in a finer-grained matrix composed of muscovite, chlorite, quartz, and albite. Small grains of pyrite and garnet are present in some samples. The magnetite is strongly magnetic and will be attracted by a small hand magnet. A small magnet suspended on a string will be pulled slightly toward a porphyroblast of magnetite.
Magnetite schists are not rare in Vermont. This rock and other magnetite-bearing rocks are unique, however, because the presence of this mineral in a rock will cause a compass needle to deviate from a normal magnetic north direction. Woodsmen, hikers, and others who depend on a compass to find their way may become disoriented where this rock is present.
The sample was collected near Jonesville, Vermont from the Underhill Formation (around 550 million years old).
12. Phyllite with pyrite, Waits River Formation, Montpelier, VT
This sample is a very fine-grained, black to dark gray, quartz-sericite phyllite with pyrite. The phyllite is a metamorphic rock that formed at thermodynamic (temperature and pressure) conditions intermediate between those that produce slate (low temperature, high pressure) and those that produce schist (higher temperature, higher pressure). Phyllite is a fine-grained, foliated (layered) rock like slate but additionally shows a "crinkly" or "wavy" character along the rock cleavage surfaces.
Large crystals of the bronze-yellow colored mineral pyrite are present in this sample. Pyrite (FeS2) is composed of both iron and sulphur. Its bronze-yellow color often causes pyrite to be mistaken for gold, and a common name for pyrite is "fool's gold".
Phyllite may be found in the eastern part of the Green Mountains and in areas of the Vermont Piedmont. This sample of phyllite with pyrite was collected near Montpelier from the Silurian-Devonian Waits River Formation (around 415-390 million years old).
13. Concretion, VT
The term concretion is a general term used for peculiar mineral segregations found in sedimentary deposits. In Vermont, concretions are found in the fine-grained silt and the very fine-grained sand layers associated with glacial lake sediments. These very fine-grained layers are commonly called varved clays.
Concretions are composed of concentrations of common sediment and cementing materials such as calcium carbonate (CaCO3) and/or iron oxide (FeO2). The cementing material fills in the pore space around the fine sediment grains and binds the sediment to form a variety of shapes. Often, the cement crystallizes around a central nucleus and forms concentrically banded, oval-shaped nodules. Concretions are most often restricted to one narrow bed or layer and thus tend to be quite flat.
Button Bay, on the east shore of Lake Champlain, was originally named "Button Mould Bay" for the numerous concretions that were found washed out of glacial lake clay beds now exposed along the Lake Champlain shoreline. The concretions reminded the early inhabitants of the molds used in the making of pewter buttons.
The concretion sample in this collection was collected from Pleistocene age (15,000 to 10,000 years old) glacial lake sediments in either East Montpelier or Putney.
14. Talc and Soapstone, Windham, VT
Talc, our State Mineral, is very soft and is used in talcum powder. Talc is a hydrous magnesium silicate, Mg6Si8O20(OH)4, and is formed during the metamorphism of ultramafic rocks such as serpentinite. The metamorphism occurs in the presence of hot fluids circulating through the rocks (hydrothermal alteration).
The Green Mountains are comprised of folded and faulted metasedimentary rocks, metamorphosed volcanic rocks and slivers of ocean crust (serpentinized ultramafic rocks). Talc, soapstone, verde antique and asbestos are usually associated with the ultramafic rocks. The bodies of serpentinite occur almost exclusively within two rock formations, the Ottauquechee and Stowe Formations (Cambrian-Ordovician in age, between 550 and 500 million years old). The isolated occurrences of talc extend the full length of the state.
Talc is an economically valuable mineral used in cosmetics, in the manufacture of paper, paint, and insecticides, in ceramics, and by the roofing materials and tire industries. It is also used to coat products which need to slide apart easily, such as nested stacks of plastic pails in a hardware store. Soapstone, composed of chlorite and talc, can withstand intense heat and is used by some Vermont companies to make woodstoves, countertops and sinks.
The sample was collected from a talc mine in Windham.
15. Quartzite, Cheshire Formation, Danby, VT
"Quartzite" is a word used interchangeably for both metamorphic quartzites (known as metaquartzite) and sedimentary quartzite (known as orthoquartzite).
This sample of metaquartzite is a greyish-white rock composed almost exclusively of intergrown quartz crystals. The rock was formed during regional metamorphism by complete recrystallization of a pure, quartz sandstone. Pure, white metaquartzite is present along the western edge of the Green Mountains. This rock type forms the "Green Mountain Front" in southern Vermont. This sample is from the Cheshire Formation of Cambrian age (around 540 million years old) and was collected near Danby from Big Brook.
16. Dolostone, Ludlow and Milton, VT
These pinkish-white and buff, mottled dolostones are slightly metamorphosed rocks that were not greatly altered from an original sedimentary state. Dolostone is the name of a sedimentary rock that is dominantly composed of the mineral dolomite (CaMg(CO3)2). Dolomite is a carbonate mineral similar to calcite (CaCO3), the dominant mineral in limestone and marble. Dolostones are easily distinguished from limestones by using the dilute hydrochloric acid (HCL) test. The reaction of a drop of this solution on the rock specimen will determine the presence of calcite (CaCO3) or dolomite [CaMg(CO3)2]. Limestone (CaCO3) effervesces easily and vigorously, but the same reaction occurs very sluggishly on dolostone. The presence of some magnesium (Mg), in place of some of the calcium (Ca), in the mineral dolomite means that the reaction of the acid is slower.
Dolostones are common in western Vermont in the Champlain Valley and the Vermont Valley ( a narrow valley located between the Taconic Mountains and the Green Mountains). Dolostones are less common in the Green Mountains and the Vermont Piedmont in central and eastern Vermont.
The mottled dolostone is the Dunham Dolomite which represents the initial deposition of carbonates on the shelf margin of Iapetus during Cambrian time. The plain buff-colored dolostone is from the Tyson Formation in Ludlow, VT. It is also Cambrian in age (550 million years old).
17. Black Graphitic Schist, Ottauquechee Formation, Morrisville, VT
This is a fine-grained, black, graphitic, quartz-albite-sericite schist with pyrite. If you rub your fingers over the surface of this rock, the rock will feel greasy and your fingers will be blackened with a fine black film of graphite. Graphite has a structure similar to mica (flaky) and a hardness similar to talc (very soft).
The graphitic schist was originally formed as a sedimentary rock that was rich in carbon (C). During metamorphism, the rock was recrystallized in an oxygen-starved environment and a rock made of carbon (seen as the mineral graphite) and iron sulfide (FeS, or pyrite) was produced.
18. Serpentinite, Rochester, VT
Serpentinite is a dark green-colored rock that is composed of serpentine, a hydrous magnesium silicate mineral, (Mg, Fe)3Si2O5(OH)4. The rock is usually crisscrossed with white veins of calcite (calcium carbonate, CaCO3) and/or magnesite (magnesium carbonate, MgCO3). Serpentinite acquires a brilliant and durable surface when polished, and is a popular dimension stone used as building facings or decorative interior walls. Verde antique is the commercial name for polished serpentinite.
Serpentinite is commonly found as isolated, ultramafic (high amounts of mafic minerals containing iron and magnesium) bodies in Vermont and occurs primarily within a narrow zone in the Green Mountains that stretches from Massachusetts to Canada. Serpentinite was produced through alteration of peridotite or dunite, ultramafic igneous rocks. Serpentinite in Vermont is believed to be remnants of oceanic crust that were incorporated with ocean sediments as continents collided and the Green Mountains were formed during the Taconic and Acadian Orogenies.
The sample of serpentinite in this collection was quarried in Rochester, Vermont.
19. Micaceous Marble, Waits River Formation, Berlin, VT
Micaceous marble (crystalline limestone) is a metamorphic rock that was derived from
calcareous mudstones and shales. The marble is termed "impure" because of the abundance of material that is not pure calcium carbonate (CaCO3).
Impure marble is common in the Waits River Formation which dominates the bedrock
geology of the eastern half of Vermont (the Vermont Piedmont). The impure marble beds and lenses are interbedded with schist, phyllite, and impure quartzite. In exposed outcrops, the weathered, impure marble has a light to dark brown color. With progressive leaching of the calcium carbonate (CaCO3), a rusty brown colored, porous, permeable, crumbly crust develops. This crust is composed of a spongy mass of iron oxide-stained mica grains. The beds with this type of weathering pattern are often referred to as the "punky brown beds" of the Waits River Formation.
The Waits River Formation probably formed in a moderately deep, marine environment as turbidite deposits or soft sediment slump deposits of calcareous shales, mudstones, and muddy sandstones. Fossils found in this formation indicate the rock is approximately 400-425 million years old, placing it in the Late Silurian-Early Devonian time period.
20. Slate, Southwestern VT
These samples are very fine-grained, medium gray-green, green-purple mottled, and purple slates. Slate is composed mainly of quartz, feldspar, carbonates, chlorite and micas. Slate may be found in a variety of colors. The green slate contains chlorite. Colors are imparted to the rock by very minor amounts of iron oxide (red), carbon (black), and other pigments or mineral constituents. The most common colors are black, grey, green, purple, and red. Vermont is somewhat unique in the production of green and purple slates.
Slate, one of the State Rocks of Vermont, is a metamorphic rock that is a product of recrystallization of shale or other fine-grained sedimentary rocks under conditions of low temperature and high pressure. The new minerals that are produced are primarily platy varieties, such as mica and chlorite, and are microscopic size.
Slate may be distinguished from shale (its sedimentary counterpart) in a number of ways. Slate has a higher luster, is somewhat harder and denser, and splits more evenly along parallel foliation surfaces.
The minerals in slate crystallize with their flat, plate-like dimension aligned at right angles to the direction of applied pressure. This alignment of minerals produces a unique structure called "slaty cleavage". Slaty cleavage may be parallel to the original layering or, as is most often the case, at a high angle to the original bedding or lamination surfaces. This can result in outcrops where the bedding (defined by compositional differences in the layers) is nearly horizontal in orientation and the cleavage is nearly vertical.
The rock is quarried for commercial purposes. Vermont has had a slate industry since the mid -1800's. Today, the slate industry is located in the southwestern part of Vermont around Poultney, Pawlet, and Fair Haven. Commercial uses for slate include roofing slate, flooring tile, and flagstones. Some older school blackboards were made from slate.