Cumberlandite is a specific type of plutonic rock called a melanocratic troctolite, or melatroctolite. It is the state rock of

Rhode Island Rhode Island ( ) is a state in the New England region of the Northeastern United States. It borders Connecticut to its west; Massachusetts to its north and east; and the Atlantic Ocean to its south via Rhode Island Sound and Block Is ...

and can be found in a lot in Cumberland, Rhode Island at Iron Mine Hill. Further traces can be found scattered throughout the

Narragansett Bay Narragansett Bay is a bay and estuary on the north side of Rhode Island Sound covering , of which is in Rhode Island. The bay forms New England's largest estuary, which functions as an expansive natural harbor and includes a small archipelago. S ...

watershed as far as

Martha's Vineyard Martha's Vineyard, often simply called the Vineyard, is an island in the U.S. state of Massachusetts, lying just south of Cape Cod. It is known for being a popular, affluent summer colony, and includes the smaller peninsula Chappaquiddick Isla ...

. Cumberlandite is not exclusive to Rhode Island, but is also found in Taberg, Sweden. It is slightly

ferrimagnetic A ferrimagnetic material is a material that has populations of atoms with opposing magnetic moments, as in antiferromagnetism, but these moments are unequal in magnitude, so a spontaneous magnetization remains. This can for example occur wh ...

due to its high concentration of iron.

Background

Colonial settlers recognized its value as ore during the 18th and 19th centuries. Historical records reveal that it was smelted as early as 1703, and it was used in forging cannons during significant events such as the Siege of Louisbourg in 1745 and possibly the

American Revolutionary War The American Revolutionary War (April 19, 1775 – September 3, 1783), also known as the Revolutionary War or American War of Independence, was the armed conflict that comprised the final eight years of the broader American Revolution, in which Am ...

. Cumberlandite weathers to a brownish black with white crystals and has secondary

chlorite The chlorite ion, or chlorine dioxide anion, is the halite (oxyanion), halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as s ...

and saussurite. It is predominantly found in glacial deposits stretching from south of its origin to the southern shores of Narragansett. Cumberlandite is denser than common granites or metamorphic rocks. Its unique origin, distinct appearance, and ease of identification contributed to its selection as the Rhode Island state rock.

Petrology

Cumberlandite is an uncommon

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

igneous rock Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

known as a melanocratic troctolite, or by IUGS classification, titaniferous

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...

melatroctolite. Bulk rock geochemistry shows the below analysis with trace Pb: Troctolites are unusual

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

-rich

pyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...

-poor

gabbro Gabbro ( ) is a phaneritic (coarse-grained and magnesium- and iron-rich), mafic intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

s common in layered mafic intrusions believed to have formed as cumulates in a

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it u ...

. Cumberlandite has light

phenocryst image:montblanc granite phenocrysts.JPG, 300px, Granites often have large feldspar, feldspathic phenocrysts. This granite, from the Switzerland, Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid sh ...

s of labradorite in a dark, fine to medium-grained

matrix Matrix (: matrices or matrixes) or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the m ...

ilmenite Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printi ...

, and hercynite spinel. Magnetite and ilmenite cumulates are also common in layered intrusions and these minerals can account for up to 70 percent of the rock's volume contributing to the rock's high density and magnetism. The preferred orientation of the

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

crystals gives the rock a lamination. The rock is part of the Esmond-Dedham Subterrane with an uncertain age ranging widely from Late Proterozoic to around the

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

. He-Magnetite dating suggest an improbable age of 1.5Ga. Given its chemical composition and the presence of inclusions from a nearby gabbro, experts believe the rock is mid-Paleozoic.

References

{{coord, 42, 0, 17.5, N, 71, 27, 13, W, type:landmark_region:US-RI, display=title Plutonic rocks Geology of Rhode Island Symbols of Rhode Island