Rhode Island State Rock

Cumberlandite

STATUS

Official

DESCRIPTION

Cumberlandite is a complex and rare igneous rock resulting from the fusion and agglomeration of two dozen minerals. A high specific gravity of 3.82-3.88 is due to predominance of magnetite (Fe₃O₄) and ilmenite (FeTiO₃), which compose up to 70 percent of the rock.

The texture is porphyritic, with larger crystals embedded in a matrix of fine crystals. The larger, light-colored crystals are plagioclase, containing varying quantities of silicon, oxygen, aluminum, calcium, and sodium. The black or dark gray groundmass is even more mixed, but is mainly iron-rich olivine with feldspar and ilmenite. The texture, density, and iron content of cumberlandite have led people to mistake specimens for meteorites.

A STATE SYMBOL

Rhode Island designated cumberlandite its official state rock in 1966 because of the rock’s uniqueness to the state. The rock had been known for centuries by the area’s native Nipmucks, who apparently recognized its rarity, holding it sacred. White settlers mined it for a variety of uses. The rock was initially called Rhodose in honor of the state, but in a paper written in 1864, the prominent geologist M.E. Wadsworth proposed the name cumberlandite for the town where it was exclusively found.  

FORMATION AND OCCURRENCE

Some sources attribute the origin of cumberlandite to a volcanic eruption 1.5 billion years ago. The United States Geological Survey gives a range of Late Proterozoic (as much as one billion years ago) to Devonian (as little as 370 million years ago) and indicates a plutonic (underground) formation, noting that in some areas, heavier minerals settled to the lower area of a mass of anorthosite magma.

Whatever the process, it was local. Cumberlandite deposits have never been found outside of Rhode Island. In fact, the only known concentration of the rock is a 3.7-acre deposit at Iron Mine Hill in the town of Cumberland, Rhode Island. Smaller specimens deposited by glacial action occur south of the source as far away as Narragansett.

USES

White settlers used the mass of cumberlandite at Iron Mine Hill for gravestones and also as an ore of iron, producing farm tools from it from the early 1700s. During the Revolutionary War, it was mined for production of cannons. The ore was difficult to smelt and required mixing with hematite for strength. The reason for the trouble would later turn out to be a valuable mineral, titanium, a component of the rock’s ilmenite (FeTiO₃); one analysis estimated that a million tons of titanium could be obtained from Iron Mine Hill.

A good deal of the rock was used for "road metal," and some was used as gravel for the racetrack parking lot at Pawtucket. It is unlikely the rock will be exploited for further industrial use, since the town of Cumberland, assisted by a grant from the state legislature, purchased the 3.7-acre parcel of land. Plans to build a park at the site will ensure public access for visiting and study.

Cumberlandite Samples

CUMBERLANDITE FACTS

Name: Cumberlandite

Chemical Formula: Fe₃O₄ (Magnatite) and FeTiO₃ (Ilmenite)

Color: Black

Author: World Trade Press