Cumberlandite
Cumberlandite is a specific type of plutonic rock called a melanocratic troctolite, or melatroctolite.<ref name="Dietrich"/> It is the state rock of Rhode Island and can be found in a Template:Convert lot in Cumberland, Rhode Island at Iron Mine Hill.<ref name="RI.gov"/><ref name="Iron Mine"/> Further traces can be found scattered throughout the Narragansett Bay watershed as far as Martha's Vineyard and Block Island.<ref name="Quinn"/> Cumberlandite is not exclusive to Rhode Island; it is also found in Taberg, Sweden.<ref name="Quinn"/> It is slightly ferrimagnetic 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.<ref name="Dietrich"/>
Cumberlandite weathers to a brownish black with white crystals and has secondary chlorite and saussurite.<ref name="USGS"/> 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.Template:Citation needed 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 igneous rock known as a melanocratic troctolite, or by IUGS classification, titaniferous magnetite melatroctolite.<ref name="Dietrich"/>
Bulk rock geochemistry shows the below analysis with trace Pb:
| Oxide | Weight % |
|---|---|
| SiO2 | 22.35 |
| TiO2 | 9.75 |
| Al2O3 | 5.26 |
| MgO | 16.10 |
| Fe2O3 | 14.05 |
| FeO | 28.84 |
| MnO | 0.43 |
| CaO | 1.17 |
| Na2O | 0.44 |
| K2O | 0.10 |
| P2O5 | 0.02 |
| V2O3 | 0.18 |
| S | 0.38 |
| Zn | 0.71 |
| Cu | 0.08 |
| CO & Ni | 0.08 |
| H2O | 0.42 |
| CO2 | 0.02 |
| Sum | 100.38 |
Troctolites are unusual olivine-rich pyroxene-poor gabbros common in layered mafic intrusions believed to have formed as cumulates in a magma chamber. Cumberlandite has light phenocrysts of labradorite in a dark, fine to medium-grained matrix of magnetite, ilmenite, olivine, 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 crystals gives the rock a lamination.<ref name="Dietrich"/>
In Rhode Island, the rock is part of the Esmond-Dedham Subterrane with an uncertain age ranging widely from Late Proterozoic to around the Devonian.<ref name="USGS"/> He-Magnetite dating suggest an improbable age of 1.5Ga.<ref name="Quinn"/> Given its chemical composition and the presence of inclusions from a nearby gabbro, experts believe the rock is mid-Paleozoic.<ref name="Quinn"/><ref name="Dietrich"/>