CUMBERLANDITE
Here’s a very rare variety of igneous
rock. This is cumberlandite. It’s more properly called
a porphyritic titaniferous magnetite melatroctolite (a.k.a.
ferrogabbro). The magnetite component is so high that a magnet easily
sticks to the rock. The whitish-gray patches are single large crystals of
plagioclase feldspar. The black component is the finely-crystalline
groundmass, consisting of magnetite, ilmenite, and olivine (partially
serpentinized by orogenic events).
This rock formed near the base of a cooling magma
chamber. Published research has indicated that the parent magma for this
rock was anorthositic gabbro. As a magma chamber cools through the
melting points of various minerals, crystals form and descend & accumulate
at the bottom of the chamber. Igneous rocks & minerals that form by
crystal settling in a magma chamber are called cumulates.
This rock has not been dated (as far as I know).
Available geologic constraints indicate it formed sometime from the late
Neoproterozoic to the late Late Devonian (~620 to ~370 m.y.).
Cumberlandite is the “official” state rock
of Rhode Island, USA. Samples are found in many places, but they all
derive from one locality. Pleistocene glaciation has moved pieces
throughout the state.
Provenance: Iron Mine Hill (Iron Rock Hill), east of Woonsocket, northeastern
Providence County, northeastern Rhode Island, USA.
Cumberlandite (6.9 cm across) from Rhode Island, USA.
Whitish-gray = plagioclase feldspar phenocrysts.
Black
= magnetite + ilmenite + partially serpentinized olivine.