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 ÒofficialÓ 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.