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.

 

 


 

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