CORUNDITE
Corundite is a remarkable metamorphic rock.
Apart from its attractive bluish color and wisps of yellow-brown, its composition
and origin are quite unusual. Corundite (a.k.a. emery rock)
is dominated by corundum, a very hard (H ≡9) aluminum oxide mineral (Al2O3).
Because itÕs blue corundum, it can be called sapphire. Rock-forming
corundum is rare (as is rock-forming diamond, but see diamondite).
This rock comes from the Naxos Emery Deposits on the
island of Naxos in the Aegean Sea. Naxos is dominated by metamorphic
rocks and some igneous rocks. Much of the island consists of marbles
(originally limestones). Some of the original limestones had lenses of
bauxite, which is a rock having aluminum hydroxy-oxide minerals. Upon
metamorphism, the limestones were converted to marbles and the bauxites were
converted to diasporites (= diaspore (AlOáOH)-dominated rocks).
The very dark green area on the left side of the rock shown below is
diasporite.
Upon further metamorphism, the diasporites were
converted to corundites plus water. High fluid pressures fractured the
rocks, and the fractures got filled up with corundite.
Metamorphism on Naxos occurred during the Cenozoic in
two main phases. A high-grade metamorphic event occurred during the
Eocene, at about 40-50 million years ago. A second, intermediate-grade
metamorphic event occurred during the Early Miocene, at 16-20 million years
ago.
Corundite (emery rock) (cut, wet surface, 10.0 cm across) with blue
corundum/sapphire and yellowish-brown calcite. The bluish-brownish
corundite is a fracture filling. The host rock can be seen on the left
side of the specimen - a chloritoid-hematite-rich diasporite (dark green
= chloritoid; red spots = hematite).
Corundite (emery rock) (broken surface, dry) with corundum/sapphire (blue) and
calcite (yellowish-brown).
Info. synthesized from:
Urai & Feenstra (2001) - Weakening associated with
the diaspore-corundum dehydration reaction in metabauxites: an example from
Naxos (Greece). Journal of Structural Geology 23: 941-950.
Feenstra & Wunder (2002) - Dehydration of
diasporite to corundite in nature and experiment. Geology 30(2):
119-122.