Emery from the Tertiary of Naxos Island, Greece. (~5.6 centimeters across along the base)
Emery is a rare metamorphic rock. Also known as corundite, the rock is dominated by the mineral corundum, a very hard aluminum oxide, Al2O3. Corundum is the definition of hardness 9 on the Mohs Hardness Scale.
This material 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, a rock having aluminum hydroxy-oxide minerals. Upon metamorphism, the limestones were converted to marbles and the bauxites were converted to diasporites (= diaspore (AlO·OH)-dominated rocks).
With 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.
The "quality" of emery in the Naxos Emery Deposits varies from relatively pure corundite, which can be blue (= sapphire rock) to corundite significantly mixed with other minerals. This emery specimen is magnetitic corundite - a magnet sticks to it.
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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.
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