Corundite (emery rock) (cut, wet surface, 10.0 centimeters 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).
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Corundite is a remarkable metamorphic rock. The sample seen here has an 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). This particular rock has blue corundum, therefore it can be called sapphire. Rock-forming corundum is rare.
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, 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 (AlO·OH)-dominated rocks) (= very dark green area on the left side of the first photo).
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.
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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.
Tags: corundite sapphire emery Naxos Aegean Sea Tertiary Eocene Mioene Greece
Corundite (emery rock) with corundum/sapphire (blue) and calcite (yellowish-brown). (broken surface, dry)
Corundite is a remarkable metamorphic rock. The sample seen here has an attractive bluish color and wisps of yellowish-brown. Its composition and origin are quite unusual. Corundite (also known as emery rock) is dominated by corundum, a very hard (H ≡9) aluminum oxide mineral (Al2O3). This particular rock has blue corundum, therefore it can be called sapphire. Rock-forming corundum is rare.
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, 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 (AlO·OH)-dominated rocks).
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.
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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.
Tags: corundite sapphire emery Naxos Aegean Sea Tertiary Eocene Mioene Greece
Quartz from an island in the Aegean Sea.
A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5100 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.
The silicates are the most abundant and chemically complex group of minerals. All silicates have silica as the basis for their chemistry. "Silica" refers to SiO2 chemistry. The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4. Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon. The resulting formula for silica is thus SiO2, not SiO4.
The simplest & most abundant silicate mineral in the Earth's crust is quartz (SiO2). All other silicates have silica + impurities. Many silicates have a significant percentage of aluminum (the aluminosilicates).
Quartz (silicon dioxide/silica - SiO2) is the most common mineral in the Earth's crust. It is composed of the two most abundant elements in the crust - oxygen and silicon. It has a glassy, nonmetallic luster, is commonly clearish to whitish to grayish in color, has a white streak, is quite hard (H≡7), forms hexagonal crystals, has no cleavage, and has conchoidal fracture. Quartz can be any color: clear, white, gray, black, brown, pink, red, purple, blue, green, orange, etc.
The dark green coloration in this cluster of quartz crystals is caused by actinolite inclusions. Actinolite is a type of amphibole with the formula Ca2(Mg,Fe)5Si8O22(OH)2 - calcium magnesium iron hydroxy-silicate.
Locality: unrecorded/undisclosed site on Serifos Island, Cyclade Islands, Aegean Sea
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Photo gallery of quartz:
www.mindat.org/gallery.php?min=3337
Tags: Quartz included inclusions actinolite Serifos Island Cyclade Islands Cyclades Aegean Sea Greece
(public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA)
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A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5500 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.
The sulfide minerals contain one or more sulfide anions (S-2). The sulfides are usually considered together with the arsenide minerals, the sulfarsenide minerals, and the telluride minerals. Many sulfides are economically significant, as they occur commonly in ores. The metals that combine with S-2 are mainly Fe, Cu, Ni, Ag, etc. Most sulfides have a metallic luster, are moderately soft, and are noticeably heavy for their size. These minerals will not form in the presence of free oxygen. Under an oxygen-rich atmosphere, sulfide minerals tend to chemically weather to various oxide and hydroxide minerals.
Galena is a lead sulfide mineral (PbS). It has a metallic luster, silvery-gray color, cubic crystals, cubic cleavage, is moderately soft, and has a very high specific gravity (it's very heavy for its size).
Galena is the most important lead ore mineral. It's principally found in hydrothermal vein systems and in Mississippi Valley-type deposits. Silver (Ag) can fall into the lead position as an impurity, sometimes in relatively high proportions. This results in argentiferous galena, (Pb,Ag)S.
Historically, lead from galena has been used as a gasoline additive, a paint ingredient, and for making bullets. Lead is also used to make various metal products, batteries, and radiation shielding.
This sample is from an old silver mining district in the Attica region of Greece and has silver impurities, making it argentiferous galena. Mining in the area started over 5,000 years ago. The silver was principally used by ancient Greeks to make coins. Sulfide mineralization is hosted in Mesozoic limestone.
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From exhibit signage:
An ore sample from the silver mines of Laurium. Silver was recovered from galena (PbS) which occurred with sphalerite (ZnS) as grains and veins in limestone matrix.
The Silver Mines of Laurium, Greece
For one brief moment in history, the development of Western civilization hung upon a few small, wooden ships and the silver mines that made them possible.
Silver from the ancient mines of Laurium (Laurion) was a strong enticement for the Persian Empire to conquer Greece. Though at first, the Persians were defeated, the Athenian statesman Themistocles convinced his fellow citizens to strengthen their defense against future attacks. They pooled their annual shares of the mines’ profits to build a fleet of new ships. Sure enough, the Persians returned with great strength in 480 B.C., and descended on Athens.
The new fleet evacuated Athens and hastened to support the Greek army’s last stand across the Isthmus of Corinth. Eight hundred Persian ships closed in upon the smaller Greek fleet of 310 ships, 200 of which were the new vessels. Amazingly, the outnumbered Greeks were victorious. The Persians retreated, burning Athens in their wake. This naval battle proved to be the most decisive of the ancient world. The following year, Greek forces destroyed the Persian rear guard, and ended the threat of Persian domination.
The wealth of Laurium, which had been used to strengthen Athens’ naval forces, was then used to rebuild Athens, ushering in the Golden Age of Greece.
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Locality: unrecorded/undisclosed site in the Laurium Silver Mines, Attica, southeastern Greece
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For more info., see:
en.wikipedia.org/wiki/Mines_of_Laurion
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Photo gallery of galena:
www.mindat.org/gallery.php?min=1641
Tags: argentiferous galena silver lead sulfide sulfides mineral minerals Laurium Mines Greece mine
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.
Tags: corundite emery magnetite magnetic Naxos Aegean Sea Tertiary Eocene Mioene Greece