Oxide Minerals

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Tin oxide
Oxide Minerals. Photo courtesy Chris Ralph via Wikimedia Commons

The oxide minerals are compounds of metallic elements plus oxygen, with two prominent exceptions: ice and quartz. Ice (H2O) always gets left out of the mineral books. Quartz (SiO2) is treated as one of the silicate minerals. Some of them are primary minerals that solidify deep in the Earth in magmas, but the most common oxide minerals form near the surface where oxygen in the air and water acts upon other minerals such as the sulfides.

The four oxides hematite, ilmenite, magnetite and rutile are often found associated with each other.

Cassiterite is tin oxide, SnO2, and the most important ore of tin. (more below)

Cassiterite ranges in color from yellow to black, but it's generally dark. Its Mohs hardness is 6 to 7, and it's a rather heavy mineral. Despite its dark color, it yields a white streak. Cassiterite occurs in crystals like this specimen as well as in brown, banded crusts called wood tin. Because of its hardness and density, cassiterite may collect in placers, where it abrades into dark pebbles called stream tin. This mineral supported the tin industry of Cornwall for thousands of years.

Other Hydrothermal Vein Minerals

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Aluminum oxide
Oxide Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

Corundum is aluminum oxide, the natural form of alumina (Al2O3). It is extremely hard, second only to diamond. (more below)

Corundum is the standard for hardness 9 in the Mohs hardness scale. This corundum crystal has a typical tapered shape and hexagonal cross section.

Corundum occurs in rocks that are low in silica, particularly in nepheline syenite, schists altered by alumina-bearing fluids, and altered limestones. It's also found in pegmatites. A fine-grained natural mixture of corundum and magnetite is called emery, which was once a widely used mineral for abrasives.

Pure corundum is a clear mineral. Various impurities give it brown, yellow, red, blue and violet colors. In gem-quality stones, all of these except for red are called sapphire. Red corundum is called ruby. That's why you cannot buy a red sapphire! Corundum gemstones are well known for the property of asterism, in which aligned microscopic inclusions create the appearance of a "star" in a round cabachon-cut stone.

Corundum, in the form of industrial alumina, is an important commodity. Alumina grit is the working ingredient of sandpaper, and sapphire plates and rods are used in many high-tech applications. However, all of these uses, as well as most corundum jewelry, employ manufactured rather than natural corundum today.

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Copper (cuprous) oxide
Oxide Minerals. Photo courtesy Sandra Powers, all rights reserved

Cuprite is a copper oxide, Cu2O, and an important ore of copper found in weathered zones of copper ore bodies. (more below)

Cuprite is the compound cuprous oxide, with the copper in a monovalent state. Its Mohs hardness is 3.5 to 4. Its color ranges from the dark red-brown of this copper ore specimen to the spectacular crimson and scarlet shades you'll see in rock-shop specimens. Cuprite is always found with other copper minerals, in this case green malachite and gray chalcocite. It forms by the weathering and oxidation of copper sulfide minerals. It may display cubic or octahedral crystals.

Other Diagenetic Minerals

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Can you say Göthe?
Oxide Minerals. Photo (c) 2011 Andrew Alden, licensed to About.com ( fair-use policy)

Goethite (GUHR-tite) is hydroxylated iron oxide, FeO(OH). It's responsible for brown colors in soil and is a major ingredient of rust and limonite. It's named for the scientist and poet Goethe and is a major ore of iron.

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Iron (ferric) oxide
Oxide Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

Hematite (also spelled haematite) is iron oxide, Fe2O3. It is the most important iron-ore mineral. (more below)

Hematite may be pronounced HEM-atite or HEEM-atite; the first is more American, the second more British. Hematite takes on several different appearances, but it's most easily identified when it is black, heavy and hard. It has a hardness of 6 on the Mohs scale and a distinctive red-brown streak. Unlike its oxide cousin magnetite, hematite does not attract a magnet except very weakly. Hematite is common in soil and sedimentary rocks, accounting for their reddish colors. Hematite is also the principal iron mineral in banded iron formation. This specimen of "kidney ore" hematite displays the reniform mineral habit.

Other Diagenetic Minerals

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Titanium ore
Oxide Minerals. Photo courtesy Rob Lavinsky via Wikimedia Commons

Ilmenite, FeTiO3, is related to hematite but substitutes titanium for half of the iron. (more below)

Ilmenite is typically black, its hardness is 5 to 6, and it is weakly magnetic. Its black to brown streak differs from that of hematite. Ilmenite, like rutile, is a major ore of titanium.

Ilmenite is widespread in igneous rocks as an accessory mineral, but is seldom concentrated or found in large crystals except in pegmatites and large bodies of plutonic rock. Its crystals are typically rhombohedral. It has no cleavage and a conchoidal fracture. It also occurs in metamorphic rocks.

Because of its resistance to weathering, ilmenite is commonly concentrated (along with magnetite) in heavy black sands where the host rock is deeply weathered. For many years ilmenite was an undesirable contaminant in iron ores, but today titanium is much more valuable. At high temperatures ilmenite and hematite dissolve together, but they separate as they cool, leading to occurrences where the two minerals are interlayered at a microscopic scale.

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Iron (ferrous and ferric) oxide
Oxide Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

Magnetite is a common iron oxide mineral, Fe3O4, named for an ancient region of Greece where metal production was prominent. (more below)

Magnetite is the only mineral that exhibits strong magnetism, although others like ilmenite, chromite and hematite may have weakly magnetic behavior. Magnetite has a Mohs hardness of about 6 and a black streak. Most magnetite occurs in very small grains. A chunk of well-crystallized magnetite like the round specimen is called a lodestone. Magnetite also occurs in well-formed octahedral crystals like the one shown.

Magnetite is a widespread accessory mineral in iron-rich (mafic) igneous rocks, especially peridotite and pyroxenite. It also occurs in high-temperature vein deposits and some metamorphic rocks.

The earliest form of the sailor's compass was a rod of lodestone mounted on cork and floating in a bowl of water. The rod aligns with the Earth's magnetic field to point roughly north-south. Magnets hardly ever point exactly north, because the geomagnetic field is tilted relative to true north, and moreover it slowly changes direction over time spans of decades. If you're navigating at sea, it's much better to use the stars and Sun, but if those are not visible, then the magnet is far better than nothing.

Other Hydrothermal Vein Minerals

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Manganese oxide
Oxide Minerals. Photo (c) 2007 Andrew Alden, licensed to About.com ( fair use policy)

Psilomelane (sigh-LOW-melane) is a catchall name for hard, black manganese oxides that form crusts like this in various geologic settings. (more below)

Psilomelane has no precise chemical formula, being a mix of different compounds, but it's approximately MnO2, the same as pyrolusite. It has a Mohs hardness of up to 6, a blackish streak, and commonly a botryoidal habit as shown along the bottom of this photo. It also adopts a dendritic habit, making up the fossil-like forms called dendrites.

This specimen is from the Marin Headlands north of San Francisco, where deep-sea chert is widely exposed. (Because the locality is in the National Park system, I left it where I found it.) It is likely that this former seafloor had at least a sprinkling of manganese nodules on it. If those compounds were mobilized during these rocks' travels in the ancient California subduction zone, this crust would be the result.

Manganese oxides are also a major ingredient in desert varnish.

Other Diagenetic Minerals

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Manganese oxide
Oxide Minerals. Photo courtesy wanderflechten of Flickr.com under Creative Commons license

Pyrolusite is manganese oxide, MnO2, the most common mineral in dendrites like these. (more below)

Identifying the manganese oxide minerals is a crapshoot without expensive lab equipment, so generally black dendrites and crystalline occurrences are called pyrolusite while black crusts are called psilomelane. There is an acid test for manganese oxides, which is that they dissolve in hydrochloric acid with the release of nasty-smelling chlorine gas. Manganese oxides are secondary minerals that form by weathering of primary manganese minerals like rhodochrosite and rhodonite or by deposition from water in bogs or the deep sea floor as manganese nodules.

Other Diagenetic Minerals

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Ruby (Corundum)

Aluminum oxide
Oxide Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

Ruby is just a special name for gemmy red corundum. Every other color of gem-quality corundum is called sapphire. (more below)

This ruby pebble, a rock-shop specimen from India, displays the clean hexagonal cross-section of corundum crystals. The flat face on this side is a parting plane, a break that results from a crystal weakness, in this case a plane of twinning. Corundum is a fairly heavy mineral, but it is extremely hard (hardness 9 on the Mohs scale) and can occur in streambeds as placer deposits, like the famous gem gravels of Sri Lanka.

The finest gem ruby stones have a red-purplish color called pigeon's blood. I've never bled a pigeon, but I think that's what this color is.

Ruby owes its red color to chromium impurities. The green mica accompanying this ruby specimen is fuchsite, a chromium-rich variety of muscovite.

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Titanium oxide
Oxide Minerals. Photo courtesy Graeme Churchard of Flickr.com under Creative Commons license

Rutile is the natural mineral form of titanium dioxide, TiO2, in plutonic and metamorphic rocks. (more below)

Rutile (ROO-TEEL, ROO-tle or ROO-tile) is generally dark red or metallic black and has a Mohs hardness of 6 to 6.5. The name rutile comes from the Latin for dark red. It forms prismatic crystals that can be thin as hairs, as in this specimen of rutilated quartz. Rutile readily forms twins and sprays of six or eight crystals. In fact, microscopic rutile needles account for the stars (asterism) in star sapphire.

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Magnesium aluminum oxide
Oxide Minerals. Photo courtesy "Dante Alighieri" via Wikimedia Commons

Spinel is magnesium aluminum oxide, MgAl2O4, that is sometimes a gemstone. (more below)

Spinel is very hard, 7.5 to 8 on the Mohs scale, and commonly forms chunky octahedral crystals. You'll typically find it in metamorphosed limestones and low-silica plutonic rocks, often accompanied by corundum. Its color ranges from clear to black and almost everything in between, thanks to the wide range of metals that can partially replace the magnesium and aluminum in its formula. Clear red spinel is a significant gemstone that can be confused with ruby—the famous jewel known as the Black Prince's Ruby is one.

Geochemists studying the mantle refer to spinel as a crystallographic structure, like that of the mineral spinel. For instance, olivine is said to adopt the spinel form at depths greater than about 410 kilometers.