Tuesday, August 23, 2011

Mineral O

  • Chemistry: SiO2; Mostly silicon dioxide with large amounts of impurities.
  • Class: Mineraloids
  • Uses: As a semiprecious stone and ornamental stone for carvings.
  • Physical Properties of Obsidian
  • Specimens

Obsidian is the result of volcanic lava coming in contact with water. Often the lava pours into a lake or ocean and is cooled quickly. This process produces a glassy texture in the resulting rock. Iron and magnesium give the obsidian a dark green to black color. Obsidian has been used by ancient people as a cutting tool, for weapons, and for ceremonial purposes and is sometimes found by archaeologists in excavations.

Obsidian has several varieties. Obsidian can contain small bubbles of air that are aligned along layers created as the molten rock was flowing just before being cooled. These bubbles can produce interesting effects such as a golden sheen, known as Sheen Obsidian or a rainbow sheen called Rainbow Obsidian. Inclusions of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern producing Snowflake Obsidian. Small nuggets of obsidian that have been naturally rounded and smoothed by wind and water are called Apache Tears.

Often confused with smoky quartz, obsidian has similar properties to quartz because of a similar chemistry. However, many properties dependant on a crystal structure are altered or absent in obsidian because it lacks any crystal structure of its own. The piezoelectric and optical properties in quartz are thus absent in obsidian. Smoky quartz usually has a splotchy or zoned distribution to its color while Obsidian's color is more uniformly distributed.



  • Color is dark green to dark brown and black, also can show sheens of gold or green, yellow, blue and/or purple coloration. Sometimes with white inclusions (Snowflake Obsidian).
  • Luster is vitreous.
  • Transparency: Obsidian is translucent in any stone of appreciable size.
  • Crystal System does not apply because obsidian is amorphous.
  • Habits include compact nodules or as massive layers between other volcanic rocks
  • Fracture is conchoidal.
  • Hardness is 5 - 5.5 (much softer than quartz).
  • Specific Gravity is approximately 2.6 (average)
  • Streak is white.
  • Other Characteristics: Generally lacks open voids or large bubbles like other volcanic rocks.
  • Notable Occurrences include Italy; Mexico; Scotland; Arizona, Colorado, Texas, Utah and Idaho, USA.
  • Best Field Indicators are color, fracture, flow bubbles, softness, association with other volcanic rocks and lack of crystal faces.




Okenite is an unusual mineral. It frequently forms "cottonball" clusters where the crystals are so thin they look like tiny fibers. The clusters are composed of straight, radiating, thread thin, crystals. These clusters can make for very attractive specimens and often accompany many fine and rare minerals such as apophyllite, gyrolite and many of the zeolites. Some volcanic bubbles called vesicles can be lined with delicate tufts of okenite and these are sometimes called "Okenite Geodes". They form a mesmerizing crystal wonderland like landscape. One note of caution, the clusters seem to bring out an urge in people to touch the fine fibers and to "test" the minerals softness. Discourage and refrain from this as the crystals are very delicate and once touched, are never the same again.



  • Color is white or colorless.
  • Luster is resinous to pearly.
  • Transparency crystals are transparent to mostly translucent.
  • Crystal System is triclinic; bar 1
  • Crystal Habits include the popular radiating accicular crystals described above, but is more commonly found as radially fiberous masses and rarely as single bladed crystals.
  • Cleavage is perfect in one direction but rarely seen because of small crystal size.
  • Fracture is splintery.
  • Hardness is approximately 5.
  • Specific Gravity is approximately 2.3+ (below average)
  • Streak is white.
  • Associated Minerals are gyrolite, calcite, apophyllite, quartz, laumontite and other zeolites.
  • Other Characteristics: crystals are bendable and fragile.
  • Notable Occurrences include Poona, India; Greenland, Chile and Ireland.
  • Best Field Indicators are crystal habit, color and associations.




  • Chemistry: Na(90-70%) Ca(10-30%) (Al, Si)AlSi2 O8, Sodium calcium aluminum silicate.
  • Class: Silicates
  • Subclass: Tectosilicates
  • Group: Feldspars
  • Uses: ornamental and semi-precious stone and as mineral specimens.
  • Specimens

Oligoclase is not a well known mineral but has been used as semi-precious stone under the names of sunstone and moonstone. Sunstone has flashes of reddish color caused by inclusions of hematite. Moonstone shows a glowing shimmer similar to labradorescence, but lacking in color. The display is produced from lamellar intergrowths inside the crystal. These intergrowths result from compatible chemistries at high temperatures becoming incompatible at lower temperatures and thus a seperating and layering of these two phases. The resulting shimmer effect is caused by a ray of light entering a layer and being refracted back and forth by deeper layers before it exits the crystal. This refracted ray has a different character than when it went in and produces the moonlike glow.
Oligoclase is a member of the Plagioclase Feldspar Group. The plagioclase series comprises minerals that range in chemical composition from pure NaAlSi3 O8, Albite to pure CaAl2 Si2 O8 , anorthite. Oligoclase by definition must contain 90-70% sodium to 10-30% calcium in the sodium/calcium position of the crystal structure. The various plagioclase feldspars are identified from each other by gradations in index of refraction and density in the absence of chemical analysis and/or optical measurements.

All plagioclase feldspars show a type of twinning that is named after albite. Albite Law twinning produces stacks of twin layers that are typically only fractions of millimeters to several millimeters thick. These twinned layers can be seen as striation like grooves on the surface of the crystal and unlike true striations these also appear on the cleavage surfaces. The Carlsbad Law twin produces what appears to be two intergrown crystals growing in opposite directions. Two different twin laws, the Manebach and Baveno laws, produce crystals with one prominant mirror plane and penetrant angles or notches into the crystal. Although twinned crystals are common, single crystals showing a perfect twin are rare and are often collected by twin fanciers.



  • Color is usually off-white or gray or pale shades of green, yellow or brown.
  • Luster is vitreous to dull if weathered..
  • Transparency crystals are translucent to transparent.
  • Crystal System is triclinic; bar 1
  • Crystal Habits include blocky, or tabular crystals. Crystals have a nearly rectangular or square cross-section with slightly slanted dome and pinacoid terminations. Twinning is almost universal in all plagioclases. Crystals can be twinned according to the Albite, Carlsbad, Manebach and Baveno laws. Oligoclase can be found as a major rock forming component in granites and syenites.
  • Cleavage is perfect in one and good in another direction forming nearly right angled prisms.
  • Fracture is conchoidal.
  • Hardness is 6 - 6.5.
  • Specific Gravity is approximately 2.64 - 2.68 (average)
  • Streak is white.
  • Associated Minerals are quartz, muscovite and K-feldspars.
  • Other Characteristics: index of refraction is 1.533 - 1.552. Lamellar twinning may cause a grooved effect on cystal and cleavage surfaces that appear as striations.
  • Notable Occurrences include Sri Lanka; New York, USA; Russia; Sweden and Canada.
  • Best Field Indicators are occurence, twinning striations, shimmer, density and index of refraction.




  • Chemistry: Cu2AsO4(OH), Copper Arsenate Hydroxide
  • Class: Phosphates
  • Uses: As mineral specimens and minor ore of copper.
  • Specimens

Olivenite is a rare secondary copper mineral that is noted for its deep olive green color, hence the name. It is found in deeply weathered, highly concentrated copper sulfide ore bodies. Olivenite is isostructural with the minerals libethenite, Cu2PO4(OH) and adamite, Zn2AsO4(OH). This means that they share the same symmetry and crystal shapes. Olivenite's olive green color and bright luster make it a popular mineral specimen.



  • Color is dark olive green.
  • Luster is resinous to nearly adamantine.
  • Transparency: Specimens are translucent to almost opaque.
  • Crystal System is orthorhombic 2/m2/m2/m
  • Crystal Habits include crystals that are diamond-shaped, often acicular, prisms that are terminated by a dome with triangular faces. Can form feniform masses similar to wavellite's typical habit. Also as tiny crystalline druzes, fiberous masses, nodules and crusts.
  • Cleavage is poor in two directions, but rarely noticed.
  • Fracture is conchoidal.
  • Hardness is 3
  • Specific Gravity is approximately 3.9 - 4.4 (above average to heavy for translucent minerals)
  • Streak is olive green.
  • Associated Minerals are malachite, libethenite, Clinoclase, limonite, adamite, agardite and other secondary copper ore minerals.
  • Other Characteristics: Soluable in hydrochloric acid.
  • Notable Occurrences: Cornwall, England; Nevada, USA and Tsumeb, Nambia.
  • Best Field Indicators are color, habits, soluability in hydrochloric acid, associations, locallities and density.


Opal has been a popular gem for many centuries and has a very interesting structure. Opal is considered a mineraloid because this structure is not truly crystalline. The chemistry of Opal is primarily SiO2 and varying amounts of water. The amount of water varies from 5 -10% and greater. This water can help geologists determine the temperature of the host rock at the time the opal formed.

Although there is no crystal structure, (meaning a regular arrangement of atoms) opal does possess a structure nonetheless. Random chains of silicon and oxygen are packed into extraordinarily tiny spheres. These spheres in most Opals are irregular in size and inconsistent in concentration. Yet in Precious Opal, the variety used most often in jewelry, there are many organized pockets of the spheres. These pockets contain spheres of approximately equal size and have a regular concentration, or structure, of the spheres. This has the effect of diffracting light at various wavelengths, creating colors. Each pocket produces a different color, with a different intensity depending on the angle from which a viewer sees it. The multicolored flashes of light that Opal emits gives it a truly beautiful and valuable look.
The name opal probably is derived from the Sanskrit name for precious stone; upala. It has been mined for centuries, at least since Roman times when they extracted the opal from areas now within the Czech Republic. The Aztecs made use of local Mexican sources as did the Spaniards when they exported the material back to Europe. Today most precious opal comes from Australia with significant sources from Mexico and the Western United States.

Not all opal is so precious however. Common opal lacks opalescence, color or luster and is . . . after all . . . common. Opal is often imitated, forged and "enhanced". Fluorescence, while somewhat unreliable is a good method to determine authenticity.



  • Color is white, colorless, pale yellow, pale red, gray or black when impurities are common. Diffraction can cause flashes of any color of the rainbow (opalescent).
  • Luster is vitreous to pearly.
  • Transparency: Specimens are transparent to translucent.
  • Crystal System: Does not apply because opal is amorphous.
  • Habits include massive, cavity-fillings such as in fractures and geodes, nodular, reniform or as a replacement of other minerals and wood.
  • Cleavage is absent.
  • Fracture is conchoidal.
  • Hardness is 5.5 - 6
  • Specific Gravity is approximately 2 - 2.5 (light)
  • Streak is white.
  • Other Characteristics: Most specimens will fluoresce white or pale green, some phosphoresce and all specimens can be very sensitive to impacts and low temperatures.
  • Associated rocks are chert (a form of microcrystalline quartz), volcanic rocks and many others.
  • Notable Occurrences include many Western USA localities; Mexico; Australia; England; Czech Republic and many other localities around the world.
  • Best Field Indicators are color play (opalescence), low density, fluorescence, fracture filling tendency and lack of cleavage or crystal faces.





Orpiment is a rare mineral that usually forms with realgar. In fact the two minerals are almost always together. Crystals of orpiment are extremely rare as it usually forms masses and crusts. The masses are sometimes transparent to a degree and have a gemmy quality to them. The yellow color is special to orpiment and can be confused only with a few other minerals. Orpiment is derived from the latin auripigmentum, or golden pigment. Its use as a dye or pigment is limited due to its instability. Over time, orpiment will deteriorate into a powder. The process takes a long time, but exposure to light will accelerate it. Specimens should be stored in dark, enclosed containers.








  • Color is orange-yellow to yellow.
  • Luster is resinous to pearly
  • Transparency crystals are translucent to transparent.
  • Crystal System Monoclinic; 2/m
  • Crystal Habit: is usually foliated or earthy masses and crusts, also fiberous and as small tabular crystals that appear orthorhombic.
  • Cleavage is perfect in one direction producing flexible, non-elastic flakes.
  • Fracture is flaky.
  • Hardness is 1.5-2.
  • Specific Gravity is 3.5
  • Streak is yellow
  • Associated Minerals realgar, calcite, stibnite, barite and gypsum.
  • Other Characteristics: orpiment is unstable in light; specimens should be stored in complete darkness. Also usually has a distinct odor similar to sulfur, but is due to the arsenic.
  • Notable Occurances Romania; Peru; Japan; Mercur, Utah, USA and Australia.
  • Best Field Indicators are crystal habit, cleavage, odor and color.





Orthoclase is a polymorph of other minerals that share the same chemistry, but have different crystal structures. If positive identification between these minerals can not be made by field methods, then the specimen may simply be referred to as a potassium feldspar or K-spar. Plagioclase feldspars lack potassium, are light colored and are usually striated. The other k-spar minerals are sanidine, microcline and anorthoclase. Orthoclase is the more common of the k-spars.

The differences between these minerals are minor in hand samples. Microcline tends to be deeper-colored and is the only one that can be, but is not always, a blue-green color (amazonite). Orthoclase does not show the lamellar twinning that is common in microcline and is occassionally present as striations on cleavage surfaces.

Sanidine and anorthoclase usually have a flattened crystal habit. Other than that, enviroment of formation is the only other hand sample clue to distinguish orthoclase from sanidine or anorthoclase. Orthoclase is the main k-spar of granites and syenites that cooled moderately quickly. Sanidine and anorthoclase are common constituents in extrusive igneous rocks such as rhyolites, where the rock cooled quickly. Optical properties and x-ray techniques are the only sure ways to distinguish orthoclase from sanidine, microcline and anorthoclase.

Orthoclase forms at intermediate temperatures between the stability fields of sanidine and microcline. At 400 degrees C or less, microcline is the stable structure for KAlSi3O8. Between approximately 500 degrees C and 900 degrees C, orthoclase is the stable structure. And above approximately 900 degrees C, sanidine is the stable structure. The difference between the structures is only in the randomness of the aluminum and silicon atoms. In microcline the ions are ordered, and this produces the lower symmetry of triclinic (yes, more order produces lower symmetry, see discussion in symmetry). With higher temperatures the positions of the aluminums and silicons become more disordered and produce the monoclinic symmetry of orthoclase and finally, sanidine.

Twinning is common in all feldspars and follow certain twin laws such as the Albite Law, the Pericline Law, the Carlsbad Law, the Manebach Law and the Baveno Law. In orthoclase, only the Carlsbad Law, the Manebach Law and the Baveno Law are seen. The Carlsbad Law twin produces what appears to be two intergrown crystals growing in opposite directions. Two different twin laws, the Manebach and Baveno laws, produce crystals with one prominant mirror plane and penetrant angles or notches into the crystal. Although twinning in general is common for orthoclase, single crystals showing a perfect twin are rare and are often collected by twin fanciers.




  • Color is off-white, yellow, or shades of red, orange to brown.
  • Luster is vitreous to dull if weathered.
  • Transparency crystals are usually opaque, may be translucent or rarely transparent.
  • Crystal System is monoclinic; 2/m
  • Crystal Habits include blocky or tabular crystals. Crystals have a nearly rectangular or square cross-section with slightly slanted dome and pinacoid terminations. Twinning is common. (see above). A psuedo-orthorhombic or psuedo-trigonal variety, found in alpine veins is called adularia, and forms more flattened tabular crystals.
  • Cleavage is good in 2 directions forming nearly right angled prisms.
  • Fracture is conchoidal or uneven
  • Hardness is 6
  • Specific Gravity is approximately 2.53 - 2.56 (average)
  • Streak is white.
  • Associated Minerals are quartz, plagioclase feldspars, micas, garnets, tourmalines and topaz.
  • Other Characteristics: some crystals may show opalescence and are called moonstone.
  • Notable Occurrences are many but these are a few of them: Salzburg, Austria; Cornwall, England and New York, Vermont, Maine and New Hampshire, USA.
  • Best Field Indicators color, lack of striations, cleavage, twinning if present and occurrence.





Osbornite is one of the rarest minerals from Earth. This can be said because it is not found in terrestrial rocks. Osbornite is only found in enstatite chondrite meteorites. These stoney meteorites contain minerals that have been extremely reduced, the opposite of oxidized. Other chemically unusual minerals that are found in these meteorites include sinoinite {SiNO}, cohenite {Fe3C}, schreibersite {(Fe, Ni)3P}, oldhamite {CaS} and troilite {FeS} as well as plagioclase, enstatite and about 10-15% iron-nickel. How osbornite forms and what produces these chemically unusual meteorites is still being studied.

Although osbornite is a compound and not an element, it is still classified in the Native Elements Class as an elemental compound (an oxymoron for sure). This is basically because it is difficult to put in any other class! Since it lacks oxygen or halides, it can not be classified in any class except the elements or the sulfides. Although it lacks sulfur, it could have been classified as a sulfide as the arsenides are. But osbornite's chemical bonds are more similar to the chemical bonds in elements like sulfur and other elemental compounds such as moissanite, carlsbergite, nierite. And so we get the unsual situation were compounds such as osbornite are classified as elements.



  • Color is bronze yellow to golden yellow.
  • Luster is metallic.
  • Transparency: Crystals are opaque.
  • Crystal System is isometric; 4/m bar 3 2/m.
  • Crystal Habits include small disseminated grains in meteorites.
  • Hardness is 7.
  • Specific Gravity is 5.3 - 5.4 (above average for metallic minerals)
  • Associated Minerals include iron-nickel, sinoinite, cohenite, schreibersite, oldhamite, troilite, plagioclase and enstatite
  • Notable Occurrences include Gorakhpur, Basti district, Uttar Pradesh, India; the site at which the Bustee meteorite was found and other enstatite chondrite meteorite sites.
  • Best Field Indicator is color, source, associations and density.





Osumilite, when first discovered as grains in volcanic rocks near Osumi, Japan, was thought to be the similar appearing cordierite. Cordierite, a magnesium aluminum silicate, is also a cyclosilicate and has a similar blue color. But cordierite had never been found in volcanic rocks before and eventually the unusual structure of the new mineral was discovered to be quite different from cordierite.

The primary structural unit of osumilite is a most unusual double ring, with a formula of Si12O30. Normal rings of cyclosilicates are composed of six silicate tetrahedrons; Si6O18. The double ring is made of two normal rings that are linked together by sharing six oxygens, one from each tetrahedron in each six membered ring (notice the loss of six oxygens in the double ring formula). The structure is analogous to the dual wheels of a tractor trailer.

Osumilite is a rare mineral and yet it is one of the two minerals that gives its name to a somewhat large group of silicates, namely the Milarite - Osumilite Group. This group is sometimes called the Milarite Group and sometimes called the Osumilite Group and of course, as is done here, the Milarite - Osumilite Group. The group is composed of similar cyclosilicate minerals that are all rare and very obscure with the exception of osumilite, milarite and sugilite. All members of this group contain in their structures the unusual double ring mentioned above.

Osumilite is in a series with a similar mineral called Osumilite-(Mg). A series is where two or more minerals having the same structure will vary their chemistry between two elements. One mineral is enriched in one element and the other mineral is enriched in the other, with all other elements in the formula staying largely unaffected. In this case osumilite is the iron rich end-member of the series and the appropriately named osumilite-(Mg) is the magnesium end member.

Both minerals contain some magnesium and iron anywise, but they are distinguished by an enrichment of either iron or magnesium. Osumilite has been suggested to be named osumilite-(Fe) which would seem unnecessary unless a third mineral that is more pure in iron is established and osumilite could represent the middle of the series.



  • Color is blue, pale blue, gray, pink, brown to black.
  • Luster is vitreous.
  • Transparency: Crystals are translucent to rarely transparent.
  • Crystal System is hexagonal; 6/m 2/m 2/m.
  • Crystal Habits include prismatic crystals with a pinacoidal termination.
  • Cleavage is absent.
  • Fracture is subconchoidal.
  • Hardness is 5 - 6
  • Specific Gravity is approximately 2.6 - 2.7 (average)
  • Streak is blue-gray.
  • Associated Minerals: Magnetite, hematite and pyroxenes.
  • Notable Occurrences include the type locality for osumilite which is Sakurazima Mountain, Sakkabira (Osumi), Hayasaki, Kyusyu, Japan and the type locality for osumilite-(Mg) of Tieveragh, Antrim County, Northern Ireland as well as MacKenzie Pass, Lane County, Oregon, USA; Namaqualand, South Africa; Sardinia, Italy and Eifel District, Germany.
  • Best Field Indicators are hardness, color, luster and environment.





  • Chemistry: CdCO3, Cadmium Carbonate.
  • Class: Carbonate.
  • Group: Calcite.
  • Uses: A minor ore of cadmium and as mineral specimens.
  • Specimens

Otavite is a rare mineral from the famous mines of Tsumeb, Otavi (hence the name), Namibia. It is one of only a few cadmium minerals as cadmium is usually a trace element in other minerals. Other cadmium minerals include: Native elemental cadmium; the oxide mineral monteponite; the sulfide minerals greenockite, hawleyite, cadmoselite, shadlunite, barquillite, cernyite and quadratite; the sulfate mineral niedermayrite, and the arsenate minerals andyrobertsite, and keyite. All are rare minerals, as is otavite. Otavite is a member of the Calcite Group of minerals. Most other members of this group are quite common and include such well known minerals as calcite, siderite, rhodochrosite, smithsonite and magnesite.



  • Color is usually white, but can also be yellowish brown, reddish brown and brown.
  • Luster is adamantine to pearly.
  • Transparency crystals are usually translucent to transparent.
  • Crystal System is trigonal; bar 3 2/m.
  • Crystal Habits are limited to crusts and tiny scalahedral crystals.
  • Cleavage is perfect in 3 non-perpendicular directions forming rhombs.
  • Fracture is conchoidal.
  • Hardness is 3.5 - 4.
  • Specific Gravity is 5 (very heavy for a translucent mineral).
  • Streak is white.
  • Other Characteristics: Effervesces with acid and some specimens have fluoresced red under shortwave UV light.
  • Associated Minerals includes azurite, malachite, smithsonite and calcite.
  • Notable Occurrences include the type locality of Tsumeb, Otavi (hence the name), Namibia; Blanchard Mine, New Mexico, USA and Broken Hill, New South Wales, Australia.
  • Best Field Indicators are density, reaction to acids, cleavage, color and locality.

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