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Saturday, March 10, 2012

Alabaster the Non-Gemstone

Church walls and gold leaf decorations made from calcitic alabaster in Poland
Photo by Beemwej

There are two distinct minerals that are called alabaster, one of them is calcium sulfate commonly called gypsum or anhydrite.  The other is a variety of calcium carbonate that is commonly known as limestone or marble.  Of these two the former is the alabaster of today, and the latter is the alabaster of the ancients.  Both of these materials are easy to work with, and have an attractive appearance making them favorites for small carvings and other works of art.

The formation of alabaster explained in Italian
by Miguel

Since we've already established that there are two different types of alabaster the best way to distinguish between them is by their relative hardness. The first type that is composed of gypsum is so soft you can scratch it would your fingernail because I have a Mohs hardness of 1.5 to 2. The calcite variety of alabaster is too hard to be scratched in this way having a Mohs hardness of 3 although you can scratch it with a knife blade. Because it is a carbonate it will effervesce when a drop of hydrochloric acid is placed on its surface. When the gypsum type of alabaster has a drop of mass placed on virtually nothing happens.

An alabaster lamp
Photo by David Dennis

Although alabaster can come in many different colors most people think of it as white, so much so that it has entered the language as a metonym for weight objects. In this sense it is often used to describe “alabaster skin” that means very light and quite transparent that is derived from the use of alabaster in building tombs.

Although alabaster is essentially too soft for use as a gemstone it often finds uses as a decorative stone for making different kinds of statuary and other objects de art. A very common use today is making the basis for electric lamps.

A piece of raw alabaster
Photo by Ra'ike

The word alabaster itself is derived from middle English as well as several other languages including Greek. The word was used to identify a type of vase made of alabaster. There is a certain amount of evidence that the word can be traced all the way back to ancient Egypt where it was named after the town Alabastron where it was commonly found. It was Alabastron where it was originally quarried, however the name of the mineral is still obscure.

The so-called “Oriental” alabaster was highly esteemed by sea ancients for making such items as small perfume bottles small point that basis that were called alabastra. This thing has also been suggested as a possible source of the mineral name. The craftsmen of this stone in ancient Egypt often used alabaster for use in candlestick chart and various other sacred and sepulchral objects. In the Sir John Soane’s Museum in London is a sarcophagus that was carved from a single block of translucent calcite alabaster that was mined Alabastron.

An alabaster carver at work.
Photo by Zyance

Both types of alabaster are essentially evaporative minerals in the case of the gypsum variety it is the mineral that evaporates from seawater before salt,  In the case of the calcitic variety is has its origin from the evaporation of lime water that has reached saturation.  Any further water loss causes the lime to come out of solution where it is deposited. A lot of this type of alabaster is found on the walls and floor of caves where it also forms the formations we call stalactites and stalagmites.

Ancient alabaster vases
Louvre Museum, Paris, France

Although most people think of alabaster as a white mineral it is also found in many other colors particularly when it is stained by mineral salts. Iron is the most common of these although copper and manganese also one of their own suite of colors to the stone. If you cut large blocks of this material into thin sheets they can be used as translucent windows such as those found in the Beineke Library of Rare Manuscripts and Books at Yale University. Throughout the ages how a master has been used to make many beautiful objects and will continue to do so into the future.

Thursday, March 8, 2012

Beryl the Genstone of Many Colors

Beryl crystals from Pakistan var. aquamarine
Photo by Gia Cassa

As a mineral beryl is beryllium aluminum cyclosilicate possessing the chemical formula Be3Al2(SiO3)6­.   As a gem it comes in many colors although pure beryl is colorless of the variety goshenite named after the town of Goshen in the central Berkshires of Massachusetts its type locality.  It is tinted by different colors causing it to be colored blue, green, red, yellow and white.  This is a mineral that is usually found in pegmatites although it is often found in biotite schists and in intruded limestone.  A rare occurrence is found in Utah called bixbite that has been colored red by manganese as an impurity.  Other gems in this family include aquamarine, emerald, green beryl, heliodor and morganite

An emerald crystal from Colombia
Photo by Gery Parent

Aquamarine is the color of sea water a bluish green with green beryl as a sub-order of aquamarine.  Emerald of course is bright green that is usually found in heavily intruded limestone although some occurrences are in biotite schist.  Heliodor is the yellow variety of beryl that is sometimes galled “golden beryl.”  Morganite is sometimes found associated with both aquamarine and golden beryl, bit is differentiated by its pink color.  Red beryl is also called bixbite or red emerald because of its red coloration.  Of all the beryl’s bixbite is the rarest, but only occurs in small crystals allowing gems cut from it to be lass then 5 carets in weight,

Golden beryl or heliodor

Beryl aside from its use as a gem is also the ore of beryllium where it occurs in granitic pegmatites.  One such crystal was a giant that occurred in the Bumpus Quarry in Albany, Maine that was about 5.5 meters long by 1.2 meters in diameter that weighed around 18 metric tons.  The largest crystal of any kind of mineral was found in Madagascar is a beryl crystal that was 18 meters long by 3.5 meters in diameter.  That is the size as the boiler on a steam locomotive.

A crystal of red beryl var. bixbite from Utah
Photo by Rob Lavinsky

Beryl is found on all the continents wherever crystalline igneous or metamorphic rocks are found.  The gems are found in the core of pegmatites where at times terminated crystals are found growing into cavities or vugs in the rock.  Some beryl like the emeralds found in Columbia are found in a peculiar black limestone that has been intruded with granite that contain in addition to the beryl an abundant amount of pyrite that certainly predates the formation of the emerald crystals.. Beryl is also found associated with biotite schist.   Many of the emeralds found in the Ural Mountains of Russia are this type of deposit.  The emeralds found at the Crabtree Corners deposit in North Carolina are also an example of this kind of deposit.

A crystal of morganite on albite and quartz.
Photo by Rob Lavinsky

One of the most important deposits of aquamarine in the United States is found in the Maryall district of New Milford, Connecticut where they occur in a large pegmatite that that was originally mined for feldspar and mica.  This mine has produced some fine specimens of both aquamarine and heliodor beryl including a 44 carat heart on display in the American Museum of Natural History in New York City.  It is also known to produce many other minerals especially large garnets and uranium minerals.  

Amblygonite one of the Gemological Garbage Cans

It is easy to see how amblygonite is often mistaken for other minerals this specimen mimics milky quartz
Photo by Ra'ike

As a mineral amblygonite is classified as a fluorophosphate with a chemical formula of (Li, Na)AlPO4(F, OH) that is composed of sodium, lithium, aluminum, phosphate, fluoride and hydroxide. This is a mineral occurs in pegmatite deposits were this easily mistaken for albite or other feldspars. It is readily distinguished by its density, cleavage and flame test for lithium. Amblygonite forms a solid solution series with montebrasite that is the low fluorine member of the series.

Faceted amblygonite from Brazil.
Photo by Ra'ike

This is a mineral found in LCT type pegmatites, high-temperature tin veins and greisens where it is found with spodumene, apatite, lepidolite, tourmaline and other lithium bearing minerals. It is often been used as an ore of lithium The chief commercial sources have been the deposits in California and France where the mineral contains about 10% lithium. 

A single crystal of faceting grade amblygonite.
Photo by Rob Lavinsky

The mineral amblygonite was originally discovered in Saxony, Germany by August Breithaupt in 1817 and named by him from the Greek words amblus (blunt) and gouia (blunt) because there is an obtuse angle between its cleavages.  Later the same mineral was found at Montebras, France and Hebron, Maine. Because there are slight optical differences in amblygonite found at these differing localities they are also called montebrasite and hebronite after the localities where it has been found.  The mineral is also found in large quantities in Pala in San Diego County, California as well in the Black Hills of South Dakota.  To date the largest single crystal of amblygonite measured 7.62 x 2.44 x 1.83 meters that weighed about 102 tons.

Amblygonite variety Montebrasite
Photo by Rob Lavinsky

When its transparent amblygonite is faceted into gemstones where it is set into jewelry, however because it is prone to breakage and abrasion as well as displaying hardness problems as well as toughness it is rarely mounted into jewelry.  This stone when it is cut often finds its way into collections as unmounted stones. The principle sources of this gemstone are Brazil and the United States however other countries have also produced faceting grade amblygonite such as Australia, France, Germany, Namibia, Norway and Spain.  Most of the gem quality amblygonite crystals that have a yellowish caste are found in pegmatite cavities.

Saturday, March 3, 2012

Benitoite the One of a Kind Gemstone

Benitoite in a matrix of neptunite from San Benito County, California.
Photo by Rob Lavinsky

Benitoite is a one-of-a-kind gem since it is only found in one place in California; it is also the state gemstone of California as of 1985.  The stone itself is a rare variety of barium-titanium silicate found in hydrothermally altered serpentine. Benitoite reacts under shortwave ultraviolet light fluorescing light blue in color.

As a mineral benitoite was described in 1907 by George D. Louderback who named it benitoite because it was found near the headwaters of the San Benito River of San Benito County, California. The stone was one of the first if not the first mineral to be studied under x-ray diffraction.

A single crystal of benitoite in matrix.
Photo by Lech Darski

This stone is usually found an unusual set of minerals including the minerals that make up its host rock. The associated minerals usually include naturolite, neptunite, joaquinite, serpentine and albite. Benitoite is very rare mineral that is only found that it few locations among them are places in San Benito County, California. It is also been found in Arkansas and Japan. It is usually found in glaucophane schist as inclusions in veins of naturolite.  Benitoite mining is allowed for a fee in the benitoite mine in the Clearwater management area in central California. Collecting is by appointment only!

This gemstone is typically colored various shades of blue although it can also appear as colorless or even have a yellowish caste.  Its crystals can vary from transparent to translucent with visible inclusions with a vitreous luster.

A single crystal of benitoite with no matrix.
Photo by Rob Lavinsky

When found its crystals are hexagonal; bar 6 m 2 that in crystal habits include six place dipyramid flattened shapes having a distinct triangle shape that is often modified by having minor faces.  At times it is also found as small grains.

Benitoite has no cleavage and its fracture is irregular.  Other distinguishing features are its hardness ranging from 6 to 6.5 on the Moh’s scale of hardness.  Its specific gravity is 3.6 making it denser then most other minerals making it capable of being separated by gravity methods such as panning.  The mineral also displays a white streak.

A faceted example of benitoite
Photo by Fastily

This mineral is counted among those that fluoresce under the influence of ultraviolet light displaying a blue light.  Minerals that are associated with benitoite include serpentine, neptunite, natrolite, joaquinite, sanbornite, taramellite, albite and fresonite. 

Benitoite gems are only found in three mines located in San Benito County, California where good to excellent crystals are found.  The only other occurrences are in Eocene aged sands in southwestern Texas along with another few localities in California.  These occurrences are only small grains found in sand.

Benitoite under ultraviolet light.
Photo by Parent Gery 

If you are searching for this mineral the best field indicators its crystal habit, the fact that it displays fluorescence in blue, its distinctive color, the minerals it is associated with and the locality.  The fact it is associated with bodies of serpentine suggests that it might be found in localities that have been identified as suture zones where geologists feel that serpentine is metamorphosed oceanic crust that has been caught up in earth movements associated with the collision of two continental plates,


Friday, March 2, 2012

Azurite Hydrous Copper Carbonate a Blue Gem.

Botrydoil Azurite from the Apex Mine of Utah.
Photo by Rob Lavinsky

Azurite is one of the lesser known copper ores that is produced by the weathering of copper deposits. The mineral is also known as chessylite named after the Chessy-les-Mines near the city of Lyon in France. This mineral has been known to the ancients and is mentioned in Pliny the Elder’s, Natural History.  Because of its deep blue color has also been used as a pigment that is a deep blue color. Azurite is usually found in the desert where the low humidity causes its cousin malachite to lose water.  This is where it is often found in the company of its cousin malachite.  Both minerals are essentially the same copper carbonate except azurite contains more water in its crystals.  Over time this water will evaporate causing the azurite to change to malachite.

Typically azurite crystals are monoclinic that when they are large enough to be seen will appear is dark blue prismatic crystals. In nature azurite crystals are massive to nodular with some of them forming stalactitic masses. Any specimen of azurite over time will lighten in color as its surface retrogrades in the malachite. The mineral is soft with a Mohs hardness of 3.5 to 4. It displays a specific gravity ranging from 3.77 to 3.89. The mineral azurite is destroyed by the application of heat losing both carbon dioxide and water forming a type of black powdery copper oxide. A drop of acid placed on the specimen will cause it to effervesce like any other carbonate.

Azurite and malachite
Photo by Rob Lavinsky

Some of the earliest uses of azurite are a blue pigment in artist’s paints. In this use it was finely ground, and then mulled with linseed oil and turpentine. The mineral is capable of producing many different shades of blue depending upon the fineness to which it is ground. When it is mixed with oil, as in oil paints it turns slightly green. On the other hand if it is mixed with egg yolk it turns a gray green. In older paintings the azurite has turned into malachite displaying a greenish color. Many times what was azurite was mislabeled as lapis lazuli that in times past was a name that was applied to many other blue pigments. True lapis lazuli that was used as another blue pigment came primarily from Afghanistan during the Middle Ages. Azurite was a common mineral found in Europe in the same timeframe.

Lapis Lazuli, notice the difference in color from azurite.

There were sizable deposits of azurite that were mined near Lyons, France during the Middle Ages. Azurite was also mined during the 12th century in Saxony and the various silver mines that were located there.

If azurite is heated it turns black whereas the more expensive natural ultramarine remains blue when it is heated. If the mineral is gently heated it produces a deep blue pigment that is often used by the Japanese in some of their painting techniques.

Azurute surrounded by malachite.
Photo by Rob Lavinsky

Occasionally azurite finds uses beads and other types of jewelry; it is also used as a decorative stone. The thing that keeps it from being used more commonly as a gemstone is its softness and the fact that it weathers easily reverting back to malachite.