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Roger Weller, geology instructor

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Alabaster
by Richard Hotchkiss
Physical Geology
Spring 2010
    

                                                      Alabaster

          Alabaster - The purest alabaster is a snow-white material of fine tiniforni grain.  Due to the characteristic color of white alabaster, the term has entered the vernacular as a metonym for white things, particularly “alabaster skin” which is very light and quite transparent.  The use to imply whiteness occurs in a line from the poem and song, America the Beautiful, and a line from a jazz ballad titled Midnight Sun. 



 

         Alabaster is a name applied to varieties of two distinct minerals: gypsum (a hydrous sulfate of calcium) and calcite (a carbonate of calcium). The former is the alabaster of the present day called true or modern alabaster ;  generally, the latter is the alabaster of the ancients, known as Oriental alabaster. 
 

         What do we know about the physical properties of these two forms of alabaster?  Calcite has the chemical formula CaCO3, while gypsum has the formula CaSO4.  Both are chemical sedimentary materials.  The four factors in the formation of sedimentary rocks are weathering (erosion), transportation, deposition, and compaction.
 

         Sedimentary materials are made from small particles of weathered igneous rocks.  The process involves the release of cations (positively charged atoms of a material) that form new materials.  
 

Olivine

photo from Roger Weller

goes to Calcite


photo from Roger Weller
 

         Olivine igneous rock giving of Mg+2 cations to form the sedimentary mineral calcite which forms the sedimentary rock Marble. 
 

         Amphibole igneous rock giving off Ca+2 cations to form the sedimentary mineral gypsum which becomes the rock of the same name.

Amphibole

photo from Roger Weller

goes to Gypsum

 

         Calcite is a carbonate rock.  Carbonate rocks are those composed of the carbonate minerals, Calcite (CaC03) and Dolomite ((Mg,Ca)C03).  Any rock dominated by Calcite is called a limestone, although there are many different types.  Limestones are almost all formed under the influence of the biota (organisms living in the water).  Some examples are clams, corals and snails, which extract the constituent chemicals from the water to form their exoskeletons.  When these animals die, they leave their skeletons behind and these particles can become incorporated into the rocks.  More commonly, Calcite is precipitated by the actions of single-celled organisms (algaes and zooplankton).  These microscopic Calcite particles make up most of Limestone.   

         Gypsum is an evaporative rock.  Evaporite rocks, in contrast to Carbonate rocks, are deposited by chemical precipitation (not biochemical).  They are classified by the dominant mineral, gypsum (rock gypsum or alabaster) and halite (rock salt). These rocks form when waters become highly saline (such as Great Salt Lake in Utah) and supersatured for these minerals.  Normal ocean waters contain about 34,500 ppm TDS (Total Dissolved Solids). If the concentration reaches about 100,000 ppm TDS, then gypsum will begin to precipitate from the water.
 

         Gypsum also occurs in a massive form called alabaster, but most gypsum occurs in massive chalky beds of rock gypsum.  It is mined for the manufacture of plaster, and household wallboard is filled with gypsum.  Plaster of Paris is a roasted gypsum with most of its associated water driven off, so it readily combines with water to return to gypsum. 
 

         Gypsum is the alabaster of the present day, while calcite is generally the alabaster of the ancients.  The two kinds are readily distinguished from each other by their relative hardness. The modern alabaster is so soft as to be readily scratched even by the finger-nail (hardness = 1.5 to 2).  Its softness enables it to be readily carved into elaborate forms, but its solubility in water renders it inapplicable to outdoor work.  The stone called alabaster by the ancients is too hard to be scratched in this way (hardness = 3), though it yields readily to a knife.  Moreover, the ancient alabaster, being a carbonate, effervesces on being touched with hydrochloric acid, whereas the modern alabaster when so treated remains practically unaffected.  
 

         Calcite alabaster is found as either a stalagmitic deposit, from the floor and walls of limestone caverns, or as a kind of travertine, similarly deposited in springs of calcareous water.  Its deposition in successive layers gives rise to the banded appearance that the marble often shows on cross-section, whence it is known as onyx-marble or alabaster-onyx, or sometimes simply as onyx — a term that should be restricted to siliceous minerals, however.  The Egyptian alabaster has been extensively worked near Suez and near Assiut; there are many ancient quarries in the hills overlooking the plain of Tell el Amarna.
 

          When the term "alabaster" is used without any qualification, it invariably means a fine-grained variety of gypsum. This mineral, or alabaster proper, occurs in England.  The early use of alabaster for vessels dedicated for use in the cult of the deity,  Bast, in the culture of the Ancient Egyptians is well documented, however, thousands of gypsum alabaster artifacts dating to the late 4th millennium BC also have been found in Tell Brak (present day Nagar), in Syria.
 

          From the Early Dynastic Period on, the Egyptians quarried a type of stone that is frequently referred to as alabaster, but is in fact travertine (a type of limestone - calcium carbonate).  It is a whitish, slightly translucent stone (often with veins of another colour) which occurs mainly in Middle Egypt, particularly between Miniya and Asiut.  The best known and most important source was the Hatnub quarries, near El-`Amarna.  One Old Kingdom quarry is situated in the Wadi Gerrawi close to Helwan.  Because of its color and durability, alabaster was regarded as a pure stone and used for chapels, pavements in temples, sarcophagi, altars and above all statues. True alabaster was quarried in the Faiyum region, but Egyptologists usually call this material gypsum (calcium sulphate).
 

          Alabaster is ageless.  It is found throughout the histories of Sumer, Babylon and Assyria and of course, Egypt.  Deposits of alabaster are not only found in Egypt also in countries like Algeria, England, United States, Belgium, India, Turkey, Spain, Cyprus, and Italy.
 

          Stonework with alabaster was one of Egypt's earliest industries (4000 BC).  Two ancient alabaster sites are known. One site was at Wadi Gerawi, and the other at Helwan.  Oriental alabaster (marble or calcium carbonate) was very popular during Egypt's New Kingdom.  The working of hard stone reached its height during the Third and Fourth Dynasties, 2600-2400 BC. Early vessels were of simple but elegant shape, often with flat board rims.  Calcite was believed to have, in a mythical sense, solar connections.  The first people to use large amounts of true alabaster were the Etruscans.  These were a people who lived in northern central Italy, in what is now Tuscany and Umbria.
 

           Black Alabaster is a rare form of the gypsum-based mineral.  This black form is only found in three veins in the world,  Italy,  the People's Republic of China, and Alabaster Caverns State Park, near Freedom, Oklahoma that is home to a natural gypsum cave in which much of the gypsum is in the form of alabaster.


 http://www.sculpt.com/catalog_98/stones/ALABASTERS.htm

 

           Alabaster is quarried either in open pits or underground.  In open pits, veins of alabaster are found 12 to 20 feet below the surface under a layer of shale which can be two or three feet deep.  The rocks have an average height of 16-20 inches and a diameter of two to three feet.  Rarely do the rocks exceed these sizes.  This material is a mineral product of two fine masters, father Time and mother Earth.  It requires both to achieve the fine grain of its smooth white translucent stone.  Working the alabaster quarries is a laborious, unpredictable job.  

Canopic Jars

photo courtesy of Richard Hotchkiss

Seti I Sacophagus
 


http://freespace.virgin.net/martin.churchil/sarcophagus_of_seti_i.htm

 

Alabaster Sphinx

photo courtesy of Patricia Hotchkiss


 

           Alabaster also was employed in Egypt for canopic  jars and various other sacred and sepulchral objects.  A sarcophagus, sculptured in a single block of translucent calcite alabaster from Alabastron, is in the Sir John Soane's Museum, London.  This was discovered by Giovanni Belzoni in 1817 in the tomb of Seti I, 19th Dynasty, 1294-1278BC, near Thebes.  It was purchased by Sir John Soane, previously having been offered to the British Museum. Pictures above are 4 miniature replicas of canopic alabasters jars, the single pieces sarcophagus, and the alabaster sphinx at Saqqara.

Alabaster Lamp

http://www.touregypt.net/featurestories/alabaster.htm

Alabaster Gift Shop

photo courtesy of Patricia Hotchkiss

 

           At the beginning of last century, ART DECO and ART-NOUVEAU designers used it for the first time as diffuser of light to design decorative fitting, taking advantage of its transparency and natural graining.  In the middle of the 80´s, the recovery of DECO´s style in interior design boosted the re-introduction of alabaster as a material, competing with crystal and other acrylic materials, and mainly contributing, as novelty, the fact that it was a "natural product made by hand”.  Alabaster used in lighting is white with beige or taupe veins and inclusions.  Pieces used for lamp bases and other small parts might be cut from lower quality, less translucent stone.
 

           Several millenniums have passed since the ancient Egyptians began to work with alabaster and in spite of various problems Egyptian artisans have not lost the knowledge of how to handle this material.  Even though branch of industry no longer represents the most important factor of the economy in Egypt, it is still a characteristic element of its culture and its history.  There aren’t many artisans left "knowing how to handle" Alabaster.

 

References used:

http://www.answers.com/topic/alabaster

http://geology.about.com/od/rocks/ig/sedrockindex/rocpicalabaster.htm

h
ttp://www.brandlighting.com/alabaster_information.html


http://www.touregypt.net/featurestories/alabaster.htm

http://geology.about.com/od/minerals/ig/minpicsulfates/minpicgypsum.htm

http://cmsc.minotstateu.edu/Labs/rockslab/sedimentary/Chemical%20sedimentary%20rocks.html

http://skywalker.cochise.edu/wellerr/aawellerweb.htm

http://freespace.virgin.net/martin.churchill/sarcophagus_of_seti_i.htm