Cochise College Student Papers in Geology
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Roger Weller, geology instructor
wellerr@cochise.edu
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
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
http://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