Diorite
Diorite is a grey to dark grey intermediate
intrusive igneous rock composed principally of plagioclase
feldspar (typically
andesine),
hornblende, and/or
pyroxene. It can also be black or bluish-grey, and frequently has a greenish cast. Varieties deficient in hornblende and other dark
minerals are called
leucodiorite. It is often described as "salt and pepper" when composed largely of light-colored minerals randomly interspersed with dark minerals. When
olivine and more
iron-rich
augite are present, the rock grades into ferrodiorite, which is transitional to
gabbro. The presence of
quartz makes the rock type quartz-diorite or
tonalite, and if orthoclase (potassium feldspar) is present the rock type grades into
granodiorite.
Diorites may be associated with either
granite or gabbro intrusions, into which they may subtly merge. Diorite results from partial melting of a
mafic rock above a
subduction zone. It is commonly produced in
volcanic arcs, and in cordilleran mountain building (subduction along the edge of a continent, such as with the
Andes Mountains). It appears in thousands of square miles of large
batholiths (mass of intrusive igneous rock believed to have solidified deep within the earth). The extrusive
volcanic equivalent is
andesite.
Diorite is an extremely hard rock, making it difficult to carve and work with. It is so hard that ancient civilizations (such as ancient
Egypt) used diorite balls to work granite. Its hardness, however, also allows it to be worked finely and take a high polish, and to provide a durable finished work. Thus, major works in diorite tend to be important.
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Micrograph of pyroxene-hornblende diorite, from Iranian National Bureau of Geology |
One comparatively frequent use of diorite was for inscription, as it is easier to carve in relief than in three-dimensional statuary. Perhaps the most famous diorite work extant is the
Code of Hammurabi, inscribed upon a seven-foot pillar of black diorite. The original can be seen today in Paris' Musée de
Louvre. A few large statues remain, including several statues of King Khafre in the Egyptian Museum. The use of diorite in art was most important among very early Middle Eastern civilizations such as Egypt, Babylonia, Assyria and Sumeria. It was so valued in early times that the first great Mesopotamian empire listed the taking of diorite as a purpose of military expeditions.
Although one can find diorite art from later periods, it became more popular as a structural stone and was frequently used as pavement due to its durability. Diorite was used by both the Inca and Mayan civilizations, but mostly for fortress walls, weaponry, etc. It was especially popular with medieval Islamic builders. In later times, Diorite was commonly used as cobblestone; today many diorite cobblestone streets can be found in England (from the Isle of Guernsey) and Scotland, and scattered throughout the world in such places as Ecuador and China. Although diorite is rough-textured in nature, its ability to take a polish can be seen in the diorite steps of
St. Paul's Cathedral, London, where centuries of foot traffic have polished the steps to a sheen.
Diorite is a relatively rare rock; source localities include
Sondrio,
Italy;
Thuringia and
Sassonia in
Germany;
Finland;
Romania; central
Sweden;
Scotland; the Andes Mountains; the Isle of
Guernsey; and the
Basin and Range province and
Minnesota in the
USA.
An
orbicular variety found in
Corsica is called
corsite.
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list of rocks
