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Staurolite
Usually found as red to brown prismatic crystals, the iron aluminium oxide mineral staurolite is very useful to metamorphic geologists, since it is fairly common and records the pressure and temperature of mineral transformation undergone in the bowels of the Earth by the precursor rocks (called protoliths). Indicating the metamorphic grade known as amphibolite facies, it commonly occurs in transformed mudstones and often forms a variety of beautifully twinned crystals such as this natural cross.
The name comes from the Greek word for cross, and the twins are two crystals that have grown together interpenetrating each other at either 60 or 90 degrees. It has been a good luck charm for Christians since antiquity, and was known in olden days as fairy stone. Nowadays it is also the state mineral of Georgia (USA). It commonly occurs associated with garnet, kyanite and micas in regional metamorphic (i.e. wide scale) schists.
Our past posts on twinned crystals: http://tinyurl.com/mvbojujhttp://tinyurl.com/l8yba54, http://tinyurl.com/ntdtdb6, http://tinyurl.com/nr7cq6n, http://tinyurl.com/kp6nluf and http://tinyurl.com/nr7cq6n
Loz
Image credit: Mineral Auctions http://www.mindat.org/min-3753.html
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earthstory reblogged
Structural Geology of a cookie!
The immortal bridge
No one really knows how these enormous rocks fell in this unusual place. Together they form a natural bridge spanning over a seemingly bottomless ravine. This so called “immortal bridge” can be found on Mount Tai in the Chinese province of Shangdong.
It is a mountain of cultural and religious significance and counts as one of the five sacred mountains of China. Mount Tai is seen as a symbol of the country’s peace and prosperity and has been visited by many Emperors who performed different ceremonies on the mountain. It is listed both as a World Natural Heritage and World Cultural Heritage by UNESCO.
Its geological history started in the Cambrian period about 500 million years ago. Mount Tai is a typical tilted fault-block mountain. A fault block is a body of rock partly or completely defined by faults and differing in elevation from its surroundings. The mountain comprises sedimentary, magmatic and metamorphic rocks. These rocks create many beautiful formations, such as the immortal bridge which is likely to have been like this since the last ice age.
Xandi Image Credit: http://bit.ly/1N5Jy6e Sources: http://www.chinahighlights.com/mount-tai/ http://whc.unesco.org/en/list/437 http://bit.ly/1J1qQ4A
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