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Andalusite, kyanite, and sillimanite all contain the same
elements. In fact, they have exactly the same chemical composition:
So why on Earth are there three different minerals? Each of these
aluminosilicate minerals has a unique internal arrangement of atomsa
result of the combined temperatures and pressures under which they form.
Andalusite is stable at low pressures and temperatures. Kyanite is
stable at high pressures and moderate to high temperatures. Sillimanite
is stable at a wide range of pressures and high temperatures.
This remarkable diagram enables geologists to estimate the temperatures
and pressures at which some rocks form. How was it created? In special
laboratories, geologists make synthetic rocks and minerals at conditions
similar to those deep within the Earth. They use the results of these
experiments to produce temperature-pressure diagrams such as this one.
Join chef Michael Holdaway, a geologist who helped concoct this diagram
Deep within this mountain range rocks are cooking. High temperatures
and pressures are transforming their minerals into new ones. And the
rocks are metamorphosing.
Deep in the heart of Texas, theres something
cooking too. A geologist is growing minerals in laboratory experiments that
simulate the conditions of earths interiors.
Professor Mike Holdaway of Southern Methodist University wants to learn
whether Andalusite or Sillimanite grows at particular temperatures and
Heres his recipe: Carefully weigh out some ground up Sillimanite
crystals and a single Andalusite crystal. Place these ingredients into a
tube made of silver alloy. Add a little water and seal the tube to form a
capsule. Drop this capsule into a pressure vessel and place it in the oven.
Set the pressure to 3500 atmospheres and the temperature to 510 degrees
Celsius. Thats about 3 times hotter than your kitchen oven.
One month later its time to see whats happened.
Have the ingredients changed? Yes! The Andalusite gained weight. Now
Mike knows that Andalusite grows from Sillimanite in earths oven at a
temperature of 510 degrees Celsius and a pressure of
He uses this information to construct diagrams like the
one in the case to your left. It show the ranges of temperatures and
pressures at which Andalusite, Sillimanite and a third mineral, Kyanite, can
grow inside the earth.
With the help of diagrams like these geologists can
pick up a rock at earths surface and by studying its minerals, figure out
how deep in the earth it formed.
They can read tales of rocks that roasted beneath towering mountains 400
million years ago. Or were toasted by invasions of hot magma when dinosaurs
walked the earth.
Each of these two rocks crossed boundaries on the diagram to the right and contains
a coded message. The key is the shapes of the large blocky and rod-shaped crystals,
which were once andalusite. In each of the rocks, one mineral replaced
another evidence that allows scientists to track part of the rocks
temperature and pressure histories.