Andalusite, kyanite, and sillimanite all contain the same elements. In fact, they have exactly the same chemical composition: Al2SiO5. So why on Earth are there three different minerals? Each of these aluminosilicate minerals has a unique internal arrangement of atoms—a 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.

Video Transcript
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, there’s something cooking too. A geologist is growing minerals in laboratory experiments that simulate the conditions of earth’s interiors.”

“Geological Gourmet.”

“Professor Mike Holdaway of Southern Methodist University wants to learn whether Andalusite or Sillimanite grows at particular temperatures and pressures.”

“Here’s 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. That’s about 3 times hotter than your kitchen oven.”

“One month later it’s time to see what’s happened. Have the ingredients changed? Yes! The Andalusite gained weight. Now Mike knows that Andalusite grows from Sillimanite in earth’s oven at a temperature of 510 degrees Celsius and a pressure of 3500 atmospheres.”

“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 earth’s 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.”

Minerals Out of Bounds 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.

		Kyanite (Pseudomorph after andalusite) The blocklike shapes in this rock were once large andalusite crystals. Now they are occupied by hundreds of white blades of kyanite that formed when the rock was deeply buried and subjected to higher pressures.
		Sillimanite (Pseudomorph after andalusite) Thousands of white, fibrous sillimanite crystals completely replaced the original rod-shaped andalusite crystals when this rock was subjected to a higher temperature.