Feature

From Rocks to Rubies

Rocks have been partially turned into rubies in a Victoria University laboratory, as part of research by mineralogist Associate Professor Rodney Grapes.

Grapes perfected his recipe, based on "stewing" greywacke rocks in a "bomb", as part of his investigations into geological processes. He aimed to replicate in a laboratory the conditions of high temperatures and pressures found deep underground, and the natural processes that form rubies.

His research was inspired partly by rubies found in a few rocks swept down from the Southern Alps. The rubies, found together with sapphires, obviously originated somewhere in the Alps, but no one has yet been able to pinpoint just where. One large boulder, found near Kaniere in gold prospecting tailings several years ago, was nearly one metre by 60cm by 40cm in size.

"When cut in half it contained a core of deep red-blue ruby/sapphire surrounded by a zone of emerald-green mica with a partially preserved skin of serpentinite," Grapes says. "Serpentinite is a rock originally formed in the Earth's mantle, ...kilometres below the surface. These serpentinite rocks occur as lenses up to several hundred metres in length within the northern part of the Southern Alps and are known as the Pounamu Formation -- they are also the source of the highest quality greenstone."

Grapes and associate Ken Palmer have studied the origin of these rocks, taking two complementary approaches by subjecting rocks to chemical analysis using x-ray fluorescence and electron microprobe methods, and also synthesising rubies and sapphires by simulating underground conditions. Ruby is corundum (colourless aluminium oxide) with some chromium thrown in, while sapphires have iron and titanium instead of chromium.

"The Southern Alps are largely composed of Wellington-type greywacke, or rather a version that has been subjected to heat and pressure deep within the crust and modified to form a laminated, mica-rich rock called a schist," Grapes says. "When slivers of this schist are caught up in the serpentinite of the Pounamu Formation, and subjected to temperatures of 400 to 500 degrees Celsius and pressures of about 5000 atmospheres, they undergo a radical change both in composition and and structure."

The process of this change, the mechanism by which it occurs and the rate at which it occurs have all been verified by the experiments at Victoria University. Powdered samples of schist and serpentinite were packed into 5cm-long gold tubes which were placed in "bombs" (vessels capable of withstanding high temperature and pressure), and left to stew there for up to eight weeks at 450^oC and 2000 atmospheres.

The result is a small amount of ruby, several tens of microns in diameter, in the middle of the tube, associated with chromian mica that is emerald green in colour. Chemicals have travelled back and forth between the greywacke and the serpentinite, helped by hot fluid acting as a transfer medium. The greywacke has lost most of its silica, leaving the aluminium oxide more concentrated, and gained chromium from the serpentinite.

"The study has shown to what degree chemical elements become mobile at high temperature and pressure when a thin film of water is present along mineral grain boundaries, in rocks of two very different compositions that are in contact with each other," Grapes says.

This type of chemical reaction has not been thoroughly studied before, and it gives insights into natural processes within the Earth. Extrapolating from his experiments, he estimates that the ruby rock from Kaniere would have taken three million years to form underground.

This is not a commercially viable method of manufacturing rubies, and was never intended to be, but large (centimetre-sized) rubies have been made in the pressure vessels of the high temperature-pressure laboratory at Victoria by simply cooking-up aluminium oxide powder to which a small amount of chromium has been added. These rubies have a beautiful colour, says Grapes, and are flawless, unlike the natural material.

Rubies and sapphires are indispensable for industry and are used in watches (as crystals and bearings), medical equipment and aerospace fibres. Synthetic sapphire plates protect supermarket check-out counters from scratching so that laser scanners can read bar-code prices. Rubies and sapphires are also at the heart of lasers and in microcircuit systems of satellite intelligence systems. They are commercially manufactured for these purposes in the US, Japan, Switzerland, Russia, France and other countries.