A geological headache is over

This is what Tellnes anorthite looks like, enlarged 50 times. The pictures have been taken with double polarized light through a thin plate. Photo: Henrik Schiellerup

After eight intense months in a laboratory in Australia, NTNU geologist Henrik Schiellerup had enough -material to prove that the rock known as anorthite is formed in the crust of the earth, and not in the mantle. This could prove to be important for the extraction of the super-material titanium.

By Nina E. Tveter

Let us compress 4.5 billion years into a couple of hours. Imagine that we are listening to a symphony. Violins, cellos, trumpets, horns, percussion instruments, clarinets, double-bass and triangles – all these make their contribution to the musical composition. Waves of harmonies as well as more demanding sounds sharpen our awareness. The clarinets tap away at the slowly-dying theme, the violins whip up rainstorms, and the oboes force out thin strips of notes under pressure. The kettledrums herald the greatest volcanic eruption of all times, and the triangle outlines its message clear as crystal. The theme is then investigated by other instruments in other keys. Many instruments have a lot to say, and they frequently discuss it with their co-players, while some others only make a single contribution. If we stretch this analogy a bit, we can say that it gives us a sense of how the minerals and rocks of the earth have been created. Some are formed in the upper layers of the mantle, others in the lower layer of the crust of the earth, and some in the upper layer of the crust from the remains of deep-layer rocks or in chemical processes involving air and water.
Many types of rock have also been through several processes of transformation, as continental plates have moved towards each other and rock-flakes have been pressed back down towards the mantle only to re-emerge on the surface with a new personality. Most rocks are the result of processes involving several geological eras – just as most instruments in the orchestra take part in creating the music of each movement. The triangle, however, seldom performs more than once in a symphony, and this is also the case for a few rocks. One of these is called anorthite, and it has created so many headaches for geologists around the world that its formation has been labelled ‘the anorthite problem’.

Accepted by the journal Nature

Tellnes in Rogaland, Norway, is one of the largest solid-rock mines extracting titanium in the world. The mine produces about seven per cent of total global production. Photo: Henrik Schiellerup

Large massifs of anorthite were formed exclusively in the period from 2500 million to 900 million years ago (in the Proterozic era). How, why and where they formed around the globe has been a hot topic for discussion among geological researchers for many decades. Was this species of rock formed in the mantle or in the crust? And why are there so many ilmenite deposits in the anorthite?
Research student Henrik Schiellerup in the Department of Geology and Mineral Resources Engineering at NTNU became interested in the anorthite problem. Funded by the Research Council of Norway, Schiellerup travelled to Monash University in Australia, where he studied the isotope combination in the exotic elements osmium (Os) and rhenium (Re) in anorthite rocks from Rogaland, Norway. He used advanced techniques and equipment possessed by only a few universities in the world.
After eight months in the laboratory, Schiellerup finally succeeded in establishing that this rock is actually formed in the lower region of the earth’s crust, and not in the mantle as most geologists have claimed. The result is so interesting that the prestigious international journal Nature accepted Schiellerup’s article on this subject last autumn, with NTNU’s Professor Tore Prestvik as co-author. Prestvik was Schiellerup’s supervisor for the doctoral work which led to these conclusions. Other co-authors of the article are Brian Robins at the University of Bergen, Rune B. Larsen at NGU, and David D. Lambert from the Australian university.
– Why is it so important to know whether this rock has its origin twenty or two hundred kilometres below the surface of the earth?
In addition to all the scientific benefits which result from having as many pieces of the ‘jig-saw puzzle’ which comprises the geological development of the earth in place as possible, Schiellerup foresees economic possibilities for mining and for business:
– There are deposits of the mineral ilmenite in anorthite. We can extract titanium from ilmenite (Ti and TiO2). Titanium is a modern raw material which is used in, for instance, white paint, aeroplane engines, frying pans, prostheses, plastic, paper, cosmetics, pills and also for military purposes. Titanium’s melting point is high and it has attractive strength-weight characteristics. It is also resistant to corrosion, he explains.
The world’s two largest solid-rock mines which extract titanium are Lac Tio in Canada and Tellnes in Rogaland, Norway. The latter alone is responsible for about seven per cent of total global production. Norway is the fourth largest titanium ore producer in the world.

Easier to choose the best

Research at NTNU shows that anorthite rock is not formed in the mantle, as was previously believed, but in the lower part of the Earth’s crust. Exact knowledge about formation processes makes it easier for the mining companies to find deposits which have the desired qualities.

Schiellerup explains that even if the deposits in Rogaland and in Canada are huge and very rich in ilmenite, the quality of the ilmenite is very varied. Knowledge of formation processes makes it easier for the mining companies to find deposits of the desired quality.
– In the future the extraction of titanium and TiO2 from ilmenite will be able to take place using two methods. The older method, the so-called sulphate process, does not demand much in terms of the quality of the raw material, other than that that it should not contain too much chrome. But this method results in some undesired by-products, such as sulphuric acid and iron sulphate. The chloride process is less hard on the environment. The drawback is that this method demands more of the raw material (the ore).
Traces of unwanted elements such as magnesium create problems for this method. If we had enough information about where in the Earth the anorthite was formed, along with knowledge about the geological processes which are linked to the anorthites, then the mining companies and the smelteries would be able to select the most appropriate qualities of material, and the best methods for the extraction of titanium. As a result, they would be able to obtain a better return on the raw material.