Chiastolite

(via Wahroongai News, Volume 24, No.3, March 1990) Australian Amateur Mineralogist writes:

One of the most intriguing minerals found in interior Australia is the chiastolite of Australian Cross Stone. In nature, the mineral occurs as cigar-shaped crystals in slates which have been ‘heat treated’ by invading granites. Crystals mostly average about 3 or 4 inches in length by an inch in diameter. As they weather from their micaceous (schistose) host rock, they have little obvious beauty. Their shape alone may attract attention.

When broken, an internal cross from is revealed traced in shades of gray, white and sometimes a delicate pink. When cut and polished, true gem status is achieved, and selected examples were once quite popular in the jewelry trade. Like most Australian gem industries, however, indiscrete swamping of markets, careless preparation, and lack of suitable advertising propaganda ruined any wide market possibilities even before they materialized. Its fate was a dismal as the Harts Range ‘rubies’ (or garnet). Even opal had from time to time suffered from similar Australian carelessness. Opal, however, now seems to be enjoying more rewarding popularity on its own merits.

Chiastolites of good internal form occur in but few places in the world, and of these the Bimbowrie Station examples from the Olary district in South Australia undoubtedly are of outstanding quality. At one time, fine specimens could be collected in quality strewn about the slopes of the low schistose hills of the locality. Shearers passing by repeatedly collected them in sugarbags full and returned them to their friends and relatives in the cities. They received wide appeal, but after a time their very abundance weighed against them. Now the picture is different. The old localities are largely worked out as far as surface occurrences are concerned. A day’s collecting will certainly provide many tens of pounds of them, but good ones are no longer over plentiful. Now it appears the stage may be set for real appreciation. The jewelry trade will soon realize their potential.

The origin of these freaks of nature, as indeed they are, is interesting. The mineral is a form of andalusite, a relatively common mineral in the field. In composition, it is a silicate of aluminum, or chemically, Al2SiO5. The reason for its interest is, of course, the cruciform internal structure—a factor of interest, particularly in matters of Christian religion—but also for beauty in its own right.

The cross structure is related to the method of growth of the individual crystal in the presence of minute amounts of carbon as impurities in the enclosing rock medium. The crystals actually grow at the expense of the enclosing aluminous sediment—a redistribution of the alumina and silica of the argillaceous slate occurring to form the crystals. In the present examples small amounts of carbonaceous material occur along with these substances which are excess to the growing crystals requirements. The effects of solution and heat responsible for the minerals growth were not sufficient to remove the carbonaceous matter so they were just pushed aside into less inconvenient situations with in the developing crystal structure.

In this way, the square form of the crystal in section (the mineral crystallizes in the orthorhombic system) imposes a cruciform distribution of the reconcentrated carbon material and leaves other areas relatively free of this substance, and therefore lighter in color.

Actually, the crystal effects redistribution of the carbon in two major patterns, both of which form ‘crosses’ of a type. One is called the ‘Howdenite’ after Mr J Howden, their prospector-discoverer, and the other is unnamed.

Prof Mawson, in his writings on ‘The Geology of the Broken Hill Area’, has dealt with this subject in great detail and earlier in this century produced excellent color reproductions of them. He, too saw their gem potential, and felt that they should have been developed to greater advantage. Perhaps one day we will appreciate them as they deserve, and our ladies will choose them in preference to artificial gems as charms and pendant adornments. I think we shall……….