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There are some mineral specimens in this world that beggar belief and the opal pineapples from White Cliffs in Australia are amongst those. They are palm-sized clusters of radiating points, looking like some bizarre fruit and have been known since the beginning of the 20^th century. Found only in the Cretaceous sediments of the White Cliffs opal field in New South Wales, they occurred in an area which has also produced opalised fossils of marine shells, pieces of wood, and even the occasional plesiosaur skeleton. The pineapples are rare, perhaps fewer than 250 have ever been found, and range in quality from gem opal displaying bright diffraction colours, to opaque white potch.

In 1978 Grant Pearson and a group of friends made the lucky discovery of a pocket of opal pineapples about 12 metres below the surface of abandoned workings. Careful excavation showed that the pineapples were restricted to an oval area about 12 by 20 metres in a layer about 2 metres thick, probably originally a localised saline pool at the bottom of the sea. The pineapples are not fossilised fruit, but actually opal pseudomorphs after clusters of crystals.

Many of the nodules were found broken, some so severely shattered that the fragments were too small even to cut as gems. Others were intact and after laborious cleaning made the most spectacular specimens. According to Grant Pearson the best specimen recovered was sold for just under AU$100 000, although one wonders how anyone could sell such a specimen, having found it oneself!

Initially the original crystals were thought to be glauberite, but more recent investigations have shown that they probably were the mineral ikaite, the hexahydrate of calcium carbonate, CaCO₃·6H₂O. This forms only in water at near freezing conditions, like that found at the bottom of deep oceans or in the polar regions. At the

time of formation of the ikaite precursors of the pineapples, Australia was further south than at present, and the area of the White Cliffs opal fields was beneath a cold sea. Although the Cretaceous polar temperatures were higher than polar temperatures today, icebergs must have rafted debris out to sea, as evidenced by dropstones found occasionally in the opal diggings, including one in Grant Pearson’s pineapple layer. The uncertainties in reconstructing the geological past mean one can’t be too specific; but we know it was cold, and wet, and there was one layer in which the conditions were just right for the formation of ikaite.

The mechanism of replacement of ikaite by opal is still unclear. The pineapple pocket was underlain by a relatively impervious clay horizon, which may have helped retain the opalising fluids in the pineapple bearing layer. Some of the pineapples show a distinct layering of the opal, while others when broken or sawn open have interior swirls of colour, indicating a complex mechanism of opal formation. The opal itself is not unusual in structure, and like all colourful opal consists of ordered layers of tiny silica spheres, precipitated from a gel. Ordered volumes of larger spheres produce red colours, and smaller spheres blue.

Opal pineapples are amongst the rarest of mineral specimens. Given that they seem to originate from only one layer in the largely worked-out White Cliffs opal field, and from isolated and widely-spaced pockets within that layer, it is unlikely that more are going to appear any time soon. Duncan Miller

Acknowledgement:

Thank you to Grant Pearson for allowing reproduction of several of his photographs of opal pineapples and for providing copies of relevant publications. These are now lodged in the club’s library, for those who may wish to read more about these fascinating specimens.

References:

Niedermayr, G. & Pearson, G.M. 1997. Die „Opal Pineapples“ von White Cliffs in New South Wales, Australien. Mineralien Welt 8(5):48.

Niedermayr, G. & Pearson, G.M. 1997. Die „Opal Pineapples“ von White Cliffs in New South Wales, Australien. Mineralien Welt 8(4):51.

Pearson, G. 1986. Opal pineapples from White Cliffs. The Australian Gemmologist 16(4):143.

Pearson, G. 1987. Opal pineapples from White Cliffs, New South Wales. Australian Mineralogist 2(2):7.