Chrysoprase from Gumigil, Queensland

Abstract

Chrysoprase from Gumigil, near Marlborough in central east Queensland, occurs in weathered ophiolite of the Marlborough Block, in veins up to 20 cm thick in the lower parts of an approximately 50 m-thick regolith profile. Serpentinised harzburgite saprolite hosting magnesite and chalcedony veins with rare chrysoprase grades upward to a plasmic zone of chalcedony, nontronite and chlorite, with a lateritised zone above consisting of nodular or vermiform ferruginous duricrust, boxwork chalcedony and kaolinite. Gem chrysoprase and the poorer quality so-called 'wetstone' are dominantly chalcedonic quartz, coloured by willemseite, which occurs as 30-100 nm-wide zones largely aligned on polycrystalline quartz grain boundaries. Individual willemseite flakes (possibly single crystals) range from 2 to 12 nm thick. The quartz grainsize in chrysoprase is finer than in wetstone, and the willemseite domains in chrysoprase are smaller and less abundant, amounting to about 5% in chrysoprase and 15% in wetstone. Willemseite composition varies slightly between samples, with an average formula (Ni2.7Mg0.3)Si4O10(OH)2.