IRON OXIDE INCLUSIONS IN THE RAINBOW LATTICE SUNSTONE FROM AUSTRALIA

The oriented ribbon-like inclusions in the orthoclase from Australia produce a spectacular aventurescence effect, making the feldspar a unique gem material, known as the rainbow lattice sunstone. Recent study reveals that the inclusions are magnetite and hematite, instead of the previously speculated ilmenite and hematite. The hematite inclusions are thin enough to be transparent, and can create rainbow colors by the interference of light reflected off the boundaries of the crystals. They are estimated to be ~100nm thick from the wavelength of the interference colors. The hexagonal flaky crystal form is common for hematite, but very unusual for magnetite, which suggests the magnetite crystals are pseudomorphs of reduced hematite. Heating fragments of the sunstone at 1000°C for 50h would turn some of the opaque inclusions into red transparent ones. Single-crystal X-ray diffraction data show that the magnetite crystals are spinel twinned, with lattice parameters deviating from cubic symmetry, suggesting strong internal strain introduced by the twin boundaries.

The host feldspar has a bulk composition of An₁Ab₁₅Or₈₃Cn₁, where the solvus of the alkali feldspar solid solution is at ~400°C. Thin film lamellae of albite up to 100μm long and 2μm thick are perfectly exsolved throughout the host feldspar crystal, producing a strong adularescence effect at a slightly different orientation from the aventurescence. The iron oxide inclusions would cut through these albite lamellae, indicating the flaky crystals were formed after the exsolution of albite thin films, at below 400°C. The albite lamellae appears perfectly pristine, with no signs of hydrothermal or deuteric alterations at all. Therefore, growth of hematite crystals due to Fe introduced by external fluid, as one of the proposed mechanism suggested, would be improbable. The flaky hematite crystals are most likely from the Fe previously dissolved in the feldspar lattice, which are expelled during the exsolution of albite and ordering of the crystal structure. The boundaries between the albite lamellae and the orthoclase matrix cannot be coherent due to the large difference in lattice parameters, and would increase the diffusion rate of Fe dramatically, which might be the reason for the exceptionally large hematite crystals compared to the sunstones found elsewhere in the world. The special relation between exsolution lamellae and iron oxide inclusions in the rainbow lattice sunstone is shedding some light on the enigmatic forming mechanism of the aventurine and red-clouded feldspars.