Revisiting Rainbow Lattice Sunstone from the Harts Range, Australia

Jia, Litao; Shen, Andy H.; Zhang, Zhiqing; Wang, Chengsi; Tian, Si

doi:10.15506/jog.2018.36.1.44

Cited by 1 publication

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“…1a) (Koivula and Gunter, 1989). Magnetite and hematite have been recently identified as the inclusions creating the "rainbow-lattice" effect in RLS (Liu et al, 2018). A strong iridescence (adularescence) effect is also observed in RLS (Fig.…”

Section: Introductionmentioning

confidence: 88%

“…Some hematite inclusions with a similar appearance as in the RLS have been found in Tanzania (Koivula and Tannous, 2003) and North Carolina, USA (Challener et al, 2017). The morphology of the iron oxide inclusion seems to be correlated with the composition and structure of the host feldspar, with acicular inclusions found only in intermediate to calcic plagioclase (Armbrustmacher and Banks, 1974;Nienaber-Roberts, 1986;Sobolev, 1990;Wenk et al, 2011;Ageeva et al, 2020;Jin et al, 2021;Bian et al, 2021) and flaky inclusions mostly in sodic plagioclase and alkali feldspar (Boone, 1969;Copley and Gay, 1979;Challener et al, 2017;Liu et al, 2018).…”

Section: Formation Of the Magnetite Filmmentioning

confidence: 99%

“…1b) at a slightly different angle from the aventurescence, indicating the presence of submicron, strain-controlled exsolution lamellae (cryptoperthite) in the feldspar crystal. The high potassian composition (Liu et al, 2018) of RLS is very rare, if not unique, for iridescent alkali feldspar because perthites with more potassian bulk compositions (more than Or 80 ) are almost always subject to hy-drothermal or deuteric alteration (Parsons and Brown, 1991), which dramatically coarsens the lamellae texture and erases any iridescence effect. This means the RLS is not only a valuable gemstone for its unusual appearance but also a very special mineral specimen with extraordinary thermal and chemical histories.…”

Section: Introductionmentioning

confidence: 99%

“…The compositions of L 2 (oligoclase spindle), L 3 (albite film) and M 3 (orthoclase matrix) are plotted on the ternary diagram, along with M 2 (average of L 3 and M 3 ) and the bulk composition (average of L 2 , L 3 and M 3 ) of the RLS. The composition of Or 96 reported byLiu et al (2018) has to be incorrect as it places the RLS on the solvus far below the closure temperature for the exsolution process.…”

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confidence: 99%

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Iron oxide inclusions and exsolution textures of rainbow lattice sunstone

2022

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Abstract. Iron oxide inclusions and exsolution lamellae in rainbow lattice sunstone (RLS) from Harts Range, Australia, are examined using optical and electron microscopy and single-crystal X-ray diffraction (SC-XRD). Laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) analyses show a bulk composition of An1.4Ab14.8Or83.0Cn0.8 with < 200 ppmw (parts per million weight) of Fe. Two stages of exsolution can be identified in RLS from the bimodal distribution in the size and shape of the exsolution lamellae. Micron-scaled Albite-twinned oligoclase spindles (An27Ab72Or1) first exsolved at ∼ 650 ∘C were followed by nanoscaled Pericline-twinned albite films (∼ Ab100) below 500 ∘C that create adularescence. The albite films inherited and preserved the monoclinic tetrahedral framework of the orthoclase matrix (An0.3Ab11.5Or87.3Cn0.9) as further ordering was completely inhibited by coherent-interface strain after exsolution. All the exsolution lamellae are pristine and strain-controlled with no signs of any deuteric or hydrothermal alteration, indicating the iron in the magnetite inclusions was not introduced by an external fluid. The magnetite inclusions nucleated around the same time as the exsolution of oligoclase spindles likely due to the reduction of Fe3+ to Fe2+ in the feldspar lattice. Magnetite films following the specific crystallographic orientation relationship (COR) of {111}Mt∥{100}Or and 11‾0Mt∥001Or grew to extraordinarily large sizes due to the near perfect lattice match at the interface with the orthoclase host. Some thinner magnetite films were oxidized into hematite during weathering of the host rock. RLS reveals a new mechanism for the formation of the flaky hematite inclusions in feldspars, which may explain the enigmatic origin of aventurescence observed in many other sunstones and red-clouded feldspars.

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Section: Introductionmentioning

confidence: 88%