Publication Date:
2016-03-15
Description:
A second locality of the milarite-group mineral brannockite (ideally KLi 3 Sn 2 Si 12 O 30 ) has been discovered at Liberty Bell/Washington Pass in the peralkaline granite of the Golden Horn Batholith, Okanogan County, Washington, USA as millimeter sized euhedral bladed clear crystals in a single miarolitic cavity. The Washington Pass brannockite shows micro-scale compositional zoning involving tetravalent A-site Sn 4+ , Zr 4+ , Ti 4+ , and Hf 4+ substitution and the coupled substitution A (X 4+ ) + B A (Fe=Al) 3+ + B (Na; K) + . In contrast, brannockite analyzed from the type locality in the late hydrothermal zone of the Li-Sn-rich pegmatite of the Foote Mine, Kings Mountain, North Carolina, USA exhibits near end-member composition with only minor Na + , Al 3+ , and Ti 4+ . Single-crystal X-ray diffraction of Washington Pass brannockite confirms the space group P 6/ mcc with unit-cell parameters a = 10.014(2) Å, c = 14.268(5) Å, and V = 1239.16(8) Å 3 . Washington Pass brannockite co-exists with its Zr-analogues, both the milarite-group sogdianite and the related tuhualite-group mineral zektzerite. Selected samples of both minerals show Sn as minor element. The sogdianite contains unusually low Na and Ti compared to most other localities and has near end-member composition. The results emphasize the high compatibility of the milarite structure with different heterovalent cations, associated with the cation-size dependent coupled distortion of the octahedral A and tetrahedral T2 sites.
Print ISSN:
0935-1221
Electronic ISSN:
1617-4011
Topics:
Geosciences
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