Crystal chemistry of brannockite, KLi3Sn2 Si12O30, from a new occurrence in the Golden Horn Batholith, Washington State, USA

Abstract

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 microscale compositional zoning involving tetravalent A-site Sn 4þ , Zr 4þ , Ti 4þ , and Hf 4þ substitution and the coupled substitution A X 4þ À Á þ B & , A Fe=Al ð Þ 3þ þ B Na; K ð Þ þ. 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 þ , Al 3þ , and Ti 4þ. Single-crystal X-ray diffraction of Washington Pass brannockite confirms the space group P6/mcc with unit-cell parameters a ¼ 10.014(2) Å , c ¼ 14.268(5) Å , and V ¼ 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.