Crystal chemistry of brannockite, KLi3Sn2Si12O30, from a new occurrence in the Golden Horn Batholith, Washington State, USA

A second locality of the milarite-group mineral brannockite (ideally KLi₃Sn₂Si₁₂O₃₀) 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 Sn⁴⁺, Zr⁴⁺, Ti⁴⁺, and Hf⁴⁺ substitution and the coupled substitution ^A(X⁴⁺) + ^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⁺, Al³⁺, and Ti⁴⁺. Single-crystal X-ray diffraction of Washington Pass brannockite confirms the space group P6/mcc with unit-cell parameters a = 10.014(2) Å, c = 14.268(5) Å, and V = 1239.16(8) ų. 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.