Museum specimens of lacroixite from the type locality (near Greifenstein, German Democratic Republic) were restudied. The mineral occurs in druses of a lithium-bearing granite in close association with viitaniemiite. Being similar in appearance it is difficult to distinguish between these two minerals, and therefore physical properties measured from viitaniemiite have been cited for lacroixite in the literature.

Lacroixite, NaAlFPO, is monoclinic, space group C2/c, a = 6.414, b = 8.207, c = 6.885Å, β = 115.47° and Z = 4. The strongest peaks in the X-ray diffractogram are 3.155(100)(112), 2.895(98)(200), 2.476(57)(022,130), 4.73(51)(110), 2.166(43)(131), 4.627(32)(111). The specific gravity is 3.29 and hardness 4–5.

The mineral is colorless or gray, biaxial with α = 1.546, β = 1.563, γ = 1.580, −2V (meas.) = 89°, −2V (calc.) = 89.1° and r < ν. The characteristic bands in the IR spectrum are 3388, 2924, 2854,1075, 567, 474, 396, 325 and 304 cm−1.

The structure of lacroixite has been solved by Patterson and Fourier methods. After the least-square refinement the final R is 0.034 for 468 (2σ observed) reflections collected on a computer-controlled four-circle diffractometer. The structure contains chains of cornersharing AlO4F2 octahedra cross-linked by PO4 tetrahedra. The resulting large cavities in the framework are filled with Na atoms in 7-fold coordination. The octahedral chains, which are parallel to the c-axis, are kinked, the Al–F–Al angle being 142.7°. Bond distance averages are [7]Na–(O,F) 2.356, [6]Al–(O,F) 1.864, [4]P–O 1.543Å.

Lacroixite is the phosphate analog of durangite, NaAlFAsO4; both are isostructural with high temperature (A2/a) titanite, CaTiOSiO4.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.