Ferro-ferri-holmquistite, □ Li 2 (Fe 2+ 3 Fe 3+ 2 )Si 8 O 22 (OH) 2 , Fe 2+ Fe 3+ analogue of holmquistite, from the Iwagi islet, Ehime, Japan

Ferro-ferri-holmquistite (IMA2022-020), ideal formula □ Li 2 (Fe32+Fe23+)Si 8 O 22 (OH) 2 , was found in albitized granite from the Iwagi islet, Ehime, Japan. Ferro-ferri-holmquistite is a C Fe 2+ Fe 3+ analogue of holmquistite and belongs to the lithium-subgroup amphiboles. It commonly occurs as acicular aggregate and/or isolated crystals in quartz, albite and K-feldspar and is blue with a bluish-grey streak and a vitreous luster. It has a Mohs hardness of 5 1/2. Its cleavage is perfect on {210}. Measured and calculated densities are Dmeas.=3.2 g cm −3 and Dcalc.=3.317 g cm −3 , respectively. Ferro-ferri-holmquistite is optically biaxial (-), with α =1.685, β =1.713 and γ =1.727, and is pleochroic, with X = pale blue ∼ pale yellowish blue, Y = deep blue ∼ brownish blue and Z = deep blue ∼ deep bluish violet; X > Z > Y . The magnetic susceptibility is similar to the associated biotite. Ferro-ferri-holmquistite is insoluble in HCl, HNO 3 and H 2 SO 4 . The empirical formula calculated on the basis of Σ(C+T) = 13 on the results obtained by electron microprobe analyzer (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is A (K 0.01 Na 0.06 ) Σ0.07 B (Li 1.95 Na 0.04 Ca 0.01 ) Σ2.00 C (Fe2.822+Fe1.393+Al 0.51 Mg 0.22 Mn0.052+Ti 0.01 ) Σ5.00 T (Si 7.98 Al 0.02 ) Σ8.00 O 22 (OH) 1.94 F 0.06 . Structure refinement converged to R 1 = 4.22 %. The space group is orthorhombic Pnma , and the unit-cell parameters are a = 18.5437(2) Å, b = 17.9222(1) Å, c = 5.3123(1) Å and V = 1765.51(1) Å 3 . Based on the refined site occupancies, the structural formula can be written as A Na 0.062 M 4 (Li 1.952 Na 0.048 ) Σ2.000 M 1 (Fe1.7702+Mg 0.230 ) Σ2.000 M 2 (Fe1.4463+Fe0.1022+Al 0.452 ) Σ2.000 M 3 (Fe0.8912+Mg 0.109 ) Σ1.000 T Si 8 O 22 (OH) 2 ( Z = 4). Three OH-stretching IR bands, centered at 3614, 3631 and 3644 cm −1 , are assigned to the local configuration M 1 M 1 M 3= FeFeFe, MgFeFe (including FeMgFe and FeFeMg) and MgMgFe (including MgFeMg and FeMgMg), respectively, based on the IR studies of the orthorhombic Pnma amphiboles.

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   Crystal structure refinement with SHELXL · 2015

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