Ferro-ferri-holmquistite, square Li-2(Fe(3)2(+)Fe(2)(3+))Si8O22(OH)(2), Fe2+Fe3+ analogue of holmquistite, from the Iwagi islet, Ehime, Japan

Ferro-ferri-holmquistite (IMA2022-020), ideal formula square Li-2(Fe(3)2(+)Fe(2)(3+))Si8O22(OH)(2), was found in albitized granite from the Iwagi islet, Ehime, Japan. Ferro-ferri-holmquistite is a (Fe2+Fe3+)-Fe-C 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 D-meas. = 3.2 g cm(-3) and D-calc. = 3.317 g cm(-3), respectively. Ferro-ferri-holmquistite is optically biaxial (-), with alpha = 1.685, beta = 1.713 and gamma = 1.727, and is pleochroic, with X = pale blue similar to pale yellowish blue, Y = deep blue similar to brownish blue and Z = deep blue similar to deep bluish violet; X>Z> Y. The magnetic susceptibility is similar to the associated biotite. Ferro-ferri-holmquistite is insoluble in HC1, HNO3 and H2SO4. The empirical formula calculated on the basis of Sigma(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)(K0.01Na0.06)Sigma(0.07) (B)(Li1.95Na0.04Ca0.01)(Sigma 2.00)(C)(Fe2.822+Fe1.393+Al0.51Mg0.22Mn0.052+Ti0.01) (T)(Sigma 5.00) (Si-7.98 Al-0.02)(Sigma 8.00) 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) angstrom, b =17.9222(1) angstrom, c = 5.3123(1) angstrom and V = 1765.51(1) angstrom(3). Based on the refined site occupancies, the structural formula can be written as A Na-0.062(M4)(Li1.952Na0.048)(Sigma 2.000)(M1)(Fe1.7702+Mg0.230)(Sigma 2.000)(M2)(Fe(1.446)(3+)Fe(0.102)(2+)Al0(.452))(Sigma 2.000) (M3)(Fe0.8912+Mg0.109))(Sigma 1.000)(Si8O22)-Si-T(OH)(2) (Z =4). Three OH-stretching IR bands, centered at 3614, 3631 and 3644 cm(-1), are assigned to the local configuration M 1M 1 M3 = FeFeFe, MgFeFe (including FeMgFe and FeFeMg) and MgMgFe (including MgFeMg and FeMgMg), respectively, based on the IR studies of the orthorhombic Pnma amphiboles.