Ferro-ferri-holmquistite, <i>□</i>Li₂(Fe²⁺₃Fe³⁺₂)Si₈O₂₂(OH)₂, Fe²⁺Fe³⁺ analogue of holmquistite, from the Iwagi islet, Ehime, Japan

Abstract

Ferro-ferri-holmquistite (IMA2022-020), ideal formula □Li2(Fe32+Fe23+)Si8O22(OH)2, was found in albitized granite from the Iwagi islet, Ehime, Japan. Ferro-ferri-holmquistite is a CFe2+Fe3+ 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, HNO3 and H2SO4. 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(K0.01Na0.06)Σ0.07B(Li1.95Na0.04Ca0.01)Σ2.00C(Fe2.822+Fe1.393+Al0.51Mg0.22Mn0.052+Ti0.01)Σ5.00T(Si7.98Al0.02)Σ8.00O22(OH)1.94F0.06. Structure refinement converged to R1= 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) Å3. Based on the refined site occupancies, the structural formula can be written as ANa0.062M4(Li1.952Na0.048)Σ2.000M1(Fe1.7702+Mg0.230)Σ2.000M2(Fe1.4463+Fe0.1022+Al0.452)Σ2.000M3(Fe0.8912+Mg0.109)Σ1.000TSi8O22(OH)2 (Z= 4). Three OH-stretching IR bands, centered at 3614, 3631 and 3644 cm−1, are assigned to the local configuration M1M1M3= FeFeFe, MgFeFe (including FeMgFe and FeFeMg) and MgMgFe (including MgFeMg and FeMgMg), respectively, based on the IR studies of the orthorhombic Pnma amphiboles.