Fe2+-rich ferro-holmquistite from Japan: structural implications for the holmquistite group
Year: 2026
Type: article
Abstract: Ferro-holmquistite with high Fe2+ content, which was previously predicted to be unlikely to occur, has been identified from a dyke composed of quartz and microcline in limestone at Mt. Hayamadake, Tamura City, Fukushima Prefecture, Japan. The specimen was collected in the 1980s and had been preserved in a private collection as an unnamed amphibole species; however, its name and features have now been revealed as ferro-holmquistite. The empirical formula, estimated on the basis of the constraints of Si = 8 is (□0.93Na0.07)Σ1(Li1.97Mg0.03)Σ2(Fe2+2.62Mg0.20Mn0.18Al1.75Fe3+0.25)Σ5Si8O22.05(OH1.96F0.04)Σ2 for the platy-crystal sample. The unit-cell parameters obtained from single-crystal X-ray diffraction measurements are a = 18.4080(3) Å, b = 17.7942(3) Å, c = 5.29463(11) Å, and V = 1734.29(6) Å3, with placement in the orthorhombic Pnma space group. Comparison of the single-crystal X-ray diffraction results with those for the holmquistite group reveals that the M1 and M3 octahedra expand with increasing Fe2+ content. In particular, the M3 octahedron exhibits marked distortion and anisotropic elongation, most notably along the b-axis. Conversely, an increase in the Fe3+ content enlarges and distorts the M2 octahedron, while mitigating deformation at the M1 and M3 sites. The tetrahedral chain demonstrates structural flexibility in response to these changes, either by adjustment of the tetrahedral volume and/or alteration in the degree of chain kinking. Through these changes, the low bond valence sums for O4A and O4B, which were once considered the starting point of structural limitation, remain consistently low. As a result, the previously held notion that geometric constraints inhibit the formation of Fe2+- and/or Fe3+-rich members should be reconsidered.
DOI: 10.2465/jmps.251128b
Institutions: The University of Tokyo, Yamaguchi University, University of Hyogo
Language: English
Subfield: Geochemistry and Petrology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: diamond
Award: G1745456642
OpenAlex ID: W7154028948