Silicon-substituted magnetite and accompanying iron oxides and hydroxides from the Kumano mine, Yamaguchi Prefecture, Japan: Reexamination of the so-called maghemite (γ-Fe2O3)

Abstract

Iron oxides and hydroxides found in the so called maghemite bearing iron oxide ores from the Kumano mine, Yamaguchi Prefecture, Japan have been investigated using polarizing microscopy, reflecting microscopy, powder X ray diffractometry (XRD), and electron probe microanalysis (EPMA) to reevaluate the previous study carried out by Shibuya (1958).The results show that the iron oxide described as maghemite by Shibuya is actually magnetite, showing a compositional zoning of Si with trace amounts of Al and Ca, and that only hematite and goethite are found as alteration products. The compositional zoning of the magnetite may indicate that the silicon bearing magnetite was formed during reducing conditions.The a lattice parameter of a = 8.376(1) Å for the silicon bearing magnetite is shorter than that of normal magnetite. This contraction in the crystal lattice is probably caused by a coupled substitution: Si 4+

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“…Negative correlations between Fe 3+ and Si 4+ and between Fe 3+ and Al 3+ were also reported in several skarn-type Fe deposits [16,17].…”

“…Several compositional trends between the components measured (FeO, Fe2O3, SiO2, Al2O3, CaO + MgO) were observed; Fe2O3 clearly shows a negative correlation with SiO2 and Al2O3 (Figure 6a,b), whereas FeO and CaO + MgO displays a positive correlation with SiO2 (Figure 6c,d). Negative correlations between Fe 3+ and Si 4+ and between Fe 3+ and Al 3+ were also reported in several skarn-type Fe deposits [16,17].…”

“…A total of 94 quantitative point analyses were carried out for the two samples, and their representative analytical results are listed in Table 1. Due to the Fe-calculation as FeO (II) in the measurements, the total weight of all the oxide components was usually around ~93%, which led to reasonable recalculated values [7,8,16].…”

“…Shimazaki [8] suggests that silician magnetite is rather a product of solid solution with a maghemite-like structure, because the Mössbauer data for a similar silician magnetite of a Japanese skarn-type deposit indicate the presence of ferrous cation vacancy in the octahedral site. Ohkawa et al [16], using X-ray powder diffraction analyses, revealed that the silician magnetite of another Japanese skarn deposit is not maghemite, but magnetite. Considering all these results, it is suitably defined that silician magnetite is an "impure" element-bearing magnetite.…”

“…Silician magnetite occurs in various types of ore deposits with high-to low-temperature conditions, as summarized by Huberty et al [23] and Xu et al [25], including porphyry-type [26], skarn-type [6][7][8][16][17][18]27,28], banded iron formation (BIF) [23,25], and volcanic-related massive sulfide [29]. The highest value of SiO 2 content in magnetite (~8.9 wt %) is recognized in a porphyry-type deposit [26].…”