TY - JOUR

T1 - Electron microprobe technique for the determination of iron oxidation state in silicate glasses

AU - Zhang, Chao

AU - Almeev, Renat R.

AU - Hughes, Ery C.

AU - Borisov, Alexander A.

AU - Wolff, Paul Eric

AU - Höfer, Heidi E.

AU - Botcharnikov, Roman E.

AU - Koepke, Jürgen

N1 - Funding information: We explored the potential effect of Ti on the flank method on differentially absorbing FeL? and FeL? within silicate glasses, we explored it within the range of TiO2 content between 0 and 25 wt%. As listed in Supplemental1 Table 2, for silicate glasses with TiO2 contents lower than 15 wt%, no systematic correlation is observed between ?Fe2+ and TiO2 content. However, sample DAT32 with extremely high TiO2 (25.09 wt%) demonstrates high ?Fe2+ (Fig. 5b), suggesting that Ti is indeed able to influence the absorption of FeL? and/or FeL?, but only for silicate glasses with very high TiO2 contents (at least >15 wt%). Although Fialin et al. (2001) emphasized the potential effect of Cr and Ti on Fe L line emission and absorption, this problem is perhaps only crucial for Cr-and/or Ti-rich phases (e.g., chromite and ilmenite). The absorption effect of Ti should be extremely weak in silicate glasses with low Ti contents, as demonstrated by the data of Fialin et al. (2004) involving silicate glasses with 0–1.8 wt% TiO2. This assumption is supported by our results.

PY - 2018/9/25

Y1 - 2018/9/25

N2 - We present a new calibration for the determination of the iron oxidation state in silicate glasses by electron probe microanalysis (EPMA) with the "flank method." This method is based on the changes in both intensity and wavelength of the FeLα and FeLβ X-ray emission lines with iron oxidation state. The flank method utilizes the maximum difference for the FeLα and FeLβ spectra observed at the peak flanks between different standard materials, which quantitatively correlates with the Fe2+ content. Provided that this correlation is calibrated on reference materials, the Fe2+/ΣFe ratio can be determined for samples with known total Fe content. Two synthetic Fe-rich ferric and ferrous garnet end-members, i.e., andradite and almandine, were used to identify the FeLα and FeLβ flank method measuring positions that were then applied to the measurement of a variety of silicate glasses with known Fe2+/ΣFe ratio (ranging from 0.2 to 1.0). The measured intensity ratio of FeLβ over FeLα at these flank positions (Lβ/Lα) is a linear function of the Fe2+content (in wt%). A single linear trend can be established for both garnets and silicate glasses with 4-18 wt% FeOT (total iron expressed as FeO). In glasses with up to 18 wt% FeOT and 15 wt% TiO2, no systematic compositional (matrix) effects were observed. A possible influence of Ti on the Fe2+ determination has only been observed in one high-Ti glass with ∼25 wt% TiO2, a content that is not typical for natural terrestrial silicate melts. The accuracy of the Fe2+/ΣFe determination, which depends on both the Fe2+ content determined with the flank method and on the total Fe content, is estimated to be within ±0.1 for silicate glasses with FeOT > 5 wt% and within ±0.3 for silicate glasses with low FeOT ≤ 5 wt%. The application of the flank method on silicate glasses requires minimization of the EPMA beam damage that can be successfully achieved by continuous movement of the sample stage under the electron beam during analysis, e.g., with a speed of 2 μm/s.

AB - We present a new calibration for the determination of the iron oxidation state in silicate glasses by electron probe microanalysis (EPMA) with the "flank method." This method is based on the changes in both intensity and wavelength of the FeLα and FeLβ X-ray emission lines with iron oxidation state. The flank method utilizes the maximum difference for the FeLα and FeLβ spectra observed at the peak flanks between different standard materials, which quantitatively correlates with the Fe2+ content. Provided that this correlation is calibrated on reference materials, the Fe2+/ΣFe ratio can be determined for samples with known total Fe content. Two synthetic Fe-rich ferric and ferrous garnet end-members, i.e., andradite and almandine, were used to identify the FeLα and FeLβ flank method measuring positions that were then applied to the measurement of a variety of silicate glasses with known Fe2+/ΣFe ratio (ranging from 0.2 to 1.0). The measured intensity ratio of FeLβ over FeLα at these flank positions (Lβ/Lα) is a linear function of the Fe2+content (in wt%). A single linear trend can be established for both garnets and silicate glasses with 4-18 wt% FeOT (total iron expressed as FeO). In glasses with up to 18 wt% FeOT and 15 wt% TiO2, no systematic compositional (matrix) effects were observed. A possible influence of Ti on the Fe2+ determination has only been observed in one high-Ti glass with ∼25 wt% TiO2, a content that is not typical for natural terrestrial silicate melts. The accuracy of the Fe2+/ΣFe determination, which depends on both the Fe2+ content determined with the flank method and on the total Fe content, is estimated to be within ±0.1 for silicate glasses with FeOT > 5 wt% and within ±0.3 for silicate glasses with low FeOT ≤ 5 wt%. The application of the flank method on silicate glasses requires minimization of the EPMA beam damage that can be successfully achieved by continuous movement of the sample stage under the electron beam during analysis, e.g., with a speed of 2 μm/s.

KW - ferric-ferrous ratio

KW - flank method

KW - Microprobe

KW - pillow glasses

KW - redox state

KW - silicate glasses

UR - http://www.scopus.com/inward/record.url?scp=85053317595&partnerID=8YFLogxK

U2 - 10.2138/am-2018-6437

DO - 10.2138/am-2018-6437

M3 - Article

AN - SCOPUS:85053317595

VL - 103

SP - 1445

EP - 1454

JO - American Mineralogist

JF - American Mineralogist

SN - 0003-004X

IS - 9

ER -