Abstract Iron electronic transitions produce intense colors in minerals with moderate to high Fe content, which phenomenon allows using color and visible spectroscopy to identify and quantify Fe-bearing minerals. This approach is especially useful for Mars investigation due to the limited analytical instruments available at Mars. The challenges are: (1) overlap between Fe electronic absorption bands of Fe-bearing minerals, (2) band wavelengths are modified by chemical and structural variables, and (3) particle size effects can modify mineral color. The magnitude of these difficulties, however, depends on the mineral mixture investigated. In this study, we further probe the potential of visible-range data for investigation of Fe-minerals. The samples were mineral mixtures from the Riotinto area (SW Spain) that resulted from acidic alteration and contained Fe in oxy-hydroxides, hydroxysulfates and phyllosilicates. Samples were studied using visible/near-infrared spectroscopy, X-ray diffraction and thermogravimetry. The wavelength range 320–1000 nm was investigated using spectral second derivatives and CIELAB color parameters. Hematite was readily detected, below X-ray diffraction detection limits, from the band at 520–560 nm. Jarosite was readily identified by the combined presence of bands at ~390 and ~ 440 nm. Thermogravimetric and CIELAB color analyses indicated goethite as the most common Fe-rich mineral in the samples. Correlations between CIELAB color parameters were controlled by goethite, where jarosite-rich specimens aligned with the correlations but some samples containing hematite plotted outside them. Quantification of goethite was attempted as calibrated against thermogravimetric data. Calibration was not possible below ~10 wt% goethite. Above this value, results from spectral second derivatives allowed quantification up to the maximum goethite content of ~35 wt% (linear correlation with R2 = 0.89). Goethite quantification with CIELAB color parameters was hindered by hematite. In samples with very low or no hematite, goethite quantification was possible with parameters L*10 (R2 = 0.83) and hab,10 (R2 = 0.65), also up to ~35 wt% goethite. Consequently, if accurate calibration is possible and depending on mineral mixture components, spectroscopic and CIELAB color parameters should allow goethite quantification in a wide concentration range.

中文翻译：

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酸化环境中多矿物混合物中铁矿物的颜色分析与检测

摘要 铁电子跃迁在中到高 Fe 含量的矿物中产生强烈的颜色，这种现象允许使用颜色和可见光谱来识别和量化含 Fe 矿物。由于火星上可用的分析仪器有限，这种方法对火星调查特别有用。挑战是：(1) 含铁矿物的 Fe 电子吸收带之间的重叠，(2) 带波长受化学和结构变量的影响，以及 (3) 粒度效应可以改变矿物颜色。然而，这些困难的程度取决于所研究的矿物混合物。在这项研究中，我们进一步探讨了可见范围数据在铁矿物研究中的潜力。样品是来自 Riotinto 地区（西班牙西南部）的矿物混合物，由酸性蚀变产生，在羟基氧化物、羟基硫酸盐和页硅酸盐中含有 Fe。使用可见光/近红外光谱、X 射线衍射和热重法研究样品。使用光谱二阶导数和 CIELAB 颜色参数研究了 320-1000 nm 的波长范围。在 520-560 nm 波段，赤铁矿很容易被检测到，低于 X 射线衍射检测限。通过在~390 和~440 nm 处结合存在的带，很容易识别黄钾铁矾。热重分析和 CIELAB 颜色分析表明针铁矿是样品中最常见的富铁矿物。CIELAB 颜色参数之间的相关性由针铁矿控制，其中富含黄钾铁矾的标本与相关性一致，但一些含有赤铁矿的样品绘制在它们之外。针铁矿的量化尝试根据热重数据进行校准。低于约 10 wt% 的针铁矿无法进行校准。高于此值，光谱二阶导数的结果允许量化高达 ~35 wt% 的最大针铁矿含量（与 R2 = 0.89 的线性相关性）。赤铁矿阻碍了使用 CIELAB 颜色参数的针铁矿量化。在赤铁矿含量极低或不含赤铁矿的样品中，针铁矿量化可以通过参数 L*10 (R2 = 0.83) 和 hab,10 (R2 = 0.65) 进行，针铁矿的重量百分比也高达 ~35%。因此，如果可以根据矿物混合物成分进行准确校准，