近红外光谱对岩矿表面黏土矿物覆层的响应

Response of near-infrared spectra to clay coatings on mineral surface

近红外光谱(NIR)技术具有测试快速、准确和对样品无接触、无损害等诸多优点,因而被广泛应用于材料科学、医学和地球科学等众多领域。在地质学和行星探测领域,近红外光谱常用来原地和遥感识别星球表面矿物组成、分布及相对含量等信息。近红外光谱对含有羟基和水的矿物,特别是黏土矿物有更强的响应,而黏土矿物主要的生成条件和矿物物化性质使其常常以岩石矿物表面覆层形式广泛存在于地表环境中。在实际测试过程中,覆层形式存在的黏土矿物极有可能会对探测目标区的岩石矿物含量和赋存状态等信息产生干扰,其干扰程度和机制尚不十分清楚。为此,本研究针对性地考察了黏土矿物覆层对矿物表面近红外光谱测试的影响。选用绿脱石、绿泥石、高岭石和伊利石4种代表性黏土矿物,覆层的厚度分别设计约为30、10、2和1μm;基体矿物选取角闪石、钾长石和石英为代表。实验结果表明,在位于2100~2370 nm处的近红外光谱吸收波段,不同厚度的黏土矿物覆层均会不同程度地影响基体矿物的反射光谱特征,导致在遥感探测过程中无法准确地获取其光谱信息,且随着覆层厚度增加,基体矿物信息被湮没的程度加剧。与富铁黏土矿物(绿脱石和绿泥石)相比,富铝黏土矿物(高岭石和伊利石)在2100~2370nm波段更易被近红外光谱识别。当因覆层厚度较小(<30μm)而导致黏土矿物的结构羟基特征吸收峰的相对深度与矿物含量相关性较低时,其特征吸收峰的二阶导数强度值可更好地指示覆层矿物的相对含量。本研究所取得的结论可为近红外光谱在矿产勘查和火星遥感探测等领域的应用提供重要参考。

Near-infrared(NIR) reflectance spectroscopy has been widely used in the fields of materials science,medicine, and earth science owing to its fast, accurate, and nondestructive testing. In the fields of geology and planetary exploration, NIR is commonly used to identify minerals that contain hydroxyl groups and water,especially clay minerals. However, NIR spectra are readily affected by many ambient factors, among which clay coating on rock/mineral surfaces is a common one. The presence of a clay coating may interfere with the determination of the content and occurrence of target minerals deduced from NIR spectra. To evaluate the effects of clay coating on NIR measurements, we prepared complexes by covering substrate minerals with clay coatings of varying thickness(30, 10, 2 and 1 μm). The clay minerals that were used as coating components were nontronite, chlorite, kaolinite, and illite, while the substrate minerals included hornblende, K-feldspar, and quartz.NIR spectral results showed that the thickness of clay coatings have a prominent effect on the absorptions of substrate minerals in the range from 2100 to 2370 nm. Compared to iron-rich clay minerals(nontronite and chlorite), aluminum-rich clay minerals(kaolinite and illite) are easier to identify using the characteristic absorptions in the NIR spectra. Compared with the relative depth of the structural OH absorption bands, their corresponding deriv2 values have a higher positive relationship with the clay mineral content in the samples when the coating thickness is less than 30 μm. These findings are useful for the application of NIR techniques in the fields of mineral prospecting and Martian remote sensing exploration.