Nurdini Nadya, Ilmi Moh Mualliful, Maryanti Evi, Setiawan Pindi, Kadja Grandprix Thomryes Marth
Division of Inorganic and Physical Chemistry, Institut Teknologi Bandung, Jl. Ganesha No. 10, Bandung 40132, Indonesia.
Department of Chemistry, Universitas Bengkulu, Jl. W.R. Supratman, Bengkulu 38371, Indonesia.
Heliyon. 2022 Aug 23;8(8):e10377. doi: 10.1016/j.heliyon.2022.e10377. eCollection 2022 Aug.
Since the prehistoric era, hematite has been known as a reddish color pigment on rock art, body paint, and decorating substances for objects discovered almost worldwide. Recently, studies about purple hematite used in prehistoric pigment have been done vigorously to investigate the origin of the purple pigment itself. These previous studies indicate that the differentiation of crystallinity, crystal size, morphology, and electronic structure can cause the color shift, resulting in purple hematite. In this study, we conducted a detailed study of the sintering temperature effects on the formation of hematite minerals. This study aims to reveal the structural, crystallography, and electronic transformation in hematite due to heating treatment at various temperatures. The hematite was synthesized using precipitation to imitate the primary method of hematite formation in nature. The sintering process was carried out with temperature variations from 600 °C to 1100 °C and then characterized by crystallographic and structural properties (XRD, Raman Spectroscopy, FTIR), particle size (TEM), as well as electronic properties (DRS, XANES). The crystallinity and particle size of hematite tend to increase along with higher sintering temperatures. Moreover, we noted that the octahedral distortion underwent an intensification with the increase in sintering temperature, which affected the electronic structure of hematite. Specifically, the → transition exhibited lower energy for hematite produced at a higher temperature. This induced a shift in the absorbed energy of the polychromatic light that led to a color shift within hematite, from red to purple. Our finding emphasizes the importance of electronic structure in explaining hematite pigment's color change rather than relying on simple reasons, such as particle size and crystallinity. In addition, this might strengthen the hypothesis that the prehistoric human created a purple hematite pigment through heating.
自史前时代起，赤铁矿就作为一种红色颜料出现在世界各地发现的岩石艺术、人体彩绘和物品装饰材料中。最近，关于史前颜料中使用的紫色赤铁矿的研究蓬勃开展，以探究紫色颜料本身的来源。此前的这些研究表明，结晶度、晶体尺寸、形态和电子结构的差异会导致颜色变化，从而产生紫色赤铁矿。在本研究中，我们对烧结温度对赤铁矿矿物形成的影响进行了详细研究。本研究旨在揭示在不同温度下进行热处理后赤铁矿的结构、晶体学和电子转变。通过沉淀法合成赤铁矿，以模拟自然界中赤铁矿形成的主要方式。烧结过程在600℃至1100℃的温度范围内进行，然后通过晶体学和结构性质（XRD、拉曼光谱、傅里叶变换红外光谱）、粒度（透射电子显微镜）以及电子性质（漫反射光谱、X射线吸收近边结构）进行表征。随着烧结温度的升高，赤铁矿的结晶度和粒度趋于增加。此外，我们注意到随着烧结温度的升高，八面体畸变加剧，这影响了赤铁矿的电子结构。具体而言，对于在较高温度下生成的赤铁矿， → 跃迁表现出较低的能量。这导致多色光吸收能量发生偏移，进而使赤铁矿内部发生颜色变化，从红色变为紫色。我们的发现强调了电子结构在解释赤铁矿颜料颜色变化方面的重要性，而不是依赖于诸如粒度和结晶度等简单原因。此外，这可能会加强史前人类通过加热创造紫色赤铁矿颜料的假说。
