近来, 一种产自老挝的新型印章石（俗称“老挝石”）涌进国内市场, 对我国印章石市场造成一定影响, “老挝石”的研究尚处于起步阶段, 对其颜色成因的研究更为缺乏。 采用漫反射光谱（DRS）结合X射线粉晶衍射（XRD）、 红外光谱（FTIR）、 X射线荧光光谱（EDXRF）等测试对红色“老挝石”的矿物成分和致色机理进行深入研究。 结果显示, “老挝石”的主要矿物成分为地开石, 并含有少量高岭石, 化学成分中的Fe含量和“老挝石”红色色调呈正相关关系, 即颜色越深Fe的含量越高。 铁质矿物呈微晶集合体浸染分布于地开石的颗粒间, 由于其含量低、 粒度细小, 常规的微区测试方法无法确认其种属。 相比之下, 漫反射光谱对微晶铁矿物的鉴定十分有效, 对可见光波段漫反射光谱处理得到导数等, 在土壤沉积物中已经被用来定量测定针铁矿和赤铁矿。 该研究中“老挝石”基体与土壤沉积物均为粘土矿物, 可以用漫反射光谱来判定“老挝石”中铁矿物种属。 漫反射光谱一阶导数法显示, 其谱峰位于565～570 nm, 由此确认铁矿物的种属为赤铁矿。 微晶赤铁矿分布于“老挝石”矿物颗粒间, 使样品产生红色, 赤铁矿含量越高, 红色调越深。

In recent years, a kind of new seal stone called “Laowo Stone” from Laos has entered the Chinese seal stone market and it has great impact on the market. However the coloration mechanism is unclear. In order to study the coloration mechanism of “Laowo Stone”, red samples were investigated with VIS DRS techniques together with XRD, FTIR and EDXRF. It was found that: the main mineral composition of “Laowo Stone” is dickite, together with small amounts of kaolinite. EDXRF data show that the concentration of the red color has a good relationship with Fe element; the spot with deeper color has higher Fe content. Fe occurs as iron mineral distributed in the dickite particles. The grain size of the muddy iron minerals is extremely small and the content is low, regular micro area study cannot identify the iron mineral. Then we study the samples with diffuse reflectance spectroscopy (DRS) which is sensitive to iron minerals. The derivative curves of DRS are potentially useful for quantitatively determining hematite and goethite concentration in soils. The main mineral composition of “Laowo Stone” is clay mineral, which is the same as soils. So DRS is used to identify the iron minerals. The first order derivative spectrum has the 565~570 nm peak and the iron mineral was identified to be hematite. Our results confirmed that the red coloration of “Laowo Stone” is caused by tiny hematite crystals distributed within the grain boundaries of dickite particles.