Mn2+和Fe3+的致色作用:来自意大利白垩纪远洋红色灰岩的启示

本文首先详细研究了含Mn2 和Fe3 的致色矿物菱锰矿、鲕状赤铁矿、云母赤铁矿和镜铁矿的可见光吸收光谱及其一阶导数谱,鲕状赤铁矿、云母赤铁矿和镜铁矿的可见光吸收一阶导数谱的红光区的吸收谷的位置的变化表明随赤铁矿结晶度的降低,吸收谷由586.4nm移至577.4nm,而菱锰矿展示出了Mn2 的因电子跃迁产生的四个典型吸收及其一阶导数谱上577nm的吸收谷的强度比赤铁矿相应谷的强度低一个数量级,表明其电子跃迁的致色机理与赤铁矿的染色机理完全不同。依据赤铁矿的结晶度和鲕状赤铁矿与大洋红层中赤铁矿的沉积成因相似的原则遴选出鲕状赤铁矿为含Fe3 致色矿物,依据菱锰矿是和方解石具有相似结构且为红色的原则选取菱锰矿为含锰致色矿物,并佐以化学纯氧化镁为基体配制了一系列的含菱锰矿、鲕状赤铁矿和菱锰矿及鲕状赤铁矿的两相或三相混合物。详细研究了三类混合物的可见光吸收光谱的一阶导数谱,发现含菱锰矿和含鲕状赤铁矿在573nm附近均存在一吸收谷,鲕状赤铁矿的重量分数低至0.05%时仍可见一明显吸收谷且该吸收谷移至565nm附近,菱锰矿在低至0.50%时也可见这一吸收且在低至0.11%时仍可显示出菱锰矿的信息,其575nm的吸收峰未见偏移;混合物可见光一阶导数吸收谱上鲕状赤铁矿的575nm附近的吸收谷的强度随鲕状赤铁矿的重量分数的升高而增强,而所有的配制混合物中该吸收谷的位置低于577.4nm的事实也表明为使致色矿物和氧化镁混合均匀的研磨降低了赤铁矿的结晶度。本研究表明Mn2 的电子跃迁激发和细小、结晶差的赤铁矿的染色共同造就了意大利白垩纪远洋红色灰岩的红色。

The Color-Causing Mechanism of Mn2+ and Fe3+: Evidence from the Italian Cretaceous Pelagic Red Limestones

It has been widely accepted that the color of red sediments results from iron oxides. Thus, the dyeing of hematite is also thought to be reason of reddish limestone in the Cretaceous Ocean Red Beds (CORB). However, as we know that manganese minerals such as rhodochrosite, spessartite, rhodonite and manganocalcite etc., are all red, pink or in varieties of the red. The ESR study showed that the valence of manganese is 2, the content is higher than that of iron cations, and the cations are placed in octahedral sites of the structure. To reveal the origin of the color, three ferric oxides, oolitic hematite, mica hematite and specularite, and a rhodochrosite were selected and measured by a Visible Absorption Spectrometer. The comparative Visual Spectra study of four minerals showed that one broad absorption shoulder was present on the spectra of three ferrous oxides and four absorption peaks at 362nm, 408nm, 445nm and 547nm appeared on the spectra of rhodochrosite. The peak at 577nm in the 1st derivative visible spectra was ascertained as the characteristic peak of the hematite, and used to identify the existence of hematite in sediments or rocks, and believed to be the color origin. The peak could be easily identified at all ferric oxides but the position shifted from 586 nm to 577nm from specularite, mica hematite to oolitic hematite. This might imply that the peak may shift to lower wavelength as the crystallized index decreased. Furthermore, this peak was also present in the derivative spectra of rhodochrosite. This might indicate that the ferric oxide was not the only mineral which could result in the red in color. The intensity of peak at or near 577nm was about 0.0065, 0.0006, 0.002 in oolitic hematite, mica hematite or specularite, rhodochrosite, respectively. This may indicated that the capability of dyeing of these four minerals followed with the order of oolitic hematite, rhodochrosite, mica hematite or specularite. According to the origin of oolitic hematite which was similar to the origin of hematite in red limestone at their sedimentary origin and the rhodochrosite was similar to manganese calcite at its structure and color, a series of biphase or triphase mixtures with different fractions in the range of 1.0% to 0.05% of oolitic hematite and rhodochrosite were measured on the visible spectrometer. The study showed that the peak at 577 nm was stable, well-resolved and the intensity decreased with the decrease of the fractions in spectra of oolitic hematite biphase. However, the peak at 577 nm and at 411 nm in the 1st derivative spectra of rhodochrosite bi-phase could be identified as lower as to 0.5% and 0.11%, respectively. The 1st derivative spectra of the oolitic hematite and periclase mixtures showed that the site of the absorption was departure from 577nm in oolitic hematite to 560nm in mixtures. This showed that the deep ground whose aim was to uniformly mix the sample resulted in the forming of amorphous hematite. It was the worse crystalline and ultrafine particles of hematite and Mn2 that resulted in the origin of red of limestone.