Mineralogical and Spectral Characteristics of Purple Boulder Opal from Australia
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摘要: 紫色砾背蛋白石是市场上较为新颖的宝石品种，学界目前对它的研究仅限于矿物组成及体色成因，并未对其化学成分、谱学特征、变彩成因进行较为深入的分析。对澳大利亚紫色砾背蛋白石的矿物学及谱学特征进行综合研究，结果表明，紫色砾背蛋白石的主要矿物组成为非晶态蛋白石，并含有少量的α-方石英; EMPA分析结果显示，蛋白石基质的主要成分为SiO2，达96%以上，紫外-可见光谱表征为407，480 nm附近存在吸收强度较强的吸收峰，522 nm之后全吸收; LA-ICP-MS和紫外-可见吸收光谱的测试结果显示，紫色砾背蛋白石凝结于铁质岩石的环境中，其蛋白石基质的紫色调与Fe含量存在一定的正相关性; SEM测试结果显示，紫色砾背蛋白石变彩区域的SiO2球粒直径范围为182~240 nm，球粒周围八面体空隙的大小约在63~150 nm范围，只允许波长为400~450 nm的紫色光通过，形成单彩的紫色蛋白石。Abstract: Purple boulder opal is a relatively new gem variety in the market. At present, the research that related to purple boulder opal is limited to the mineral components and the genesis of its colour. There is no in-depth analysis on the chemical compositions, spectral characteristics and causes of play-of-colour effect. The mineralogical and spectral characteristics of purple boulder opal from Australia are comprehensively studied in this paper. The results showed that the main mineral component of purple boulder opal is amorphous opal, and it contains a small amount of α-cristobalite. The results of EMPA analysis showed that the main component of opal matrix is SiO2, which is more than 96%. The UV-Vis spectrum showed that there are absorption peaks of strong absorption intensity at 407 nm and 480 nm, and totally absorbed above 522 nm. Based on the results of LA-ICP-MS and UV-Vis absorption spectra, the saturation of the purple colour of purple boulder opal is positively correlated with Fe content. SEM results showed that the diameter of SiO2 spheres in the play-of-colour effect area of the purple boulder opal is about 182-240 nm, and the octahedral space around the sphere is about 63-150 nm. Only violet light with wavelength of 400-450 nm can pass through the sphere to form the pure purple opal.
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图 2 大溪地黑珍珠不同体色紫外光谱Figure 2. UV-Vis spectra of different body colour of black pearls from Tahiti -
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图 1 紫色砾样蛋白石样品的基本特征
a.样品OP-A蛋白石基质; b.样品OP-B蛋白石基质; c.样品OP-C蛋白石基质; d.样品OP-C围岩部分
Figure 1. Basic characteristics of the purple boulder opal samples
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图 4 紫色砾背蛋白石样品OP-C的差热分析曲线
Figure 4. Differential thermal analysis curve of the purple boulder opal sample OP-C
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图 5 紫色砾背蛋白石样品的紫外-可见吸收光谱
Figure 5. UV-Vis absorption spectra of the purple boulder opal samples
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图 6 样品OP-A紫色变彩区域的扫描电子显微镜图像
Figure 6. SEM images of purple play-of-colour area of sample OP-A
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图 7 样品OP-C紫色变彩区域的扫描电子显微镜图像
Figure 7. SEM images of purple play-of-colour area of sample OP-C
表 1 紫色砾背蛋白石样品的化学成分测试结果
Table 1 Chemical compositions of the purple boulder opal samples
wB/% 样品号 SiO2 MgO Al2O3 K2O CaO FeO MnO OP-A 97.85 1.242 0.416 0.121 0.122 0.076 0.001 OP-B 96.19 1.027 0.402 0.155 0.154 1.910 0.020 OP-C 98.31 0.880 0.391 0.135 0.168 0.012 0.002 OP-B(围岩) 39.09 1.550 0.795 0.379 0.252 57.093 0.502 
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表 2 紫色砾背蛋白石样品的微量元素含量特征
Table 2 Content characteristics of trace elements in purple boulder opal samples
/10-6 样品号 Sc Ti V Cr Mn Fe Co Ni Cu Rb Sr Zr Ba Pb OP-A 6.194 4.359 0.162 0.266 4.913 39.687 0.109 0.042 1.179 9.026 15.522 1.670 37.734 0.284 OP-B 5.926 4.656 0.042 0.167 4.952 51.870 0.111 0.953 2.039 11.389 11.014 7.336 42.303 2.901 OP-C 6.504 7.422 0.013 - 7.070 50.379 0.200 1.168 1.735 9.993 23.758 12.520 84.035 0.213 OP-C 5.551 7.549 - 1.185 6.338 56.507 0.263 0.335 1.247 11.273 23.422 11.483 87.770 0.179 OP-C(变彩) 7.275 8.390 0.243 3.933 7.533 69.146 0.091 0.281 2.178 11.322 30.552 17.031 107.887 0.083 OP-B(围岩) 7.046 460.334 57.052 16.220 5 962.683 368 448.060 13.713 37.186 63.058 4.588 36.215 10.534 17.961 6.798
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