蛇纹石质“卡瓦石”与软玉籽料对比研究

Comparison Research between Serpentine “Cava Stone”and Nephrite Pebble

软玉市场近期出现的一种名为“卡瓦石”的仿软玉籽料,其外观特征与软玉籽料非常相似.采用常 规宝石学测试手段,并结合红外光谱、紫外-可见吸收光谱和激光拉曼光谱等现代测试方法,对“卡瓦石”常规 宝石学特征、矿物组成等方面进行研究分析,并与软玉籽料对比.结果表明,仿软玉籽料“卡瓦石”与软玉籽料 样品颜色均为黄色,磨圆度好,微透明,肉眼观察极为相似,不易区分.但是“卡瓦石”样品折射率为1.56（点 测）,相对密度为2. 52-2. 60,摩氏硬度为5-5. 5,紫外灯下无荧光.其折射率、相对密度以及硬度均低于软 玉籽料样品.紫外-可见吸收光谱分析显示,“卡瓦石”和软玉籽料的光谱非常相似,两者的紫外-可见光吸收光谱均在320-480 nm范围内呈现较宽吸收峰带,位于紫外或近紫外区,使样品呈现其补色--黄色.此外,软玉具有位于730nm附近处的弱吸收峰,是 Fe^2＋→Fe^3 ＋的电荷转移所致,使软玉籽料样品带有微弱绿色 调,但是 Fe^3＋电子跃迁的强度大于 Fe^2＋→Fe^3＋的电荷转移的强度,所以软玉籽料样品整体呈现黄色调.红 外光谱分析表明,“卡瓦石”由蛇纹石矿物系列组成,主要矿物为叶蛇纹石,具有由OH伸缩振动引起的 3 685 cm^-1和 Si-O伸缩振动引起的1071、986 cm^-1特征吸收峰,而软玉籽料具有由OH伸缩振动引起的 3 629 cm^-1附近的吸收带,由 Si一0伸缩振动引起的1200-640 cm^-1范围内的吸收带,以及由 Si-O 弯曲振 动、 M-O振动和 OH平动的耦合振动引起的位于低频区601和487 cn^1附近的吸收带.激光拉曼光谱进 一步证明了“卡瓦石”由蛇纹石矿物组成,具有位于376、685、1 085 cm^-1处的特征峰.不同于由透闪石矿物 组成的软玉籽料,其特征峰位于1059、673和222 cm^-1处 .

Recently, a so-called “cava stone” is used to imitate nephrite seed material,which has similar appearance characteristics to nephrite pebble. In order to differentiate “cava stone” from nephrite pebble, this paper made comparison between them in gemmological and mineralogical characteristics by using standard gemmological testing methods, infrared spec-troscopy, UV-Vis absorption spectroscopy and Raman spectroscopy. Standard gemological testing showed “cava stone” and nephrite pebble both are yellow and sub translucent, with good abrasion, which are hard to differentiate from each other when observed in naked eyes.However, the refractive index （1. 56）, specific gravity（2. 52 - 2. 60） and hardness（5 - 5. 5） of “cava stone” are all lower than these of nephrite pebble. UV-Vis spectrum revealed two jades had similar spectra with broad absorption band between 320 - 480 nm, located in purpleor near-purple area which influenced the complementary colour--yellow. Besides,nephritehas a weak peak around 730 nm, which is caused by charge transfer between Fe^2＋ and Fe^3＋ , resulting in weak green colour. The intensity of Fe^3＋ electron transition is stronger than the charge transfer between Fe^2＋ and Fe^3＋,leading to a whole yellow hue of the seed material. Infrared spectrum displayed the main constituent of ＂cava stone＂ belongs to serpentine min-eral pieces that is antigorite,attributed to 3 685 cm^-1 caused by OH stretching vibration and 1 071,986 cm^-1 led by Si -O stretching vibration. In comparison,the nephrite pebble has peaks at 3 629 cm^-1 caused by OH stretching vibration and the absorption band at 1 200 - 640 nm which is caused by Si-O stretching vibration,as well as absorption bands near 601, 487 cm^-1 resulted by coupled vibrations of Si-O bending vibration, M-O vibration and OH translation. Raman spectrum further demonstrated the composition of the “cava stone” is serpentine minerals and the typical peaks at 376, 685 and 1 085 cm^-1. In contrast, the neph-rite pebble material constituted of tremolite has representative peaks at 1 059 ,673 and 222 cm^-1.