QU Mengwen, ZHONG Yuan, Andy Hsitien Shen. Gemmological Characteristic of Purple-Brownish Red Garnet from Zambia[J]. Journal of Gems & Gemmology, 2021, 23(4): 20-28. DOI: 10.15964/j.cnki.027jgg.2021.04.003
Citation: QU Mengwen, ZHONG Yuan, Andy Hsitien Shen. Gemmological Characteristic of Purple-Brownish Red Garnet from Zambia[J]. Journal of Gems & Gemmology, 2021, 23(4): 20-28. DOI: 10.15964/j.cnki.027jgg.2021.04.003

Gemmological Characteristic of Purple-Brownish Red Garnet from Zambia

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  • Received Date: September 12, 2020
  • The Magodi mining area in Zambia is a new source of purple-brownish red garnets, but there are only a few studies on this locality. In this paper, the garnets from Zambia were systematically studied by conventional gemmological tests, electron probe micro-analysis (EPMA), Raman spectrometer (Raman) and ultraviolet-visible absorption spectrometer (UV-Vis). The refractive index of garnets from Zambia is about 1.750-1.772, and the relative density is about 3.77-3.92. The garnets correspond to the almandine-pyrope series, which contain minor mineral components like grossularite and spessartite. Besides, inclusions are rich in variety, such as transparent euhedral to semi-euhedral mineral inclusions, round ablation inclusions, dense rod or granular inclusions, long needle-like inclusions with parallel arrangement, "fingerprint-like" healed fractures, etc. Raman spectra showed that the mineral inclusions include rutile, zircon and anatase. The ultraviolet-visible absorption spectra are mainly related to the transition of d-d orbitals of Fe2+, Fe3+ and Mn2+ ions. High content of Fe2+ produces the main absorption in yellow-green spectrum region, leading to higher transmittance in red region and blue-violet region, which mix into the purple hue of some samples. The other samples with brownish-red hue produce stronger absorptions at 368 nm and 425 nm that related to Fe3+ than the purple samples, which decreases the transmission of blue and violet light, thus relatively more light transmits through the red region, causing the brownish-red hue. The chemical composition, absorption spectrum and inclusions characteristics provide references for studying the deposit genesis and geological background of garnets from Zambia, as well as the basis for determination of provenance.
  • 石榴石是全球范围内被广泛使用的宝石,因其丰富艳丽的颜色和较强的表面光泽,深受大家的喜爱,具有广阔的市场发展前景。石榴石属于岛状硅酸盐矿物,结构较为复杂。由于存在广泛的类质同象替代,大量成分不同的端元组分使得石榴石亚种丰富,主要包括铝质和钙质系列,镁铝榴石、铁铝榴石、锰铝榴石属于前者,钙铝榴石、钙铁榴石、钙铬榴石属于后者,二价和三价阳离子种类和含量的不同[1],使其外观颜色、宝石特性以及光谱特征都存在区别。紫红色-棕红色石榴石属于市场上的热门品种,主要为镁铝-铁铝榴石系列,在澳大利亚、坦桑尼亚和莫桑比克等国家均有产出[1-3]

    赞比亚Magodi地区是紫红色-棕红色石榴石的新产地,该地的石榴石主要来产自于Sangos矿,这些石榴石不论是颜色还是净度,都有较好的表现,而目前国内外关于该产地的研究还很少。本文选取该产地石榴石样品,对其主要化学成分和谱学性质进行研究,为赞比亚Magodi紫红色-棕红色石榴石的产地鉴别提供基础的特征信息。

    32颗石榴石样品来自赞比亚交易中心,根据其颜色大致分为两类(P组和R组):一类为偏向紫红色的P组样品,一类为偏向棕红色的R组样品。P组共计19颗,其中17颗切成平行双面并抛光,2颗切割成刻面(1颗水滴形刻面,1颗椭圆形刻面);R组共计13颗,其中11颗切成平行双面并抛光,2颗切割成椭圆形刻面(图 1)。

    Figure  1.  Garnet samples from Zambia

    常规宝石学测试包括折射仪、分光镜、偏光镜、显微镜、紫外荧光仪、静水称重等。谱学测试在中国地质大学(武汉)珠宝学院完成,电子探针测试在中国地质大学(武汉)地球科学学院全球大地构造中心完成。

    紫外-可见吸收光谱使用Jasco MSV-5200紫外-可见-近红外光谱仪进行测试,测试条件:测试范围330~780 nm,数据间隔1 nm,扫描速度267 nm/min,纵坐标用吸光度(A)表示。拉曼光谱使用Bruker Sentera R200L拉曼光谱仪进行测试,测试条件:激发光波长532 nm,分辨率3~5 cm-1,测量范围45~1 550 cm-1,积分时间8 s,积分次数5次,激光能量10 mW。

    选取4颗P组石榴石样品和4颗R组石榴石样品磨制薄片进行电子探针测试。电子探针型号为JEOL JXA-8230型,测试条件:电压15 kV,电流20 mA,束斑直径3 μm,峰位的计数时间10 s,前后背景值的计数时间5 s,X射线强度使用ZAF校正法进行校正。

    常规宝石学测试表明,棕红色R组样品的相对密度和折射率比紫红色的P组偏大;两组样品都具有“铁铝窗”特征吸收,如表 1

    Table  1.  Gemmological characteristics of garnet samples from Zambia
    基本特征 R组石榴石样品 P组石榴石样品
    颜色 棕红色 紫红色
    光泽 强玻璃光泽 强玻璃光泽
    光性特征 异常消光(均质体) 异常消光(均质体)
    相对密度 3.82~3.92 3.77~3.90
    折射率 1.762~1.772 1.750~1.763
    分光镜 “铁铝窗”(黄绿区三条吸收线) “铁铝窗”(黄绿区三条吸收线)
    荧光 惰性 惰性
     | Show Table
    DownLoad: CSV

    Hanneman[4]发现铁铝-镁铝榴石系列两个端元的比例和相对密度以及折射率之间均存在线性相关性。赞比亚样品符合这一研究规律(图 2),根据成分测试,R组样品比P组样品含有更多铁铝榴石的成分,这是导致两组样品相对密度和折射率差别的重要原因。

    Figure  2.  Garnet samples from Zambia plotted according to refractive index and relative density

    赞比亚样品中可见种类、数量丰富的内含物,根据显微观察结果(图 3)发现:(1)可见指纹状或栅栏状的愈合裂隙;(2)P组样品和部分R组样品中可见密集的短棒状、点状包裹体,短棒状包裹体常呈多组定向排列;(3)多组定向排列的长针状包裹体,在反射光下会显示出彩色干涉色;(4)常成群聚集在一起的透明浑圆状包裹体或自形包裹体,周围伴有应力裂隙,在R组中更为常见;(5)黑色半自形包裹体在少量样品中存在。

    Figure  3.  Common inclusions in garnet samples from Zambia

    此外,两组样品各自存在一些形态、颜色等明显差异的内含物。P组样品中存在一些被熔蚀的粒状包裹体和短针状包裹体、褶皱状纹理、黑色类四方柱状包裹体(图 4)。R组样品中发现了长管状包裹体、带晕彩的长条形片状包裹体、红色短柱状包裹体(图 5)。

    Figure  4.  Special inclusions in garnet samples of group P
    Figure  5.  Special inclusions in garnet samples of group R

    根据拉曼光谱测试结果,赞比亚石榴石中的矿物包裹体主要为金红石(图 6a)、锆石(图 6b)和锐钛矿(图 6c)。石榴石中的长针状包裹体一般为金红石或钛铁矿,富含这些针状包裹体的石榴石常产自于超高压变质岩、地幔橄榄岩和辉石岩以及高压麻粒岩中[5-8]。赞比亚的样品中大量的金红石针常呈多组定向排列(图 3f图 3g),这可能和这些石榴石产自Sangos矿的区域变质岩中,经历了麻粒岩相高压条件下的变质作用有关。

    Figure  6.  Raman spectra of mineral inclusions in garnet samples from Zambia and standard minerals in RRUFF database

    石榴石晶体化学式为X3Y2[SiO4]3,其中X位主要为二价的Mg、Fe、Mn、Ca离子,Y为主要为三价的Al、Fe、Cr离子。利用电子探针测试结果(表 2),通过阳离子法推导出赞比亚石榴石中Fe2+、Fe3+及其他阳离子的系数(表 3)[9],根据系数计算出P组和R组样品的平均化学式(表 4)。

    Table  2.  EPMA results of garnet samples from Zambia  wB/%
    成分 P1 P13 P2 P12 R3 R11 R9 R6
    Al2O3 22.83 22.75 24.20 23.79 22.31 22.23 23.73 23.05
    SiO2 41.38 40.89 40.92 40.54 40.33 40.69 39.94 39.95
    K2O 0.01 0.01 - - 0.01 - - -
    NiO 0.03 0.08 - - 0.02 0.03 - -
    CaO 1.17 0.77 1.35 1.35 1.90 1.31 1.58 1.72
    Na2O 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.02
    MgO 15.92 16.59 14.91 15.05 12.96 14.03 15.00 14.28
    TiO2 0.05 - - - 0.06 - 0.04 0.02
    FeOT 18.62 18.03 18.53 19.00 21.33 20.59 18.98 20.37
    MnO 0.33 0.60 0.18 0.15 0.52 0.16 0.43 0.41
    Cr2O3 0.03 0.04 0 0 0 0.02 - -
    P2O5 - - 0.02 0.04 - - 0.03 0.03
    Total 100.38 99.78 100.13 99.94 99.46 99.07 99.75 99.86
    注:FeOT为全铁质量百分数;“-”表示该元素的质量分数低于检出限
     | Show Table
    DownLoad: CSV
    Table  3.  Cation coefficients of the samples calculated by cation calculation method
    元素 P1 P13 P2 P12 R3 R11 R9 R6
    Si 3.031 3.003 3.009 2.988 3.033 3.053 2.951 2.968
    Ti 0.003 - - - 0.003 - 0.002 0.001
    Cr 0.002 0.002 - - - 0.001 - -
    Al 1.971 1.969 2.097 2.067 1.978 1.966 2.067 2.018
    Fe3+ - 0.028 - - - - 0.017 0.037
    Fe2+ 1.177 1.079 1.256 1.225 1.39 1.362 1.156 1.228
    Mn 0.021 0.037 0.011 0.01 0.033 0.01 0.027 0.026
    Mg 1.738 1.816 1.634 1.654 1.454 1.569 1.652 1.581
    Ca 0.091 0.061 0.106 0.106 0.153 0.105 0.125 0.137
    Na 0.002 0.003 0.003 0.002 0.002 0.003 0.001 0.003
    K 0.001 0.001 - - 0.001 - - -
    Ni 0.002 0.002 - - - 0.001 - -
    P - - 0.001 0.003 - - 0.002 0.002
    Total 8.039 8.002 8.117 8.056 8.048 8.07 8.002 8.002
     | Show Table
    DownLoad: CSV
    Table  4.  Average chemical formula of the two groups of samples
    样品组别 平均化学式
    P组 (Mg1.711Fe1.184Ca0.091Mn0.020)3.006(Al2.026Fe0.007)2.033Si3.008O12
    R组 (Mg1.564Fe1.284Ca0.130Mn0.024)3.002(Al2.007Fe0.014)2.021Si3.001O12
     | Show Table
    DownLoad: CSV

    通过阳离子系数计算得到石榴石各端元组分的摩尔百分比,结果(表 5)显示,赞比亚石榴石样品以镁铝榴石Prp和铁铝榴石Alm组分为主(Prp:48.5~60.5%,Alm:36.0~46.3%),属于镁铝-铁铝榴石类质同象系列,两组分含量之和基本在95%以上,仅有个别R组样品两组分含量之和低于95%。镁铝榴石组分的含量均大于铁铝榴石,R组整体上相较于P组含有更多铁铝榴石组分。

    Table  5.  Mole percentage of different components in two groups of samples  mol/%
    组分 P组样品 R组样品
    铁铝榴石(Alm) 36.0~41.9 38.5~46.3
    镁铝榴石(Prp) 54.5~60.5 48.5~55.1
    锰铝榴石(Sps) 0.3~1.2 0.3~1.1
    钙铝榴石(Grs) 0.5~3.5 2.7~5.1
    钙铁榴石(Adr) 0~1.4 0~1.9
    钙铬榴石(Uvt) 0~0.1 0~0.1
    Alm+Prp 95.9~97.1 93.6~97.7
    Alm/(Alm+Prp) 37.3~43.5 41.1~48.8
     | Show Table
    DownLoad: CSV

    赞比亚紫红色-棕红色石榴石的紫外-可见吸收光谱谱形基本相同,如图 7,390~480 nm有一系列吸收程度相近的谱峰,425 nm附近有较明显的吸收,500~580 nm有三处较强的吸收峰,617 nm附近有较弱的肩峰,696 nm附近有一个宽缓的吸收带。

    Figure  7.  UV-Vis absorption spectra of garnet samples from Zambia

    铝系石榴石中,Fe2+和Mn2+周围有8个氧原子作为配体,构成了近似六方双锥状的配位场,Fe3+在晶体中周围有六个氧原子,构成了八面体配位场[10-15],铝系石榴石的紫外-可见吸收光谱主要与配位场中的Fe2+、Fe3+和Mn2+d-d轨道跃迁有关,也可能有Fe2+-Fe3+的电荷转移、Fe3+-O的电荷转移的参与。

    结合化学成分分析认为, 398、461、503、523、574、617、696 nm附近的吸收峰代表Fe2+d-d自旋禁阻跃迁,407 nm处的吸收峰代表Mn2+d-d自旋禁阻跃迁, 368 nm处的吸收峰以及425 nm处的吸收峰来源于Fe3+d-d自旋禁阻跃迁, 各谱峰具体的配位谱项能级跃迁如表 6。关于461 nm处吸收峰的形成,还有一种说法,即相邻的八配位Fe2+和六配位的Fe3+发生了电荷转移,但是这样形成的是一个较宽的谱峰,需要较多的Fe3+含量,赞比亚石榴石样品不符合这样的要求[13]

    Table  6.  UV-Vis absorption bands and their counterpart of ions and energy level transitions
    波长/nm 离子 能级跃迁[12]
    368 Fe3+ 6A1g4Eg(D)
    398 Fe2+ 5Eg3A1g(3G)
    407 Mn2+ 6A1g4A1g+ 4Eg(4G)
    425 Fe3+ 6A1g4A1g+ 4Eg(4G)
    461、503、523 Fe2+ 5Eg3E1g(3H)
    574、617、696 Fe2+ 5Eg3T1g(3H)
     | Show Table
    DownLoad: CSV

    赞比亚样品中的Fe主要是以Fe2+为主,在500~600 nm的黄绿光区产生了最主要的吸收,导致红光区和蓝紫光区透过较多的光,呈现出紫红色-棕红色调。比较两组样品的368 nm和503 nm两处峰位(图 7)可以发现,R组样品在368 nm处有更强的吸收,该吸收峰延伸至了可见光区内,结合425 nm处的吸收,削弱了蓝紫光的透过,导致R组样品偏向棕红色调,而P组368 nm处的吸收相对较弱,透过了更多的蓝紫光,呈现出紫红色。368 nm和425 nm处的吸收均和Fe3+离子有关,推测R组样品具有更高的Fe3+/Fe占比。此外,赞比亚石榴石中Mn2+含量较低,吸收较弱,对颜色的影响不显著。

    (1) 赞比亚Magodi矿区的紫红色-棕红色石榴石的相对密度和折射率呈一定的正相关性,棕红色石榴石相对密度和折射率整体较紫红色石榴石偏大,这和棕红色石榴石具有更高的Fe含量有关。

    (2) 赞比亚紫红色-棕红石榴石为铁铝-镁铝榴石系列,总体偏向于镁铝榴石端元。棕红色石榴石较紫红色石榴石有更多的铁铝榴石组分。通过阳离子法计算,还可能存在少量的Fe3+

    (3) 赞比亚紫红色-棕红石榴石含有种类丰富的包裹体,包括自形-半自形状的透明晶体包裹体、浑圆状包裹体、密集的短棒状和粒状包裹体、平行排列的长针状包裹体、“指纹状”的愈合裂隙等。拉曼光谱测试表明矿物包裹体主要为金红石、锆石和锐钛矿,其中丰富的金红石针可能说明该产地的石榴石经过了高压的麻粒岩相变质作用。

    (4) 紫外-可见吸收光谱测试表明,棕红色和紫红色石榴石的颜色差异主要与蓝紫区和黄绿区的吸收相对强弱有关。棕红色石榴石蓝紫区吸收更强,从而显示棕红色;紫红色石榴石黄绿区吸收更强,从而显示紫红色。368 nm吸收和Fe3+有关,推测棕红色石榴石有更高的Fe3+/Fe。

  • [1]
    Hollis J D, Sutherland F L, Pogson R E. High pressure minerals and the origin of the Tertiary Breccia Pipe, Ballogie gem mine, near Proston, Queensland[J]. Records of the Australian Museum, 1983(35): 181-194.
    [2]
    Zwaan P C. Garnet, corundum and other gem minerals from Umba, Tanzania[J]. Scripta Geologica, 1974(20): 1-41. http://www.repository.naturalis.nl/document/148690
    [3]
    Sangsawong S, Raynaud V, Pardieu V. Purple pyrope-almandine garnet from Mozambique[J]. Gems & Gemology, 2016, 52(3): 321-323.
    [4]
    Hanneman W W. A new classification for red-to-violet garnets[J]. Gems & Gemology, 1983, 19(1): 37-40. http://www.researchgate.net/publication/274675103_A_New_Classification_for_Red-To-Violet_Garnets
    [5]
    Larsen R B, Eide E A, Burke E A J. Evolution of metamorphic volatiles during exhumation of microdiamond-bearing granulites in the Western Gneiss Region, Norway[J]. Contributions to Mineralogy and Petrology, 1998, 133(1-2): 106-121. doi: 10.1007/s004100050441
    [6]
    Obata M. Material transfer and local equilibria in a zoned kelyphite from a garnet pyroxenite, Ronda, Spain[J]. Journal of Petrology, 1994, 35(1): 271-287. doi: 10.1093/petrology/35.1.271
    [7]
    O'Brien P J, Rötzler J. High-pressure granulites: formation, recovery of peak conditions and implications for tectonics[J]. Journal of Metamorphic Geology, 2003, 21(1): 3-20. doi: 10.1046/j.1525-1314.2003.00420.x
    [8]
    Ague J J, Eckert Jr J O. Precipitation of rutile and ilmenite needles in garnet: Implications for extreme metamorphic conditions in the Acadian Orogen, USA[J]. American Mineralogist, 2012, 97(5-6): 840-855. doi: 10.2138/am.2012.4015
    [9]
    赵珊荣. 结晶学及矿物学[M]. 2版. 北京: 高等教育出版社, 2011.
    [10]
    Novak G A, Gibbs G V. The crystal chemistry of the silicate garnets[J]. American Mineralogist: Journal of Earth and Planetary Materials, 1971, 56(5-6): 791-825. http://www.researchgate.net/publication/304109313_The_crystal_chemistry_of_the_silicate_garnets
    [11]
    Meagher E P. Silicate garnets[J]. Reviews in Mineralogy, 1982, 5(1): 25-66.
    [12]
    Moore R K, White W B. Electronic spectra of transition metal ions in silicate garnets[J]. The Canadian Mineralogist, 1972, 11(4): 791-811. http://canmin.geoscienceworld.org/content/11/4/791
    [13]
    Taran M N, Dyar M D, Matsyuk S S. Optical absorption study of natural garnets of almandine-skiagite composition showing intervalence Fe2++Fe3+ → Fe3++Fe2+ charge-transfer transition[J]. American Mineralogist, 2007, 92(5-6): 753-760. doi: 10.2138/am.2007.2163
    [14]
    Manning P G. The optical absorption spectra of some andradites and the identification of the 6A14A14E(G) transition in octahedrally bonded Fe3+[J]. Canadian Journal of Earth Sciences, 1967, 4(6): 1 039-1 047. doi: 10.1139/e67-070
    [15]
    Krambrock K, Guimarães F S, Pinheiro M V B, et al. Purplish-red almandine garnets with alexandrite-like effect: Causes of colors and color-enhancing treatments[J]. Physics and Chemistry of Minerals, 2013, 40(7): 555-562. doi: 10.1007/s00269-013-0592-6
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