Atom Probe Tomography (APT) and Its Application in Ore Deposits
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摘要:
原子探针层析技术(APT)是一种能够以亚纳米分辨率提供定量的三维元素和同位素分析的测试分析技术,具有极高的空间分辨率和低的检出限。虽然原子探针主要用于材料科学和半导体领域,但随着近年来在矿床研究中应用的不断增加,正逐渐成为矿床研究的有用手段。与传统的地质分析技术相比,原子探针具有独特的技术优势,可以测量体积<0.0007μm3的矿物的元素组成,能够在纳米尺度上揭示矿物成分的复杂性,为理解地质演化过程提供全新的认识。本文在简述原子探针层析技术的基本原理、样品的选择和处理以及针尖样品制备的基础上,重点从成矿元素赋存状态、纳米尺度包裹体和稳定同位素组成三个方面阐述了原子探针在矿床研究中的代表性应用成果。迄今为止,原子探针在矿床学中的应用主要集中在成矿元素赋存状态的分析上,尤其是与金矿相关的黄铁矿或其他化学组成相对简单的矿物。而在纳米尺度包裹体和稳定同位素组成方面,原子探针应用成果虽不如前者丰富,但也取得了一些重要的全新认识,表现出良好的应用前景。原子探针在矿床学领域迅速发展的同时,也存在一些亟需解决的问题,如复杂质谱峰的标定、三维重建失真等。尽管如此,相信随着技术的不断进步,原子探针将逐渐成为矿床研究的重要工具。
Abstract:Atom Probe Tomography (APT) is a test analysis technique that provides quantitative three-dimensional element and isotope analysis at subnanometer resolution, with extremely high spatial resolution and low detection limits[13]. Compared with traditional geological analysis techniques, APT has unique technical advantages, which can be used to analyze the elemental composition of minerals <0.0007μm3 in volume[14], reveal the complexity of mineral composition at the nanoscale, and provide a new understanding of the geological evolution process. APT has been in development for over 50 years, and continuous technological advancements have led to its wider application range. At the beginning of APT design, it was only used for conductive materials. From the end of the 20th century to the beginning of the 21st century, the application of laser pulse mode enabled APT to be applied to semiconductors and insulating materials[15-19], and the application of Local Electrode TM Atom Probe (LEAP) improved several key parameters such as the data acquisition rate and mass resolution of APT by several orders of magnitude[20]. At present, most of the geological application work of APT is carried out by LEAP in laser-assisted mode[13]. In recent years, the unique technical advantages of APT have attracted increasing attention in geological research, and their advantages in ore deposit research have become more prominent. Some important research results have been published[21-31]. However, on the whole, its application in ore deposits and even geology is still in its infancy. The development history, basic principle, selection method of area of interest and needle tip sample preparation of APT are briefly introduced in this paper. Based on this, representative application achievements of APT in ore deposit research by domestic and foreign scholars in recent years are collected and summarized. In ore deposit research, APT is mainly applied in three aspects: the occurrence states of ore-forming elements, nanoscale inclusions, and stable isotope composition[21-31]. At present, most research results focus on the analysis of the occurrence status of ore-forming elements, especially pyrite or other minerals with simple chemical composition related to gold deposits. APT has successfully revealed three main occurrence states of ore-forming elements on the atomic scale: uniform distribution, nanoparticle and enrichment at low angle grain boundaries and dislocations[21-25]. For example, gold can be uniformly distributed in the form of dispersed lattice bound gold in the arsenic-rich overgrowth rim of pyrite[21], and can form nanoclusters of different sizes in arsenopyrite[22]. It can also host in the low angle boundary of pyrite related to deformation[24]. In terms of nano inclusions and stable isotope composition, the research mainly focuses on pyrite nano fluid inclusions and S isotopes[26-31]. For example, nano telluride inclusions along pyrite fractures in low-sulfidation type epithermal Au-Ag-Te deposit[26] and the method for obtaining quantitative δ34S measurement value from APT datasets of pyrite[29]. The relevant results are shown in Fig.E.1. So far, the applications of APT in ore deposits research have mainly focused on the occurrence state of ore-forming elements, achieving three-dimensional visualization of atomic scale element distribution that was previously unimaginable, providing a new perspective for people to understand and explain the ore-forming process. In terms of nano inclusions and stable isotope composition, although the applications of APT are not as rich as the former, some important new understandings have been obtained, showing a good application prospect. While APT is rapidly developing in the field of ore deposits, there are still many problems to be solved in its practical application. For example, the extremely small sample volume, time-consuming selection of specific areas, the background noise carried by the mass spectrometry itself, the correct interpretation of complex spectral peaks, and the accuracy of data three-dimensional reconstruction. However, it is foreseeable that with the continuous progress of technology, APT will become more popular and easier to use, increasing numbers of deposit researchers will pay attention to APT, and more ore deposit samples with complex types, structures and chemical compositions will apply this technology for in-depth research, which may change or even completely subvert our understanding of some basic scientific problems in ore deposits.
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花岗伟晶岩是稀有金属的主要赋矿岩石,常富集锂铍铌钽铷铯铀钍铊锡和稀土等多种有用金属元素,花岗伟晶岩型矿床是锂铍铷铯铌钽钨锡等金属的重要成矿类型[1]。该类岩石中主要的金属矿物有锂辉石、钽铌铁矿、锆石、绿柱石、锡石、氟碳铈矿、尖晶石、锐钛矿;脉石矿物主要有微斜长石、正长石、石英、白云母、黄玉、电气石等,其中锆石、绿柱石、锡石、尖晶石、锐钛矿、黄玉、电气石等均属于难溶矿物[2]。矿石中稀有金属等元素赋存状态复杂、不同矿物含量差异大、样品分解难度大、化学性质不稳定等因素为样品的分解和含量准确测定带来了很大困难[3-4]。
实现样品的完全消解是获得准确的多元素分析结果的重要前提。文献[5-7]采用盐酸-硝酸-氢氟酸-高氯酸敞开消解试样,采用酒石酸-稀盐酸-过氧化氢溶液定容,用电感耦合等离子体发射光谱法(ICP-OES)测定稀有金属矿中的锂铍铌钽锡等元素。该消解方法操作简便,但对地质样品中的镧铈镨钕钆钇等稀土元素以及铌钽锆铪等元素分解不完全,测定结果往往偏低。通过在消解体系中引入硫酸,利用硫酸的高沸点性,对于难溶的副矿物相有很好的溶解效果,能提高溶出率[8]。文献[9-10]采用氢氟酸-硝酸-盐酸-高氯酸-硫酸敞开分解样品,以氢氟酸-硫酸-过氧化氢提取体系替代常规的酒石酸体系,避免了酒石酸用量过大时溶液中的盐分过高,易导致仪器管路堵塞、仪器熄火等问题,实现了ICP-OES同时测定稀有金属矿选矿产品中的铌钽锂铍铷铁钛等元素。从目前文献报道来看,主要关注锂铍铷铯铌钽等稀有金属元素,缺少锆铪钍铀钡钛铅以及稀土元素,而这些元素可用于绘制稀土元素分布型式图和微量元素蛛网图,在花岗伟晶岩的源区性质、成岩成矿时代和构造环境等研究中具有重要意义[11-13]。对于花岗伟晶岩样品,文献[14-15]采用硼酸盐高温熔融法处理样品,形成的玻璃体于硝酸-盐酸-氢氟酸中消解后,采用ICP-MS测定了镓锶铀钍和稀土元素。硼酸盐熔融法实现了样品完全分解,但也有文献表明使用硼酸盐熔融分解法,由于分解温度很高(1000 ℃以上),无法测量锡、铊和铅等挥发性元素,且给ICP-MS的雾化系统带来严重的锂和硼的记忆效应[16-17]。密闭酸溶法在高温高压下长时间溶样,能保证大多数难溶元素的完全分解,同时易挥发元素在密封条件下也不易损失,被广泛应用于各类地质样品的分解[18-20]。胡兰基等[21]采用硝酸-氢氟酸高压密闭酸溶消解花岗伟晶岩地质样品,ICP-MS同时测定锂铍铌钽铷铯等6种元素,该文仅测定了6种元素,且在残渣复溶时使用的是硝酸。有文献研究表明锆铪铷钍铀和稀土等元素用硝酸复溶较困难,回收率会偏低30%左右[18-20]。
本文在参考相关文献[18-19]的基础上,对比了盐酸-硝酸-氢氟酸-高氯酸敞开消解法(四酸法)、盐酸-硝酸-氢氟酸-高氯酸-硫酸敞开消解法(五酸法)、硝酸-氢氟酸密闭消解法(密闭法)的分解效果。在硝酸-氢氟酸密闭消解法中,使用王水代替硝酸进行残渣复溶,利用硝酸的氧化性和氯离子的络合作用,促进铌钽锆铪和稀土等元素的复溶,建立了一种密闭酸溶ICP-OES/MS测定花岗伟晶岩中锂铍铷铯铌钽锆铪等稀有金属以及稀土共32种元素的方法,应用于8种标准物质和三种实际样品的测定,验证了该方法的可行性。
1. 实验部分
1.1 仪器及工作条件
iCAP 7400 Radial MFC型电感耦合等离子体发射光谱仪(美国ThermoFisher公司)。仪器工作条件为:射频功率1150W,雾化气流速0.5L/min,辅助气流速0.5L/min,泵速50r/min,长波扫描时间5s,短波扫描时间15s。待测元素分析谱线为:Li 670.776nm,Rb 780.023nm,Ti 338.376nm,Al 396.152nm。
X-Series Ⅱ型电感耦合等离子体质谱仪(美国ThermoFisher公司)。仪器工作条件为:射频功率1200W,冷却气流速13.0L/min,辅助气流速0.7L/min,雾化气流速0.82L/min,测量通道3,驻留时间10ms,扫描次数为40,取样堆孔径1.0mm,截取堆孔径0.7mm,模拟电压2000V,脉冲电压3600V,采样深度180mm。待测元素的同位素分别为:7Li、9Be、71Ga、85Rb、89Y、90Zr、93Nb、118Sn、133Cs、137Ba、139La、140Ce、141Pr、146Nd、147Sm、153Eu、157Gd、159Tb、163Dy、165Ho、166Er、169Tm、172Yb、175Lu、178Hf、181Ta、182W、205Tl、208Pb、232Th、238U。在ICP-MS仪器调谐时,控制Ba2+/Ba为代表的双电荷离子产率低于3%,控制CeO/Ce为代表的氧化物产率低于2%,以降低氧化物干扰及双电荷离子干扰。对153Eu、157Gd、159Tb进行干扰校正,其干扰离子的校正方程为:−0.0006×137Ba、−0.004×140Ce−0.008×141Pr、−1.46×(161Dy−0.76×163Dy)。
防腐型密闭消解罐:内罐为容积30mL的聚四氟乙烯容器,外罐为防锈的高硬质合金材质。
1.2 标准溶液和主要试剂
锂铍镓铷钇锡铯钡铊铅铝和稀土等元素标准储备溶液(1000µg/mL,山东省冶金科学研究院);铀钍元素标准储备溶液(100µg/mL,核工业北京化工冶金研究院);锆铌铪钽钨钛(100µg/mL,国家有色金属及电子材料分析测试中心)。ICP-MS测试时逐级稀释为混合标准溶液1(锂镓铷钇锡铯钡铊铅钍铀)、混合标准溶液2(钇镧铈镨钕钐铕钆铽镝钬铒铥镱镥)、混合标准溶液3(锆铌铪钽钨)。标准溶液系列浓度为0、10、20、50、100ng/mL,溶液介质为5%王水。内标溶液为10ng/mL的铑溶液(2%硝酸介质)。ICP-OES测试时,逐级稀释为校准溶液系列分别为:锂铷(0、0.5、1.0、2.0、5.0、10.0µg/mL);钛(0、2、10、50、100µg/mL);铝(0、10、50、150、300µg/mL)。
硝酸、盐酸、氢氟酸、高氯酸、硫酸、过氧化氢均为优级纯。实验用水为电阻率>18MΩ·cm的去离子水。
1.3 实验样品
标准物质GBW07103(花岗岩,中国地质科学院地球物理地球化学勘查研究所研制),GBW07125(伟晶岩,国家地质实验测试中心研制);GBW07152(锂矿石)、GBW07153(锂矿石)、GBW07154(铌钽矿石)、GBW07155(铌钽矿石)、GBW07184(锂矿石)、GBW07185(铌钽矿石)均为原地质矿产部沈阳综合岩矿测试中心研制。上述花岗岩、伟晶岩标准物质与本文研究样品基体相似,同时考虑到样品中锂铷铯铌钽等稀有金属含量较高,又选择了一系列稀有金属标准物质,用于分解方法的验证及方法精密度、准确度的考察。
三种花岗伟晶岩实际样品:HWJY-1(含锂辉石花岗伟晶岩,采集自河南蔡家沟矿区);HWJY-2(含锡石花岗伟晶岩,采集自河南火炎沟矿区);HWJY-3(含红色电气石花岗伟晶岩,采集自河南卢氏南阳山矿区)。将采集的矿石样品磨细,过74µm筛后混匀后备用。上述花岗伟晶岩实际样品,主要用于考察分析方法的适用情况。
1.4 实验方法
1.4.1 四酸敞开消解-王水提取法(简称“四酸法”)
准确称取0.1000g样品于聚四氟乙烯坩埚中,加入6mL盐酸、3mL硝酸,在控温电热板上120℃分解试样1h。加入6mL氢氟酸、1mL高氯酸,控温150℃分解1h,升温至240℃,加热蒸至白烟冒尽,加入5mL王水(50%),在电热板上140℃加热复溶15min,将溶液转移到25mL塑料比色管中,用水稀释至刻度,摇匀。在仪器工作条件下采用ICP-OES直接测定,分取5.0mL试液于25mL塑料比色管中,用2%硝酸定容摇匀后用ICP-MS测定。
1.4.2 五酸敞开消解-氢氟酸-硫酸-过氧化氢体系提取法(简称“五酸法”)
称取0.1000g样品于聚四氟坩埚中,准确加入5mL氢氟酸、2mL硫酸(50%)、10mL混合酸(盐酸∶硝酸∶高氯酸= 3∶2∶1.5),在260℃电热板上加盖分解30min,取下盖子,逐步升温至 330℃溶解至硫酸白烟冒尽,取下。在温热状态下加入3~5滴氢氟酸、10mL提取剂(5%过氧化氢-5%硫酸),在电热板上200℃加热提取,然后用1%硝酸定容至 25mL容量瓶中。分取5.0mL试液于25mL塑料比色管中,补加1mL硝酸后,用2%硝酸定容摇匀后分别用ICP-OES和ICP-MS测定。
1.4.3 密闭酸溶-王水密闭复溶法(简称“密闭法”)
准确称取0.1000g试样于30mL聚四氟乙烯內罐中,加入2mL硝酸、6mL氢氟酸,盖上聚四氟乙烯盖,装入外罐,拧紧外罐盖置于控温干燥箱控温185℃,保持48h。冷却后取出內罐置于控温电热板上140℃蒸干,加入2mL硝酸再次蒸干,并重复一次赶除氢氟酸,加入5mL王水(50%)后再于185℃密闭条件下复溶8h,冷却后取出转移至25mL塑料比色管中,用水稀释至刻度,摇匀。在仪器工作条件下采用ICP-OES直接测定,分取5.0mL试液于25mL塑料比色管中,用2%硝酸定容摇匀后用ICP-MS测定。
2. 结果与讨论
2.1 三种方法分解效果的对比
花岗伟晶岩是成分与花岗岩类似的浅成岩,以晶粒巨大为特征。矿物成分除与花岗岩所固有的成分相同外,还经常伴生出现黄玉、绿柱石、电气石等稀有元素的矿物[1-2]。因此,选择与样品基体相似的花岗岩标准物质GBW07103、伟晶岩标准物质GBW07125进行试验,考虑到花岗伟晶岩样品中锂铷铯铌钽等稀有金属的含量较高,又选择了GBW07152、GBW07153、GBW07154、GBW07155、GBW07184、GBW07185等一系列稀有金属标准物质以及实际样品进行消解方法实验。三种分解方法应用于多种标准物质的测量结果见图1和图2。在图1和图2中以测定值(4次测量平均值)与标准值的比值绘制曲线,考察三种方法的分解效果。
由图1和图2可见,对于四酸消解法,锂铍铷铯镓测定值与标准值基本相符,但铌钽锆铪钨和稀土元素尤其是重稀土元素严重偏低,这是因为其分解能力不强,不能完全溶解难溶矿物。
对于五酸消解法,锂铍铷铯镓的测定值与标准值基本相符,铌钽钨等易水解元素的测定结果也与标准值相符,这是由于采用硫酸-氢氟酸-过氧化氢体系进行提取,有效地防止了铌钽钨的水解,甚至GBW07185中的极高含量钽(8354µg/g)也能获得很好的回收率。存在的问题主要是锆、铪测定结果普遍偏低,这可能是因为其分解能力有限,不能完全分解锆石等难溶矿物[22]。此外,标准物质GBW07125、GBW07152、GBW07154、GBW07155和GBW07184中镧铈镨钕等轻稀土元素测定结果偏低,甚至低于四酸法结果。一般认为,在溶矿体系中加入高沸点的硫酸有利于提高稀土元素的回收率[8,23]。本实验中,轻稀土元素测定结果偏低可能是提取溶液中的氢氟酸与稀土元素生成了少量氟化物沉淀[23]。实验还发现,与四酸法、密闭法测定值以及标准值相比,钡、铅元素的测定精密度很差,有时严重偏低70%以上,这可能是因为提取时加入的硫酸产生了硫酸盐沉淀导致的。铊元素的测定值普遍偏高10%~30%,有文献[24-25]认为试样溶液存在高浓度铅时,由于204Pb和206Pb的强峰拖尾,会干扰205Tl的测定,使测定结果偏高,推荐在溶液中加入硫酸,以硫酸铅形式沉淀铅来消除干扰。但是本研究样品中的铅含量较低,且提取体系中也含有硫酸,不应当是铅对铊的干扰引起,具体原因有待进一步研究。
密闭酸溶法对锂铍铷铯镓以及铌钽锆铪钨和稀土等32种元素均实现良好分解和回收,测定值基本与标准值相符合,包括铌钽含量较高的标准物质GBW07153(铌含量为301µg/g;钽含量为573µg/g)。存在唯一问题是对于铌钽含量极高的标准物质GBW07185(铌含量为3635µg/g;钽含量为8354µg/g),由于未采取加入酒石酸或氢氟酸来防止铌、钽的水解,使用密闭法和四酸法在溶样过程中出现少量白色沉淀,估计是水解产生的铌、钽氧化物沉淀,导致铌、钽、钨等元素测定结果偏低。对于钛元素,三种消解方法测量值与标准值基本相符,但对于标准物质GBW07125,四酸法、五酸法测定值均远低于标准值,只有密闭法结果与标准值基本相符,这表明钛可能以金红石等难溶矿物形式存在。
因此,本文优选密闭酸溶-王水密闭复溶法进行花岗伟晶岩样品的分解和稀有金属、稀土等32种元素的同时测定。
2.2 分析技术指标
2.2.1 方法检出限
按硝酸-氢氟酸密闭消解法操作步骤制备12份全流程空白溶液,以测定结果的3倍标准偏差对应的含量值作为方法检出限。从表1可以看出,32种元素的检出限为0.004~2.50μg/g。本文方法检出限优于文献报道的过氧化钠碱熔法或混合硼酸锂盐熔融法的检出限[14,26]。与其他密闭消解法文献报道的检出限基本一致[20]。
表 1 硝酸-氢氟酸密闭消解法的检出限Table 1. Detection limits of nitric acid-hydrofluoric acid closed digestion method元素 检出限(μg/g) 元素 检出限(μg/g) Li 0.04 Eu 0.005 Be 0.03 Gd 0.006 Ti 2.50 Tb 0.006 Ga 0.30 Dy 0.005 Rb 0.50 Ho 0.02 Y 0.02 Er 0.005 Zr 0.05 Tm 0.02 Nb 0.02 Yb 0.006 Sn 0.05 Lu 0.006 Cs 0.02 Hf 0.007 Ba 0.20 Ta 0.07 La 0.02 Tl 0.02 Ce 0.05 Pb 0.30 Pr 0.02 W 0.02 Nd 0.01 Th 0.02 Sm 0.004 U 0.01 2.2.2 方法精密度和准确度
选择国家标准物质GBW07125和GBW07184考察方法精密度和准确度。分别称取12份样品按照密闭酸溶法进行样品消解,用ICP-OES和ICP-MS测定32种元素,计算方法精密度和准确度,结果见表2。各元素测定值的相对标准偏差(RSD)在1.0%~8.3%之间,能够满足《地质矿产实验室测试质量管理规范》(DZ∕T 0130—2006)的要求。标准物质的测定平均值与标准值基本一致,相对误差除各别低含量元素外均小于10%,说明本方法具有较高的精密度和准确度。
表 2 硝酸-氢氟酸密闭消解分析方法的精密度和准确度Table 2. Precision and accuracy tests of nitric acid-hydrofluoric acid closed digestion method元素 GBW07125 GBW07184 测定平均值
(µg/g)标准值与
不确定度
(µg/g)RSD
(%)相对误差
(%)测定平均值
(µg/g)标准值与
不确定度
(µg/g)RSD
(%)相对误差
(%)Li 14.06 14.4±1.1 3.2 −2.4 1.83** 1.81±0.07** 1.7 0.4 Be 1.23 1.3±0.3 4.8 −5.4 62.8 59.1±5.1 1.6 6.3 Ti 3650 3657 1.0 −0.2 165 174 6.0 −5.2 Ga 14.90 13.5±0.7 4.4 10.4 79.1 / 2.4 / Rb 159 155±8 2.6 2.6 1.23** 1.13±0.04** 1.4 8.3 Y 1.77 1.6±0.3 6.4 10.6 2.55 2.36± 4.8 8.1 Zr 29.6 29.3* 5.4 1.0 12.6 / 7.1 / Nb 15.6 14.6±1.8 2.5 6.8 61.8 56.6±7.5 3.2 9.2 Sn 3.36 3.5±0.9 3.2 −4.0 151 152±3 1.6 −0.7 Cs 1.79 1.8±0.2 2.2 −0.6 2925 2830±95 1.8 3.4 Ba 767 (728) 1.2 5.4 14.7 / 5.2 / La 3.59 (3.3) 4.6 8.8 0.89 0.96±0.20 5.6 −7.3 Ce 5.07 (5) 4.0 1.4 1.45 (1.52) 2.6 −4.6 Pr 0.53 0.48±0.10 5.2 10.4 0.37 0.38±0.06 7.2 −2.6 Nd 1.62 1.5±0.2 3.6 8.0 1.54 1.42±0.15 5.0 8.5 Sm 0.25 (0.24) 3.4 4.2 0.43 0.46±0.03 5.6 −6.5 Eu 0.17 (0.16) 7.6 6.3 0.091 0.083±0.08 2.2 9.6 Gd 0.26 0.22±0.04 6.2 11.2 0.50 0.49±0.06 3.0 2.0 Tb 0.042 (0.04) 7.2 5.0 0.096 0.085±0.009 5.4 12.9 Dy 0.22 0.20±0.05 3.0 10.0 0.44 0.43±0.06 4.6 2.3 Ho 0.041 (0.04) 5.4 2.5 0.089 0.082±0.005 7.2 8.5 Er 0.11 0.12±0.01 7.4 −8.3 0.22 0.21±0.04 5.2 4.8 Tm 0.021 (0.02) 8.3 5.0 0.032 0.033±0.004 7.0 −3.0 Yb 0.23 0.21±0.09 4.1 9.5 0.21 0.19±0.03 2.9 10.5 Lu 0.033 0.03±0.01 7.0 10.0 0.036 0.032±0.004 8.2 12.5 Hf 0.83 (0.8) 2.2 3.7 2.57 / 4.0 / Ta 1.28 1.3±0.5 4.4 −1.5 117 108±11 1.8 8.3 Tl 1.23 / 5.8 / 65.1 / 1.4 / Pb 36.9 34.6 7.1 6.6 8.03 / 6.2 / W 3.19 3.2±0.2 2.8 −0.3 80.0 79.0±5.6 1.6 1.3 Th 0.71 0.66±0.10 3.0 7.6 3.31 / 2.4 / U 0.76 (0.75) 6.3 1.3 2.87 / 3.8 / 注:标注“*”的数据来自文献[27];标注“**”数据的单位为10−2。 2.3 实际样品的测定与方法比对
将本文方法应用于HWJY-1(含锂辉石花岗伟晶岩)、HWJY-2(含锡石花岗伟晶岩)和HWJY-3(含较多电气石花岗伟晶岩)三种类型实际样品的测定,对容易水解的铌钽钨元素以及锡元素,分别与五酸法[9]、过氧化钠碱熔-盐酸酸化法[26]的测试结果进行比对,结果见表3。
表 3 实际样品不同消解方法测定结果比对Table 3. Comparison of analytical results of different digestion methods for actual samples元素 样品HWJY-1测定值(μg/g) 样品HWJY-2测定值(μg/g) 样品HWJY-3测定值(μg/g) 密闭法
(本文方法)五酸法 碱熔法 密闭法
(本文方法)五酸法 碱熔法 密闭法
(本文方法)五酸法 碱熔法 Nb 71.9 77.2 79.1 80.0 71.8 83.2 80.7 76.4 81.9 Ta 66.3 67.3 70.1 66.6 64.2 67.5 1111 1080 1184 W 8.13 8.16 8.05 5.96 5.79 5.958 12.9 14.0 13.35 Sn 85.5 79.8 86.7 239 83.0 242 15.4 14.0 16.2 对于铌钽钨等元素的测定,密闭法的测定值与五酸法及碱熔法基本一致,尤其是对于HWJY-3样品,其钽含量(约1000µg/g)相对较高,该结果表明在此含量水平下的钽也未发生水解情况,采用本法能实现准确测定。众所周知,铌、钽元素在稀硝酸溶液中容易发生水解,尤其是钽元素,通常向待测溶液中加入适量酒石酸、氢氟酸或盐酸形成络合物,可起到稳定铌钽的作用[28-29],在ICP-MS分析中,酒石酸的引入会增加基体效应,氢氟酸会损害进样系统。已有研究表明,当溶液中铝铁钙镁等基体元素与铌、钽比值达到104时,铌、钽能稳定存在[30]。本文方法中的样品溶液为王水体系,氯离子的络合作用以及足量的铝铁钙镁等基体元素,均能起到稳定铌、钽的作用,这应当是本实验中较高含量的铌、钽未发生水解的原因。本方法无需引入酒石酸或氢氟酸,更适合ICP-MS分析。
对于锡元素的测定,五酸法对锡的测定值远低于密闭法和碱熔法,表明五酸法未能完全分解样品中的锡石。而密闭消解法与碱熔法的测定值基本一致,说明密闭消解法实现了样品中锡石的完全消解,能实现锡元素的准确测定。
作为对本研究中测定的分析数据质量的附加评估,以测得的三种实际样品中稀土元素、大离子亲石元素和高场强元素测定值等数据绘制稀土元素分布型式图和微量元素蛛网图。由图3可见,三类样品虽有不同稀土配分曲线,但稀土曲线均很平滑,符合花岗伟晶岩中稀土元素分配的一般特征。微量元素蛛网图总体上呈现富集Rb、Nb、Ta、Zr、Hf、U,亏损Ti 等高场强元素,相对亏损Ba等大离子亲石元素的特征,与文献报道的地质规律是一致的[11-13],也说明本文方法测定结果准确可靠。
3. 结论
对比了四酸消解法、五酸消解法和密闭消解法三种方法的分解效果,结果表明四酸消解法的分解能力不强,铌钽锆铪钨和重稀土元素结果严重偏低;五酸消解法由于使用硫酸-氢氟酸-过氧化氢体系提取,有效地防止了铌、钽的水解,测定结果准确,但会造成钡铅和轻稀土元素测定结果偏低,也无法有效地消解难溶矿物,锆、铪元素测定结果偏低。密闭消解法中使用王水代替硝酸进行残渣复溶,利用氯离子的络合作用,促进了铌钽锆铪和稀土等元素的复溶,由此建立了ICP-OES和ICP-MS测定花岗伟晶岩中稀有金属和稀土等32种元素的方法,应用于三种实际样品及稀有金属标准物质的测定,取得良好效果。
实验中发现,应用本文建立的硝酸-氢氟酸密闭溶矿法处理铌钽极高含量的矿样,如标准物质GWB07185,由于无法避免铌钽的水解,铌钽钨测定结果偏低,应当单独取样后采用碱熔法进行样品分解,化学法或者ICP-OES/MS进行测定。
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图 1 常见微束分析技术的分析体积对比(据Fougerouse等[14]修改)
Figure 1. Comparison of analysis volumes for common microanalysis techniques. Left side panel: each volume is represented on a schematic monazite grain (diagram to scale). APT is capable of measuring the isotopic compositions of minerals at the nanoscale for volumes <0.0007μm3 (Modified from Fougerouse, et al[14]).
图 2 原子探针工作原理及分析结果示意图
a—原子探针仪器工作原理图(据Gault等[33]修改);b—原子探针分析质量谱峰图(据Wu等[35]修改);c—原子探针样品元素分布图(据Wu等[35]修改)。
Figure 2. Schematic diagram of working principle and analysis results of APT.
a—Working principle of APT (Modified from Gault,et al[33]); b—Mass spectrum by APT (Modified from Wu,et al[35]); c—Distribution of elements by APT (Modified from Wu, et al[35]).
图 4 制备APT针尖样品流程(据Gault等[33]修改)
a—样品表面目标微区Pt气相沉积;b—切割目标微区;c—纳米操作手提取目标样品;d—将目标样品固定在APT专用的硅阵列尖端上;e—对样品进行环形切割和精细抛光。
Figure 4. Processes for the preparation of APT tip sample (Modified from Gault, et al[33]).
a—Pt deposition of target microzone on the sample surface; b—Cut the target microzone; c—Target sample is extracted by micromanipulator; d—Immobilize target sample on APT-specific silicon array tips; e—Circular cutting and fine polishing of target sample.
图 5 结构金和微纳米金颗粒
a—黄铁矿富砷边部的APT三维原子图,每个球体代表一个原子,红色为Au,绿色为As。Au原子均匀分布(据Gopon等[21]修改);b—毒砂中Au的APT三维原子图,每个球体代表一个Au原子。Au原子呈簇状分布,团簇之间相互分离。左上角插图为最大金团簇的俯视图(据Fougerouse等[22]修改)。
Figure 5. Structurally bound Au and discrete nanoparticles of Au.
a—APT 3D atom map of the arsenic-rich rim of Pyrite. Each sphere represents an atom, red is Au, green is As. The Au atoms are evenly distributed (Modified from Gopon,et al[21]). b—APT 3D atom map of the Au in arsenopyrite. Each sphere represents an Au atom. Au atoms are segregated in clusters. The top left illustration is 5nm slice through the largest Au cluster (Modified from Fougerouse, et al[22]).
图 6 锗在闪锌矿中不同的赋存状态(据Fougerouse等[23]修改)
a—Saint-Salvy样品的APT三维原子图,每个球体代表一个原子。Cu和Ge呈细条带状分布;b—Arre样品的APT三维原子图,每个球体代表一个原子。Cu和Ge不均匀分布,呈灰锗矿纳米颗粒。
Figure 6. Different occurrence states of Ge in sphalerite (Modified from Fougerouse, et al[23]).
a—APT 3D atom maps of the Saint-Salvy specimen M22. Each sphere represents an atom. Cu and Ge are distributed in fine bands. b—APT 3D atom maps of the Arre specimens M1, M7, M18 and M19. Each sphere represents an atom. The Cu and Ge distribution is heterogenous with nanoscale clusters of briartite (Cu2(Zn, Fe)GeS4).
图 7 低角度晶界的纳米级成像(据Fougerouse等[24]修改)
a、b—每个球体代表一个原子。低角度晶界面由近水平的富集Au、Ni、Cu和Bi的位错组成;c、d—砷的一维浓度曲线,只显示位错一侧或另一侧的亏损。at%为原子百分含量。
Figure 7. Nanoscale imaging of low-angle boundary (Modified from Fougerouse, et al)[24].
a, b—Each sphere represents an atom. Low-angle boundary plane is composed of sub-horizontal Au-, Ni-, Cu-, and Bi-enriched dislocations; c, d—One-dimensional (1-D) concentration profiles for As showing depletion on one side of dislocation or the other, exclusively. “at%” represents the atomic percent.
图 8 流体包裹体的纳米级成像(据Dubosq等[27]修改)
a—样品的APT三维重构图显示了Fe、As和O元素的组成,揭示了球状高密度特征(流体包裹体)和一个连接两个较大的流体包裹体的线性特征(位错);b—分别用沿x、y和z轴的As的二维等值线图裁剪位错周围的感兴趣区域,显示了沿位错及其连接的流体包裹体升高的As浓度;c—流体包裹体(X-X’)上的成分剖面证实了As和O含量的升高,并显示了内部的Na和K含量变化。at%为原子百分含量。
Figure 8. Nanoscale imaging of fluid inclusions (Modified from Dubosq, et al[27]).
a—Reconstruction of APT specimen displays the distribution of Fe (pink dots), As (turquoise isosurfaces) and O (red isosurfaces) compositions, revealing globular high-density features (fluid inclusions) and one linear feature linking two larger high-density features. b—Clipping of region of interest surrounding linear feature with 2D contour plots of As along x-, y-, and z-axes revealing elevated As concentration along the linear feature and linked high-density features. c—Composition profiles across a high-density feature (X-X') confirming the elevated As- and O-rich compositions and revealing Na and K within the feature. “at%” represents the atomic percent.
E.1. The working principle of APT and its representative applications in ore deposits research. a—Working principle of APT. Modified from Gault, et al[33]; b—APT 3D atom map of the Au in arsenopyrite. Each sphere represents an Au atom. Au atoms are segregated in clusters. The top left illustration is 5nm slice through the largest Au cluster. Modified from Fougerouse, et al[22]; c—Nanoscale imaging of fluid inclusions. Reconstruction of APT specimen displays the distribution of Fe (pink dots), As (turquoise isosurfaces) and O (red isosurfaces) compositions, revealing globular high-density features (fuid inclusions) and one linear feature linking two larger high-density features. Modified from Dubosq, et al[27]; d—APT datasets from R47_01719 chlorapatite. Selected isoconcentration surfaces (ICS) are shown for each whole dataset (Fe=blue, Cl=green, Mn=red). Modified from Darling, et al[31].
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