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首页> 外文期刊>The American mineralogist >New insight into the structural transformation of partially dehydroxylated pyrophyllite
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New insight into the structural transformation of partially dehydroxylated pyrophyllite

机译：对部分脱羟基叶蜡石结构转变的新见解

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Two pyrophyllite samples, one S037, from the Coromandel region of New Zealand and the other, S076, from Berosovska, Ural, Russia, were studied by thermogravimetric (TG and DTG), infrared (IR), and X-ray diffraction (XRD) methods to investigate structural transformations of these samples at different stages of their partial dehydroxylation. The samples were heated at different temperatures during 45 min and the degree of dehydroxylation was estimated as a ratio of mass loss of each particular heated specimen to the total mass loss of the sample during total dehydroxylation. Sample S076 consists only of trans-vacant (tv) layers because its DTG curve and IR spectrum contain a single dehydroxylation maximum at Tmax = 723 ?C and an OH stretching band 3675 cm-1, respectively. In sample S037 tv and cis-vacant (cv) layers are interstratified at random and its DTG and IR spectrum contain, respectively, two dehydroxylation maxima at 595 and 760 ?C and two stretching bands at 3675 and 3668 cm-1. The positions and intensities of the reflections in the experimental powder XRD patterns of sample S037, as well as the refined parameters of the unit cell, almost coincide with those determined for 1A pyrophyllite. However, the XRD patterns contain an "additional" peak with d = 4.454(6) ?, which is absent in normal 1A pyrophyllite. This peak can be considered as an indicator of a structure in which tv and cv layers are interstratified. The XRD patterns from the oriented specimens of the studied samples heated above 500 ?C show splitting of the basal reflections. Simulation of the XRD patterns from oriented specimens of the S076 sample show that the phase composition of the specimens is the same independent of the heating temperature and represents a physical mixture of a non-treated original pyrophyllite and an almost completely dehydroxylated phase, which contains 5% of non-dehydroxylated layers. The higher the temperature, the higher the content of the dehydroxylated-rich phase in the heated sample. Simulation of the XRD patterns from the heated S037 samples show that the phase composition of each specimen is a physical mixture of low- and high-dehydroxylation phases, referred to as LD and HD phases, respectively. Each of these phases is a mixed-layered structure in which the original non-dehydroxylated (ND) and completely dehydroxylated (CD) layers are interstratified at random. The main difference between these phases is that in the LD phase the content of ND layers prevail, whereas in the HD phase CD layers dominate. The increase in temperature and degree of dehydroxylation change the relative content of the LD and HD phases as well as the relative amount of ND and CD layers in these phases. Interstratification of ND and CD layers in the LD and HD phases is not consistent with the commonly accepted model according to which during dehydroxylation of pyrophyllite and related layer silicates, non-dehydroxylated and dehydroxylated domains can coexist within one layer. As a result of this inconsistency, the model explaining the wide temperature interval of pyrophyllite dehydroxylation needs to be reconsidered. A similar inconsistency arises with the model predicting the formation of several intermediate semi-dehydroxylated structures during pyrophyllite dehydroxylation because evidence for such phases was not observed. A new model of pyrophyllite dehydroxylation is presented, which is consistent with the hypothesis, that the reaction is homogeneous and spontaneous nucleation and growth of completely dehydroxylated layers takes place during pyrophyllite dehydroxylation. In terms of this model, the large temperature interval of pyrophyllite dehydroxylation is related to particle size distribution as well as to structural disorder. A mechanism for the formation of the mixed-layered structures of the LD and HD phases is proposed.
机译：通过热重分析（TG和DTG），红外（IR）和X射线衍射（XRD）研究了两个叶蜡石样品，一个是来自新西兰Coromandel地区的S037，另一个是来自俄罗斯乌拉尔州Berosovska的S076。种方法来研究这些样品在部分脱羟基不同阶段的结构转变。将样品在不同的温度下加热45分钟，然后将脱羟基程度估计为每个特定加热样品的质量损失与样品在总脱羟基过程中的总质量损失之比。样品S076仅由透空（tv）层组成，因为其DTG曲线和IR光谱分别在Tmax = 723°C和OH拉伸带3675 ​​cm-1处包含单个脱羟基最大值。在样品S037中，电视和顺式（cv）层是随机分层的，其DTG和IR光谱分别在595和760°C下包含两个最大的脱羟基作用，在3675和3668 cm-1上包含两个拉伸带。样品S037的实验粉末XRD图案中反射的位置和强度，以及晶胞的精确参数，几乎与针对1A叶蜡石确定的那些一致。但是，XRD图谱包含一个d = 4.454（6）？的“附加”峰，在正常的1A叶蜡石中是不存在的。该峰可以被认为是tv和cv层被层化的结构的指标。加热到500°C以上的研究样品的定向样品的XRD图谱显示出基础反射的分裂。对来自S076样品的定向样品的XRD图谱的模拟显示，样品的相组成与加热温度无关，是相同的，代表未经处理的原始叶蜡石与几乎完全脱羟基的相的物理混合物，其中包含5非脱羟基层的％。温度越高，加热的样品中富含脱羟基的相的含量越高。对来自加热的S037样品的XRD图谱的模拟表明，每个样品的相组成是低脱羟基相和高脱羟基相的物理混合物，分别称为LD和HD相。这些相中的每一个都是混合的层结构，其中原始的非脱羟基（ND）和完全脱羟基（CD）层随机分层。这些阶段之间的主要区别在于，在LD阶段中，ND层的含量占优势，而在HD阶段中，CD层占主导。温度和脱羟基程度的增加改变了LD和HD相的相对含量以及这些相中ND和CD层的相对含量。 LD和HD相中ND和CD层的互层化与普遍接受的模型不一致，根据该模型，在叶蜡石和相关层硅酸盐的脱羟基过程中，非脱羟基和脱羟基域可共存于一层内。由于这种不一致的结果，需要重新考虑解释叶蜡石脱羟基的宽温度区间的模型。预测叶蜡石脱羟基过程中几个中间半脱羟基结构形成的模型也出现类似的不一致，因为未观察到此类相的证据。提出了一种叶蜡石脱羟基的新模型，该模型与以下假设相一致：在叶蜡石脱羟基过程中，反应是均匀的，自发成核和完全脱羟基层的生长。根据该模型，叶蜡石脱羟基的较大温度间隔与粒径分布以及结构紊乱有关。提出了形成LD相和HD相的混合层结构的机理。

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