Influence of the X-site composition on tourmaline’s crystal structure: investigation of synthetic K-dravite, dravite, oxy-uvite, and magnesio-foitite using SREF and Raman spectroscopy
Influence of the X-site composition on tourmaline’s crystal structure: investigation of synthetic K-dravite, dravite, oxy-uvite, and magnesio-foitite using SREF and Raman spectroscopy
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DOI：
10.1007/s00269-015-0776-3
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发表时间：
2016-02
影响因子：
1.4
通讯作者：
Eleanor J. Berryman;B. Wunder;A. Ertl;A. Ertl;M. Koch‐Müller;D. Rhede;K. Scheidl;G. Giester;W. Heinrich
中科院分区：
文献类型：
--
作者：
Eleanor J. Berryman;B. Wunder;A. Ertl;A. Ertl;M. Koch‐Müller;D. Rhede;K. Scheidl;G. Giester;W. Heinrich
用偏振拉曼光谱、单晶结构精修（SREF）、X射线衍射（XRD）、X射线衍射（XRD）和X射线衍射（XRD）等方法研究了合成的钾镁矾[XKYMg 3 ZAl 6 TSi 6 O 18（BO 3）3V（OH）3 W（OH）]、镁矾[XNaYMg 3 ZAl 6 TSi 6 O 18（BO 3）3V（OH）3 W（OH）]、氧镁矾[XCaYMg 3 ZAl 6 TSi 6 O 18（BO 3）3V（OH）3 WO]和镁黄铁矿[X YMg 2Al）ZAl 6 TSi 6 O 18（BO 3）3V（OH）3 W（OH）]的晶体结构，和粉末X射线衍射。利用电子探针分析，其特征在于组成简单的电气石便于确定的SREF和带分配的拉曼光谱中的占位。合成的钾镁铁矾、氧镁矾和镁铁矾在其八面体位置之间具有显著的Mg-Al无序，这由它们各自的平均键长<Y-O>和<Z-O>键长表示。利用氧镁橄榄石（YMg_（1.52）Al_（1.48）（10）和ZAl_（4.90）Mg_（1.10）（15））和镁黄铁矿（YAl_（1.62）Mg_（1.38）（18）和ZAl_（4.92）Mg_（1.08）（24））的Y位和Z位的电子密度，对它们的Y位和Z位组成进行了细化。Y和Z位的Mg-Al比也可以从O-H伸缩振动范围（3250-3850 cm-1）内的拉曼谱带的相对积分峰强度确定，产生的值与SREF数据非常一致。电气石的晶胞体积从镁黄铁矿（1558.4（3）× 10 ~ 3）→镁钠闪石（1569.5（4）~ 1571.7（3）× 10 ~ 3）→氧镁钠闪石（1572.4（2）× 10 ~ 3）→钾镁钠闪石（1588.1（2）× 10 ~ 3）逐渐增大，这主要是由于c轴的拉长。X位配位多面体的尺寸从镁橄榄石（Na）到K-镁橄榄石（K）的增加在晶体结构中被局部地容纳，导致相邻的O 1-H1键缩短。在氧镁矾中，Ca ~（2+）与去质子化的W（O_1）位点局部缔合，而空位X位点则被质子化的W（O_1）位点取代。增加X位占据离子的尺寸并不会明显影响其他位置之间的键合;然而，在氧镁矾中，Ca和去质子化的W（O 1）位置的更高电荷与晶格振动拉曼光谱（100-1200 cm−1）的变化相关，特别是对于T6 O 18环的谱带。镁菱铁矿的拉曼光谱显示出在晶格和O-H伸缩振动范围（分别为100-1200和3250-3850 cm-1）上与钾镁镁橄榄石、镁橄榄石和氧镁橄榄石的显著偏离。空位X位点与晶体结构中的长程和短程变化相关，即，T_6O_（18）环的变形和O_1-H_1和O_3-H_3键的延长。然而，在K-镁电气石，镁电气石，和oxy-uvite的X-位空缺的结果，只有在延长的neighboringO 1-H1键，并没有导致可识别的变化，在晶格键合环境。
The crystal structures of synthetic K-dravite [XKYMg3ZAl6TSi6O18(BO3)3V(OH)3W(OH)], dravite [XNaYMg3ZAl6TSi6O18(BO3)3V(OH)3W(OH)], oxy-uvite [XCaYMg3ZAl6TSi6O18(BO3)3V(OH)3WO], and magnesio-foitite [X☐Y(Mg2Al)ZAl6TSi6O18(BO3)3V(OH)3W(OH)] are investigated by polarized Raman spectroscopy, single-crystal structure refinement (SREF), and powder X-ray diffraction. The use of compositionally simple tourmalines characterized by electron microprobe analysis facilitates the determination of site occupancy in the SREF and band assignment in the Raman spectra. The synthesized K-dravite, oxy-uvite, and magnesio-foitite have significant Mg–Al disorder between their octahedral sites indicated by their respective average <Y–O> and <Z–O> bond lengths. TheY-andZ-site compositions of oxy-uvite (YMg1.52Al1.48(10)andZAl4.90Mg1.10(15)) and magnesio-foitite (YAl1.62Mg1.38(18)andZAl4.92Mg1.08(24)) are refined from the electron densities at each site. The Mg–Al ratio of theYandZsites is also determined from the relative integrated peak intensities of the Raman bands in the O–H stretching vibrational range (3250–3850 cm−1), producing values in good agreement with the SREF data. The unit cell volume of tourmaline increases from magnesio-foitite (1558.4(3) Å3) to dravite (1569.5(4)–1571.7(3) Å3) to oxy-uvite (1572.4(2) Å3) to K-dravite (1588.1(2) Å3), mainly due to lengthening of the crystallographicc-axis. The increase in the size of theX-site coordination polyhedron from dravite (Na) to K-dravite (K) is accommodated locally in the crystal structure, resulting in the shortening of the neighboringO1–H1bond. In oxy-uvite, Ca2+is locally associated with a deprotonatedW (O1)site, whereas vacantXsites are neighbored by protonatedW(O1) sites. Increasing the size of theX-site-occupying ion does not detectably affect bonding between the other sites; however, the higher charge of Ca and the deprotonatedW(O1) site in oxy-uvite are correlated to changes in the lattice vibration Raman spectrum (100–1200 cm−1), particularly for bands assigned to theT6O18ring. The Raman spectrum of magnesio-foitite shows significant deviations from those of K-dravite, dravite, and oxy-uvite in both the lattice and O–H stretching vibrational ranges (100–1200 and 3250–3850 cm−1, respectively). The vacantXsite is correlated with long- and short-range changes in the crystal structure, i.e., deformation of theT6O18ring and lengthening of theO1–H1andO3–H3bonds. However,X-site vacancies in K-dravite, dravite, and oxy-uvite result only in the lengthening of the neighboringO1–H1bond and do not result in identifiable changes in the lattice-bonding environment.