风化作用对铁陨石矿物学特征影响的探讨

本文对南丹IIICD铁陨石的矿物学特征进行了研究,并与同为铁陨石但化学分类不同的阿根廷IAB铁陨石和西伯利亚IIB铁陨石进行了对比,重点探讨了风化作用对铁陨石矿物学特征的影响.首先用偏光显微镜、静水称重、扫描电镜观察了样品的基本矿物学特征和微形貌特征,然后用振动式样品磁强计、X射线衍射与电子探针能谱半定量测试研究了样品的磁学性质、物相和化学组成.研究结果表明,南丹铁陨石在较强的自然风化作用下,光泽变弱为土状光泽,相对密度降低;风化产生的反铁磁性物质会使陨石的磁性下降;另外,样品表面物相组成也发生较大变化,以针铁矿(FeOOH)和磁铁矿(Fe₃O₄)等铁的次生矿物为主;但风化壳以下的矿物物相及化学成分均未发生明显变化,以Fe、Ni为主的铁纹石、镍纹石物相存在.     

Tilting and distortion of CaSnO<Subscript>3</Subscript> perovskite to 7 GPa determined from single-crystal X-ray diffraction

The structural changes of CaSnO₃, a GdFeO₃-type perovskite, have been investigated to 7 GPa in a diamond-anvil cell at room temperature using single-crystal X-ray diffraction. Significant changes are observed in both the octahedral Sn–O bond lengths and tilt angles between the SnO6 octahedra. The octahedral (SnO6) site shows anisotropic compression and consequently the distortion of SnO6 increases with pressure. Increased pressure also results in a decrease of both of the inter-octahedral angles, Sn–O1–Sn and Sn–O2–Sn, indicating that octahedral tilting increases with increasing pressure, chiefly equivalent to rotation of the SnO6 octahedra about the pseudocubic <001>_p axis. The distortion in the CaO12 and SnO6 sites, along with the octahedral SnO6 tilting, is attributed to the SnO6 site being less compressible than the CaO12 site.Acknowledgments The authors acknowledge with gratitude the financial support for this work from NSF grant EAR-0105864. Ruby pressure measurements were conducted with the Raman system in the Vibrational Spectroscopy Laboratory in the Department of Geosciences at Virginia Tech with the help of Mr. Charles Farley.     

Synchrotron X-ray analysis of norbergite, Mg2.98Fe0.01Ti0.02Si0.99O4(OH0.31F1.69) structure at high pressure up to 8.2?GPa

The natural norbergite, Mg_2.98Fe_0.01Ti_0.02Si_0.99O₄(OH_0.31F_1.69) is examined by synchrotron X-ray diffraction analysis at pressures up to 8.2 GPa. The measured linear compressibilities of the crystallographic axes are β _a  = 2.18(4) × 10⁻³, β _b  = 2.93(7) × 10⁻³, and β _c  = 2.77(7) × 10⁻³ (GPa⁻¹), respectively and the calculated isothermal bulk modulus of the norbergite is K _T = 113(2) GPa based on the Birch–Murnaghan equation of state assuming a pressure derivative of K′ = 4. The crystal structures of norbergite are refined at room temperature and pressures of 4.7, 6.3, and 8.2 GPa, yielding R values for the structure refinements of 4.6, 5.3, and 5.3%, respectively. The bulk moduli of the polyhedral sites are 293(15) GPa for the tetrahedron, 106(5) GPa for the M2 octahedron, 113(2) GPa for the M3 octahedron, and 113(3) GPa for the total void space. The bulk modulus exhibits a good linear correlation with the filling factor for polyhedral sites in structures of the humite minerals and forsterite, reflecting the Si⁴⁺ + 4O²⁻ ⇔ □ + 4(OH, F)⁻ substitution in the humite minerals. Moreover, two simply linear trends were observed in the relationship between bulk modulus and packing index for natural minerals and dense hydrous magnesium silicate minerals. This relationship would reflect that the differences in compression mechanism were involved with hydrogen bonding in these minerals. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

SrTiO3基钙钛矿结构X射线衍射谱特征

通过对常规电子陶瓷工艺合成的Sr1-xCaxTiO3和Sr1-xPbxTiO3钙钛矿结构陶瓷进行研究X射线衍射谱特征、晶体结构和衍射强度I100/I110进行研究,并根据原子散射因子与衍射强度关系,提出用衍射强度I100/I110变化规律预测ABO3钙钛矿结构中置换A位离子种类的方法.     

Structural and vibrational behaviour of fluorapatite with pressure. Part I: in situ single-crystal X-ray diffraction investigation

Using single-crystal X-ray diffraction from a diamond anvil cell, the compressibility of a synthetic fluorapatite was determined up to about 7?GPa. The compression pattern was anisotropic, with greater change along a than c. Unit cell parameters varied linearly with β _a =3.32(8)?10^?3 and β _c =2.40(5)?10^?3 GPa^?1, giving a ratio β _a :β _c =1.38:1. Data fitted with a third-order Birch-Murnaghan EOS yielded a bulk modulus of K ₀=93(4)?GPa with K′=5.8(1.8). The evolution of the crystal structure of fluorapatite was analysed using data collected at room pressure, at 3.04 and 4.72?GPa. The bulk modulus of phosphate tetrahedron is about three times greater than the bulk modulus of calcium polyhedra. The values were 270(10), 100(4) and 86(3) GPa for P, Ca1 (nine-coordinated) and Ca2 (seven-coordinated) respectively. While the calcium polyhedra became more regular with pressure, the distortion of the phosphate tetrahedron remained unchanged. The size of the channel extending along the [001] direction represented the most compressible direction. The Ca2–Ca2 distance decreased from 3.982 to 3.897?Å on compression from 0.0001 to 4.72?GPa. The anisotropic compressional pattern may be understood in terms of the greater compressibility of the channel size over the polyhedral units. The reduction of the channel volume was measured by the evolution of the trigonal prism, having the Ca2–Ca2–Ca2 triangle as its base and the c lattice parameter as its height. This prism volume changed from 47.3?ų at room pressure to 44.78?ų at 4.72?GPa. Its relatively high bulk moduli, 86(3) GPa, indicated that the channel did not collapse with pressure and the apatite structure could remain stable at very high pressure.     

X射线粉晶衍射仪测定萤石晶胞参数

利用X射线粉晶衍射仪对纯萤石矿物进行全谱扫描,在确定萤石矿物衍射5强峰基础上,准确标出萤石5强峰相应的d值,按2θ角值由大到小的顺序排成纵行,计算出不同d值的1/d2值,用1/d2值最小一条线的值去除其余各条线的1/d2值,得出的数值D仍依次排列;据D值小数部分是0.36或0.68数值,推断实验所测萤石低角度的第一条线是{111}网面衍射产生的,其相应的h2+k2+l2值为3,以第一条线h2+k2+l2值3分别乘以其余各条线的D值,得出的数据h2+k2+l2值仍依次排列,分别为8.04,11.04,16.08和19.08,由于测量的误差,舍去小数点后数据,保留整数位,即可得到相应的h2+k2+l2值3、8、11、16和19,据h2+k2+l2值查出相应网面指数为{111}、{220}、{311}、{400}和{331},根据晶胞参数公式:a=d(hkl)(h2+k2+l2)1/2计算出萤石晶胞参数值为0.546 nm。     

Variable190Os/184Os ratio in acid residues of iron meteorites

In residual materials obtained on dissolution of iron meteorites in 2M H₂SO₄, the ratio of¹⁹⁰Os/¹⁸⁴Os has been measured by radiochemical neutron activation analysis. Most residues have a normal isotopic ratio (to within ±2%). However, in some residues both positive and negative deviations in the isotopic ratio are seen. The most spectacular deviations are in the insoluble fragments (nuggets) from Sikhote Alin iron meteorite where the¹⁹⁰Os/¹⁸⁴Os ratio is about 50% of the normal value. The new results confirm our earlier observations that iron meteorites contain pre-solar grains.     

Crystal structure and compression of an iron-bearing Phase A to 33 GPa

Fe-bearing dense hydrous magnesium silicate Phase A, Mg_6.85Fe_0.14Si_2.00O₈(OH)₆ has been studied by single-crystal X-ray diffraction at ambient conditions and by high-pressure powder diffraction using synchrotron radiation to 33 GPa. Unit cell parameters at room temperature and pressure from single crystal diffraction are a=7.8678 (4) Å, c=9.5771 (5) Å, and V=513.43 (4) ų. Fitting of the P–V data to a third-order Birch-Murnaghan isothermal equation of state yields V ₀=512.3 (3) ų, K _T,0=102.9 (28) GPa and K′=6.4 (3). Compression is strongly anisotropic with the a-axes, which lie in the plane of the distorted close-packed layers, approximately 26% more compressible than the c-axis, which is normal to the plane. Structure refinement from single-crystal X-ray intensity data reveals expansion of the structure with Fe substitution, mainly by expansion of M-site octahedra. The short Si2–O6 distance becomes nearly 1% shorter with ~2% Fe substitution for Mg, possibly providing additional rigidity in the c-direction over the Mg end member. K _T obtained for the Fe-bearing sample is ~5.5% greater than reported previously for Fe-free Phase A, despite the larger unit cell volume. This study represents a direct comparison of structure and K _T–ρ relations between two compositions of a F-free dense hydrous magnesium silicate (DHMS) phase, and may help to characterize the effect of Fe substitution on the properties of other DHMS phases from studies of the Fe-free end-members.     

Experimental study of the bcc-fcc phase transformations in the Fe-rich system Fe-Si at high pressures

In situ X-ray diffraction experiments at high pressure were carried out up to 8.9 GPa and 1100 °C to study phase transformations of iron and two iron-silicon alloys Fe_0.91Si_0.09 and Fe_0.83Si_0.17. For iron, the transformation from the bcc phase to the fcc phase was observed at pressures 3.8–8.2 GPa and temperatures that are consistent with previous in situ X-ray diffraction studies. Reversal of the transformation of iron was found to be sensitive to temperature; hysteresis of the transformation increased from 25 °C at 3.8 GPa to 100 °C at 7.0 GPa, primarily because the bcc-fcc phase boundary has a negative Clayperon slope. In the binary system Fe-Si, the observations of the present study indicate that the ferrite (bcc phase)-stabilizing behavior of silicon persists at high pressures and that the maximum solubility of silicon in the fcc phase increases with increasing pressure: (1) the transformation from the bcc phase to the fcc phase was observed in Fe_0.91Si_0.09 at 6.0, 7.4 and 8.9 GPa and the temperatures measured at the onset of the transformations were 300 °C higher than those in iron at similar pressures, (2) the transformation rate in Fe_0.91Si_0.09 was extremely sluggish compared to that of iron, and (3) the bcc-fcc phase transformation was not observed in Fe_0.91Si_0.09 at 4.7 GPa up to 1000 °C and in Fe_0.83Si_0.17 at 8.2 GPa and 1100 °C. Received: 1 June 1998 / Revised, accepted: 9 October 1998     

Silicate-bearing iron meteorites and their implications for the evolution of asteroidal parent bodies

Silicate-bearing iron meteorites differ from other iron meteorites in containing variable amounts of silicates, ranging from minor to stony-iron proportions (∼50%). These irons provide important constraints on the evolution of planetesimals and asteroids, especially with regard to the nature of metal–silicate separation and mixing. I present a review and synthesis of available data, including a compilation and interpretation of host metal trace-element compositions, oxygen-isotope compositions, textures, mineralogy, phase chemistries, and bulk compositions of silicate portions, ages of silicate and metal portions, and thermal histories. Case studies for the petrogeneses of igneous silicate lithologies from different groups are provided. Silicate-bearing irons were formed on multiple parent bodies under different conditions. The IAB/IIICD irons have silicates that are mainly chondritic in composition, but include some igneous lithologies, and were derived from a volatile-rich asteroid that underwent small amounts of silicate partial melting but larger amounts of metallic melting. A large proportion of IIE irons contain fractionated alkali-silica-rich inclusions formed as partial melts of chondrite, although other IIE irons have silicates of chondritic composition. The IIEs were derived from an H-chondrite-like asteroid that experienced more significant melting than the IAB asteroid. The two stony-iron IVAs were derived from an extensively melted and apparently chemically processed L or LL-like asteroid that also produced a metallic core. Ungrouped silicate-bearing irons were derived from seven additional asteroids. Hf–W age data imply that metal–silicate separation occurred within 0–10 Ma of CAI formation for these irons, suggesting internal heating by ²⁶Al. Chronometers were partly re-set at later times, mainly earlier for the IABs and later for the IIEs, including one late (3.60 ± 0.15 Ga) strong impact that affected the “young silicate” IIEs Watson (unfractionated silicate, and probable impact melt), Netschaëvo (unfractionated, and metamorphosed), and Kodaikanal (fractionated). Kodaikanal probably did not undergo differentiation in this late impact, but the similar ages of the “young silicate” IIEs imply that relatively undifferentiated and differentiated materials co-existed on the same asteroid. The thermal histories and petrogeneses of fractionated IIE irons and IVA stony irons are best accommodated by a model of disruption and reassembly of partly molten asteroids.     

Thermal expansion and crystal structure of FeSi between 4 and 1173 K determined by time-of-flight neutron powder diffraction

The thermal expansion and crystal structure of FeSi has been determined by neutron powder diffraction between 4 and 1173?K. No evidence was seen of any structural or magnetic transitions at low temperatures. The average volumetric thermal expansion coefficient above room temperature was found to be 4.85(5)?×?10^?5?K^?1. The cell volume was fitted over the complete temperature range using Grüneisen approximations to the zero pressure equation of state, with the internal energy calculated via a Debye model; a Grüneisen second-order approximation gave the following parameters: θ_D=445(11)?K, V ₀=89.596(8)?ų, K ₀′=4.4(4) and γ′=2.33(3), where θ_D is the Debye temperature, V ₀ is V at T=0?K, K ₀′ is the first derivative with respect to pressure of the incompressibility and γ′ is a Grüneisen parameter. The thermodynamic Grüneisen parameter, γ_th, has been calculated from experimental data in the range 4–400?K. The crystal structure was found to be almost invariant with temperature. The thermal vibrations of the Fe atoms are almost isotropic at all temperatures; those of the Si atoms become more anisotropic as the temperature increases.     

Rhombohedral ilmenite group nickel titanates with Zn, Mg, and Mn: synthesis and crystal structures

Binary, ternary, and quaternary rhombohedral ordered titanates, Ni_1/2Mn_1/2TiO₃, Ni_1/2Mg_1/2TiO₃, Ni_1/3Zn_1/3Mg_1/3TiO₃, and Ni_1/4Zn_1/4Mg_1/4Mn_1/4TiO₃, were obtained by solid-state synthesis at 1095°C at ambient pressure in a nitrogen atmosphere. All of the compounds adopt ATiO₃ (A = Ni, Mn, Zn, and Mg) stoichiometry. Crystal structures were refined by the Rietveld method from powder X-ray diffraction data. Unit cell parameters and unit cell volumes decrease with decreasing average radius of the ^vi A ²⁺ cation. All the synthetic titanates adopt the space group and the ilmenite structure consisting of distorted AO₆ and TiO₆ octahedra. The divalent cations and Ti⁴⁺ are distributed in layers of octahedra alternating along c with no evidence for disorder. In common with pyrophanite, NiTiO₃, and ilmenite sensu stricto, the distortion of the AO₆ octahedra is less than that of the TiO₆ octahedra. The Ti⁴⁺ and A-site cations in the titanates are off-centred within the coordination polyhedra. Deviation of the z positional parameters from their theoretical values for the A and Ti atoms indicate that in the titanates with the larger A ²⁺ cations and Goldschmidt tolerance factors, t ≥ 0.745, the AO₆ octahedral layer is more “puckered” above and below planes parallel to (001) than that of the TiO₆ octahedra, and vice versa in the titanates with smaller R _A ²⁺ for which t≤0.745. Data are given for the volumes and distortion indices of all the coordination polyhedra. This study confirms the existence and stability of complex solid solutions between ordered rhombohedral titanates of Ni and first-row transition metals at ambient conditions over a range of t from 0.786 to 0.737. These experimental data suggest that the formation of ilmenite-type titanates enriched in Ni is possible in exotic mineral-forming systems at low pressure and/or in extraterrestrial rocks.     

Iron-water reaction at high pressure and temperature, and hydrogen transport into the core

We observed a direct reaction of metallic iron with water to form iron hydride and iron oxide, 3Fe + H₂O–>2FeH_x + FeO, at pressures from 6 GPa to 84 GPa and temperatures above 1,000 K in diamond anvil cell (DAC). Iron hydride is dhcpFeH_x or -FeH_x, and iron oxide has the rhombohedral or B1 structure at pressures at least up to 37 GPa. The formation of an assembly composed of dhcpFeH_x and FeO with the B8 structure was observed at 84 GPa. In primordial Earth, water formed by dehydration of the low temperature primitive materials reacts with metallic iron in the high temperature component to form iron hydride FeH_x and iron oxide FeO. The former would be incorporated in the iron forming the core. Thus hydrogen could be an important element of the Earths core. This reaction would be essential for transport of hydrogen into the core in the accretion stage of the Earth.     

Neutron diffraction study of aluminous hydroxide δ-AlOOD

We have determined the position of deuterium atoms in δ-AlOOD by neutron powder diffraction at ambient pressure. As previously reported by theoretical and experimental studies, the deuterium atoms are located in the tunnel formed by the chains of AlO₆ octahedra. The data are best fit with the P2₁ nm structure, producing bond lengths of D–O1 of 1.552(2) Å, O2–D of 1.020(2) Å and O1–O2 of 2.571(2). This study confirms that the hydrogen bond is asymmetric at ambient conditions in agreement with recent single-crystal synchrotron study for δ-AlOOH.     

Tantalum-bearing titanite: synthesis and crystal structure data

Synthetic titanite, CaTiOSiO₄, and the series of (Ca_1−x Na _x )(Ti_1−x Ta _x )OSiO₄ and Ca(Ti_1−2x Ta _x Al _x )OSiO₄ solid solutions have been prepared by ceramic methods, and their crystal structure determined by the Rietveld analysis. At ambient conditions, titanite can contain up to 20 mol% NaTaOSiO₄ or 60 mol% Ca(Al_0.5Ta_0.5)OSiO₄. These limits might differ in natural samples due to combination with substitutions involving fluorine and/or hydroxyl replacing oxygen together with vacancies at cationic sites. All cations located at the ^vii X- and ^vi Y-sites in the structures of tantalian titanite are disordered. Expansion of the <Si–O> bond from 1.618 to 1.621 Å in CaTi_0.8Ta_0.1Al_0.1OSiO₄ and CaTi_0.6Ta_0.2Al_0.2OSiO₄ to 1.644 Å in the CaTi_0.4Ta_0.3Al_0.3OSiO₄ titanite suggests the possible presence of some Al³⁺ in the tetrahedral site replacing Si⁴⁺ in the latter. All tantalian titanites crystallize in the space group A2/a. This implies that both single-site and complex double-site substitutional schemes induce P2₁/a → A2/a phase transition(s). The (Ca_1−x Na _x )(Ti_1−x Ta _x )OSiO₄ substitution scheme incorporates larger cations at both the ^vii X and ^vi Y sites, whereas the Ca(Ti_1−2x Ta _x Al _x )OSiO₄ scheme involves only ^vi Y-site (Al³⁺,Ta⁵⁺) cations with a slightly smaller “average” radius. Unit cell dimensions change insignificantly or increase incrementally with increase of average cationic radii in the (Ca_1−x Na _x )(Ti_1−x Ta _x )OSiO₄ series, and with an insignificant decrease in the ^viR _Y average cationic radii in the Ca(Ti_1−2x Ta _x Al _x )OSiO₄ series. Both Ta-doped titanite and CaTiOSiO₄ consist of distorted polyhedra with the XO₇, YO₆ coordination polyhedra and the SiO₄ tetrahedron in tantalian titanite being less distorted compared to those of the pure CaTiOSiO₄.     

爽身粉中石棉鉴定方法的研究

利用X射线衍射分析结合偏光显微镜观察,通过特征衍射峰和纤维形态可以判断试样中石棉的存在,并可判断石棉类型。该方法用于爽身粉中石棉的鉴定得到满意的结果。     

衍射指标h和k的互换问题

通过衍射线指标和面网指标的联系讨论了晶体的对称性对衍射指标h和k互换的影响。在下列情况下衍射指标h和k是不能互换的:在低级晶族;在四方晶系C_(4h)—4/m劳厄点群,六方品系C_(6h)—6/m劳厄点群和立方晶系T_h—m3劳厄点群对于hko和kkl型衍射;在三方晶系选取六方晶胞的情况下,在C_(3i)—3劳厄点群对于hol,okl,hko和hkl型衍射,在D_(3d)—3m劳厄点群对于holokl和hkl型衍射,以及在菱面体点阵除非h和k都是三的倍数。     

高压下天然菱铁矿的压缩性和电子结构研究

采用同步辐射光源和金刚石对顶砧(DAC)技术,对天然菱铁矿的压缩性和电子结构进行了原位X射线衍射(XRD)和X射线吸收近边结构谱(XANES)测试研究。在室温下随着压力逐渐升高至50.2 GPa,菱铁矿保持方解石型结构不变,但是逐渐向Na Cl型结构转变;刚性[CO3]2-基团平行于ab-平面定向排列使c轴的压缩性大于a轴。菱铁矿在44.6~47.1 GPa之间发生电子由高自旋态(HS)向低自旋态(LS)的转变,表现为体积塌陷8%。HS菱铁矿的等温状态方程参数为K0=112(5)GPa和K'0=4.6(3)。首次采用XANES技术对菱铁矿中Fe2+的电子结构进行了研究,结果表明:随着压力升高至37.3 GPa,Fe2+的配位和局域对称并未发生明显变化;此后电子结构开始转变,Fe2+的3d轨道分裂能降低,电子跃迁概率增大,呈现LS特性。     

Pressure-induced phase transition in synthetic trioctahedral Rb-mica

 The crystal structure of a synthetic Rb analog of tetra-ferri-annite (Rb–TFA) 1M with the composition Rb_0.99Fe²⁺ _3.03(Fe³⁺ _1.04 Si_2.96)O_10.0(OH)_2.0 was determined by the single-crystal X-ray diffraction method. The structure is homooctahedral (space group C2/m) with M1 and M2 occupied by divalent iron. Its unit cell is larger than that of the common potassium trioctahedral mica, and similar lateral dimensions of the tetrahedral and octahedral sheets allow a small tetrahedral rotation angle α=2.23(6)°. Structure refinements at 0.0001, 1.76, 2.81, 4.75, and 7.2 GPa indicate that in some respects the Rb–TFA behaves like all other micas when pressure increases: the octahedra are more compressible than the tetrahedra and the interlayer is four times more compressible than the 2:1 layer. However, there is a peculiar behavior of the tetrahedral rotation angle α: at lower pressures (0.0001, 1.76, 2.81 GPa), it has positive values that increase with pressure [from 2.23(6)° to 6.3(4)°] as in other micas, but negative values −7.5(5)° and −8.5(9)° appear at higher pressures, 4.75 and 7.2 GPa, respectively. This structural evidence, together with electrostatic energy calculations, shows that Rb–TFA has a Franzini A-type 2:1 layer up to at least 2.81 GPa that at higher pressure yields to a Franzini B-type layer, as shown by the refinements at 4.75 and 7.2 GPa. The inversion of the α angle is interpreted as a consequence of an isosymmetric displacive phase transition from A-type to B-type structure between 2.81 and 4.75 GPa. The compressibility of the Rb–TFA was also investigated by single-crystal X-ray diffraction up to a maximum pressure of 10 GPa. The lattice parameters reveal a sharp discontinuity between 3.36 and 3.84 GPa, which was associated with the phase transition from Franzini-A to Franzini-B structure. Received: 21 October 2002 / Accepted: 25 February 2003     

Crystal chemistry of titanite-structured compounds: the CaTi1-x Zr x OSiO4 (x≤0.5) series

Inhomogeneous aggregates of late-stage titanite enriched in Zr have been described recently from post-magmatic parageneses in silica-undersaturated rocks. In the natural samples, simple isovalent substitution of the large Zr (^[vi]R⁴⁺=0.72 Å) for Ti (^[vi]R⁴⁺=0.605 Å) is limited to an empirical maximum of 0.25 afu (15.3 wt.% ZrO₂). As the natural material is not suitable for crystallographic study, a series of CaTi_1-xZr_xOSiO₄ titanite samples have been synthesized by standard ceramic methods at ambient pressure in air, and their crystal structure determined by Rietveld refinement of laboratory powder X-ray diffraction patterns. All of the synthetic Zr-doped titanite varieties adopt space group A2/a and consist of distorted CaO₇ polyhedra together with less distorted (Ti_1-xZr_x)O₆ octahedra and SiO₄ tetrahedra. Cell dimensions and atomic coordinates together with volumes and distortion indices are given for all polyhedra. The empirical limit for Zr substitution in synthetic (F,OH)-free titanite is 0.5 afu (29.6 wt.% ZrO₂). The existence of a Zr analogue of titanite in nature is considered to be unlikely.