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1.
Cleaved and mechanically polished surfaces of olivine from peridotite xenoliths from San Carlos, Arizona, were chemically etched using the techniques of Wegner and Christie (1974). Dislocation etch pits are produced on all surface orientations and they tend to be preferentially aligned along the traces of subgrain boundaries, which are approximately parallel to (100), (010), and (001). Shallow channels were also produced on (010) surfaces and represent dislocations near the surface that are etched out along their lengths. The dislocation etch channel loops are often concentric, and emanate from (100) subgrain boundaries, which suggests that dislocation sources are in the boundaries. Data on subgrain misorientation and dislocation line orientation and arguments based on subgrain boundary energy minimization are used to characterize the dislocation structures of the subgrain boundaries. (010) subgrain boundaries are of the twist type, composed of networks of [100] and [001] screw dislocations. Both (100) and (001) subgrain boundaries are tilt walls composed of arrays of edge dislocation with Burgers vectors b=[100] and [001], respectively. The inferred slip systems are {001} 〈100〉, {100} 〈001〉, and {010} 〈100〉 in order of diminishing importance. Exploratory transmission electron microscopy is in accord with these identifications. The flow stresses associated with the development of the subgrain structure are estimated from the densities of free dislocations and from the subgrain dimensions. Inferred stresses range from 35 to 75 bars using the free dislocation densities and 20 to 100 bars using the subgrain sizes. 相似文献
2.
Simon J. Haward Mark M. Smits Kristín Vala Ragnarsdóttir Jonathan R. Leake Terence J. McMaster 《Geochimica et cosmochimica acta》2011,75(22):6870-6881
We have used a direct imaging technique, in situ atomic force microscopy (AFM), to observe the dissolution of the basal biotite surface by oxalic acid over a range of temperatures close to ambient conditions, using a specially designed AFM liquid cell and non-invasive intermittent contact mode of operation. From the 3-dimensional nanometre-resolution data sets, we observe a process characterised by the slow formation of shallow etch pits in the (0 0 1) surface and fast growth of etch pits from the resulting steps, which represent proxies for the {h k 0} surface. Measurements of dissolution rates as a function of temperature allow a determination of an apparent activation energy (Ea,app) for the process, via mass-loss calculations from image analysis. We obtain a value of Ea,app = 49 ± 2 kJ mol−1, which is consistent with separate calculations based on planar area etch pit growth, and measurements of etch pit perimeters, indicating that this value of Ea,app is representative of {h k 0} surface dissolution. The measurement of etch pit perimeters also enables an estimation of apparent activation energy as a function of step density indicating substantially higher apparent activation energy, up to Ea,app = 140 kJ mol−1, on extrapolation towards a pristine surface with no defects. We suggest that this higher value of Ea,app represents the slow formation of etch pits into the (0 0 1) surface. 相似文献
3.
津巴布韦金刚石独特的形态及其“指纹”特征的意义 总被引：1，自引：1，他引：0
津巴布韦马朗金刚石矿床为近年来发现的特大型高品位金刚石砂矿。对津巴布韦马朗金刚石产区的金刚石样品进行了矿物学和宝石学研究。结果显示,津巴布韦金刚石的晶体形态、表面微细特征等与世界上主要产地的金刚石有较大差异。其中最新发现的“十字架”形熔蚀图像和红色斑点是该产地金刚石的典型特征。“十字架”是由大量大小不等的正方形熔蚀坑沿晶体[100]重叠排列而成的,红色斑点为铁质氧化物沿熔蚀坑棱线生长而成的。研究结果表明,津巴布韦马朗金刚石的形态特征具有产地“指纹特征”的意义,对中国地勘单位赴境外进行金刚石找矿具有借鉴意义。 相似文献
4.
为研究山东蒙阴金伯利岩型金刚石晶体中氮片晶的分布特征,采用红外光谱仪对116粒宝石级金刚石晶体样品中的氮片晶进行分析,并采用微分干涉显微镜观察氮片晶在金刚石表面所具有的物理化学性质及其表面微细结构。结果表明,80.2%的金刚石样品中具有氮片晶;在个别浅褐色、八面体金刚石样品{111}面上观察到平行于[100]晶带方向的长条状蚀像,这些蚀像相互平行,大小不等。显微红外光谱对具长条状蚀像的金刚石样品{111}面的测试结果表明,该类晶体均具有较强的氮片晶的吸收峰（1359~1375cm-1）。综合浅褐色、八面体金刚石样品的红外光谱及其表面微形貌特征,推测长条状蚀像是由氮片晶的出露点受优先选择性腐蚀而致。 相似文献
5.
黄铁矿成因形态学 总被引：25，自引：5，他引：20
在1981—1986年野外工作的基础上,作者对胶东三县四个金矿床黄铁矿晶体形态进行了系统研究,对其中2495粒晶体进行了统计,对300粒晶体进行了测角,对60粒晶体进行了微形貌观察。根据以上实际工作及国内外有关资料提出了黄铁矿晶休常见单形出现频率的定量资料与晶体微形貌的特点,特别是{210}面上负条纹的特征与成因探讨。并讨论了黄铁矿晶体形态与分带性,与形成温度,与主、微量成分,与共生组合及与矿化等的关系。查明黄铁矿形态及其晶面微形貌的发育与硫逸度、温度、冷却速度等密切相关。在以上基础上提出了胶东四个金矿床中蚀变岩型金矿及石英脉型金矿两种成因类型黄铁矿形态特征上的五点差异。最后对黄铁矿研究方法要点进行了总结。 相似文献
6.
In situ Atomic Force Microscopy study of dissolution of the barite (0 0 1) surface in water at 30 °C
Yoshihiro Kuwahara 《Geochimica et cosmochimica acta》2011,75(1):41-51
The dissolution behavior of the barite (0 0 1) surface in pure water at 30 °C was investigated using in situ Atomic Force Microscopy (AFM), to better understand the dissolution mechanism and the microtopographical changes that occur during the dissolution, such as steps and etch pits. The dissolution of the barite (0 0 1) surface started with the slow retreat of steps, after which, about 60 min later, the <hk0> steps of one unit cell layer or multi-layers became two-step fronts (fast “f” and slow “s” steps) with a half-unit cell layer showing different retreat rates. The “f” step had a fast retreat rate (≈(14 ± 1) × 10−2 nm/s) and tended to have a jagged step edge, whereas the “s” step (≈(1.8 ± 0.1) × 10−2 nm/s) had a relatively straight front. The formation of the “f” steps led to the formation of a new one-layer step, where the front of the “s” step was overtaken by that of the immediate underlying “f” step. The “f” steps also led to the decrease of the <hk0> steps and the increase in the percentage of stable steps parallel to the [0 1 0] direction during the dissolution.Etch pits, which could be observed after about 90 min, were of three types: triangular etch pits with a depth of a half-unit cell, shallow etch pits, and deep etch pits. The triangular etch pits were bounded by the step edges parallel to [0 1 0], [1 2 0], and [] and had opposite orientations in the upper half and lower half layers. Shallow etch pits that had a depth of two or more half-unit cell layers had any two consecutive pits pointing in the opposite direction of each other. The triangular etch pit appeared to grow by simultaneously removal of a row of ions parallel to each direction from the three step edges. At first, deep etch pits were elongated in the [0 1 0] direction with a curved outline and then gradually developed to an angular form bounded by the {1 0 0}, {3 1 0}, and (0 0 1) faces. The retreat rate of the (0 0 1) face was much slower than those of the {1 0 0} and {3 1 0} and tended to separate into two rates ((0.13 ± 0.01) × 10−2 nm/s for the deep etch pits derived from a screw dislocation and (0.07 ± 0.01) × 10−2 nm/s for those from other line defects).The changes in the dissolution rate of a barite (0 0 1) surface during the dissolution were estimated using the retreat rates and densities of the various steps as well as the growth rates, density, and areas of the lateral faces of the deep etch pits that were obtained from this AFM analysis. Our results showed that the dissolution rate of the barite (0 0 1) surface gradually increased and approached the bulk dissolution rate because of the change in the main factor determining the dissolution rate from the density of the steps to the growth and the density of the deep etch pits on the surface. 相似文献
7.
Chemical etching of dislocations has been studied in natural and synthetic quartz single crystals, in deformed synthetic quartz and in naturally and experimentally deformed quartzites. The ability of different etchants to produce polished or preferentially etched surfaces on quartz is described. Dislocation etching was achieved on all crystal planes examined by using a saturated solution of ammonium bifluoride as the etchant. Appropriate etching times were determined for etching quartzites for grain size, subgrain boundaries, deformation lamellae, dislocations and twins. Growth and polished surfaces of synthetic single crystal quartz were similarly etched and dislocation etch pits, characteristic of various orientations were found. The use of ammonium bifluoride proved to be expecially advantageous for the basal plane, producing a polished surface with etch pits, suitable for dislocation etch pit counting. “Double” etch pits have been found on Dauphiné twin boundaries on the basal plane and the first order prism, using this etchant. Slip lines and deformation bands were suitably etched on deformed synthetic crystal surfaces for identification of the slip planes. Other acidic etchants have been explored and their application to the study of deformation structures in quartz crystals is discussed. 相似文献
9.
Whiskers and thin platelets of pyrite have been found growing coherently on the face of small cubic pyrite crystals from the Strashimir and Gradishte hydrothermal lead-zinc deposits. The host crystals formed framboids and spheroids of macroscopic size inside large crystals of chalcopyrite from which they are separated by concentric gaps. The whiskers, one to several μm in width, are elongated along [001] and bounded by {100}, partly by {210} and {111} faces. Scanning electron microscopic (SEM) observation revealed longitudinal steps and grooves, flat rectangular pits, drop-like and elongated bulges on their side faces. High-voltage electron microscopic (HVEM) studies showed that the thin whiskers are perfect and dislocation-free, although some ribbon-like whiskers and platelets contain internal longitudinal channels and small isometric fluid inclusions. Based on these observations, the following growth mechanism is proposed. Whiskers grow in the restricted volume of the internal cavities in chalcopyrite from stagnant solutions under a diffusion regime. The growth proceeds in two stages: (1) Initially, pyramidal “pedestals” are formed due to morphological instability of the host crystal surfaces and multiapex growth, (2) Whiskers grow rapidly by unidirectional supply of material and competition between neighbouring individual whiskers, the growth taking place at the tip by a two-dimensional nucleation mechanism. The whiskers and thin platelets are considered to be peculiar highly anisometric skeletal crystals. 相似文献
10.
Hydrothermal grossular-andradite garnets from contact aureoles in the Oslo region show morphological transitions from planar via cellular to hopper-like structures. Dodecahedral surfaces {110} dominate during the planar growth stage, whereas the stable crystal faces, developed during the cellular and hopper stages also includes the ikositetrahedron {211} and possibly the hexoctahedron {321}. Faceted cells develope when initially ‘wavy’ perturbations on the dodecahedral surfaces become tangential to lower-index planar surfaces. Inclusion patterns and morphologies of almandinerich garnets from Magerøy (northernmost Norway) that formed during a period of rapid heating, suggest an early stage of cellular growth followed by planar growth. The morphological transitions suggest that the hydrothermal garnets experienced an increase in the overstepping of the garnet precipitation reaction at some stage during their growth whereas the opposite was the case during growth of the Magerøy garnets. The present observations put constraints on the garnet growth rates and emphasize the importance of growth kinetics during metamorphic processes. 相似文献
11.
测量研究了锆石、黄石、石榴石、羟硅铍石和符山石的形态和微形貌，并分别列出了这些晶体的化学成分和其它鉴定参数，观察到的生长和溶蚀螺纹证实螺旋生长机制在硅酸盐矿物晶体生长中具有重要意义。 相似文献
12.
黄铁矿表面的扫描隧道显微镜研究 总被引：3，自引：0，他引：3
为了探讨硫元素在煤中的赋存，对煤硫元素的主要载体黄铁矿的表面进行了扫描隧道显微镜研究，在硅油保护下得到了黄铁矿表面高分辨率图象。根据黄铁矿晶体结构的几何分析，所观察到的是黄铁矿｛２２０｝表面的原子分布，黄铁矿的能带结构资料及前人在这方面的研究均表明，用扫描隧道显微镜应首先观察到黄铁矿表面的铁原子，但作者首先观察到的却是黄铁矿表面的硫原子，此外，作者根据所获得的实验结果认为。无论是在｛２１０｝文向还 相似文献
13.
The dissolution rate and mechanism of three different cleavage faces of a dolomite crystal from Navarra (near Pamplona), Spain, were studied in detail by vertical scanning interferometry techniques. A total of 37 different regions (each about 124 × 156 μm in size) on the three sample surfaces were monitored as a function of time during dissolution at 25°C and pH 3. Dissolution produced shallow etch pits with widths reaching 20 μm during 8 h of dissolution. Depth development as a function of time was remarkably similar for all etch pits on a given dolomite surface.On the basis of etch pit distribution and volume as a function of time, the calculated dissolution rate increases from near zero to 4 × 10−11 mol cm−2 s−1 over 5 h. The time variation is different for each of the three cleavage surfaces studied. In addition, the absolute dissolution rates of different parts of the dolomite crystal surface can be computed by using a reference surface. The different surfaces yield an “average” rate of 1.08 × 10−11 mol cm−2 s−1 with a standard deviation of 0.3 × 10−11 mol cm−2 s−1 based on about 60 analyses. The mean absolute rate of the dolomite surface is about 10 times slower than the rate calculated from etch pit dissolution alone. On the other hand, earlier batch rate data that used BET surface areas yield rates that are at least 30 to 60 times faster than our directly measured mean dissolution rate for the same pH and temperature.A conceptual model for mineral dissolution has been inferred from the surface topography obtained by the interferometry investigations. In this model, mineral dissolution is not dominated by etch pit formation itself but rather by extensive dissolution stepwaves that originate at the outskirts of the etch pits. These stepwaves control the overall dissolution as well as the dependence on temperature and saturation state. 相似文献
14.
J.M. Astilleros C.M. Pina L. Fernndez-dazA. Putnis 《Geochimica et cosmochimica acta》2002,66(18):3177-3189
This paper deals with the growth behaviour of the Mn-Ca-CO3-H2O solid solution-aqueous solution system on calcite {104} surfaces. This system represents a model example, which allows us to study the effect of a number of controlling factors on the crystallisation: (1) the supersaturation function, β(x), and nucleation rate function, J(x), for the Mn-Ca-CO3-H2O system, (2) the relationship of such functions to the molecular scale growth mechanisms operating on growing surfaces, and (3) the surface structure of the calcite {104} faces. In situ atomic force microscopy (AFM) growth experiments revealed a wide variety of surface phenomena, such as the transition between growth mechanisms, anisotropic changes in the step rates, and the influence of the Mn-bearing newly formed surface on subsequent growth (step stoppage followed by the formation of two-dimensional nuclei and the reproduction of the original calcite {104} surface microtopography). These phenomena result from the interplay between the controlling parameters and are explained in those terms. 相似文献
15.
Etch pit formation on iron silicate surfaces during siderophore-promoted dissolution 总被引：1，自引：0，他引：1
Understanding the effects of microbiota on mineral alteration requires the ability to recognize evidence of bacteria-promoted dissolution on mineral surfaces. Although siderophores are known to promote mineral dissolution, their effects on mineral surfaces are not well known. We have utilized atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Mirau vertical scanning interferometry (VSI) to investigate surfaces after incubation with the siderophore desferrioxamine-B mesylate (DFAM) and under colonies of bacteria. Iron-silicate glass planchets chemically similar to hornblende were incubated in buffered growth medium with siderophore-producing bacteria (Bacillus sp.) for 46 days with parallel abiotic experiments conducted with and without 240 μM DFAM, with and without 0.01 g l− 1 of microbially produced extracellular polysaccharides (EPS, alginate or xanthan gum). Some glass planchets were protected by dialysis tubing from direct contact with the EPS. Weekly sampling and analysis of all filtered sample solutions showed negligible Fe and Al release in the control experiments and significant release of Fe and Al in the presence of DFAM, with negligible changes in pH. Concentration of Fe in the filtered solutions after incubation with bacteria was below detection, consistent with uptake of Fe by cells. Release of Fe, Al, and Si in control, xanthan-only, and alginate-only experiments was negligible. Release of these elements was enhanced in all experiments containing DFAM, and greatest in alginate + DFAM experiments.
AFM and VSI analyses reveal widespread, small etch pits and greater root mean squared roughness on siderophore-exposed surfaces and fewer, localized, larger etch pits on bacteria-exposed surfaces. This is the first documented case of etch pit development during siderophore-promoted dissolution. Roughness was not affected by the growth medium, alginate, or xanthan gum alone. The roughness trends among samples correlate with trends in Fe depletion documented by XPS. Enhanced dissolution and roughness cannot be attributed to direct contact with EPS because no significant chemical or physical differences were observed between surfaces directly exposed to EPS and those protected by dialysis tubing. Acetate released from the EPS may have enhanced the siderophore-promoted dissolution. Siderophores produced by Bacillus sp. may be responsible for some of the ‘biopits.’ The difference in size and distribution of the biopits may be related to colonization. 相似文献
16.
E. I. Zhimulev V. M. Sonin A. I. Chepurov A. A. Tomilenko 《Geology of Ore Deposits》2009,51(3):243-246
The results of chromatographic study of the formation of rhombododecahedral diamonds synthesized in the Fe-Ni-(Ti)-C system at 5.5–6.0 GPa and 1350–1450°C are presented, including crystals with rounded surfaces of the rhombododecahedron with parallel striation, which are morphological analogues of natural diamonds abundant at various kimberlite, lamproite, and placer deposits. Chromatography was performed at 150°C with mechanical breakup of diamonds. The stable release of methane when diamonds of habit {110} are crushed is established. It is concluded that the appearance of the habit rhombododecahedron may be related not only to the effect of temperature and pressure on crystal growth but also to reductive conditions of crystallization. At the same time, the appearance of significant amounts of hydrocarbons in the system probably results in stopping of the growth of faces {110} and {100} and, instead, formation of specific surfaces that are composed of microscopic accessories faced by planes {111}. 相似文献
17.
板状绿柱石产于花岗岩云英岩化边部或晶洞壁的白云母-钠长石-绿柱石-水晶的矿物组合中,钠长石呈自形晶,绿柱石的洁净度与透明度相对较差。晶体测量表明,晶体的单形晶面发育依次为c{0001}→s{11 2-1}→p{10 1-1}→m{10 1-0}→v{21 3-1}、n{45 9-4}、a{11 2-0}。微分干涉显微镜(DIC)及扫描电镜(SEM)研究表明:各单形晶面上生长纹(微形貌)反映面网结构的对称性,c{0001}单形晶面上的六边形溶蚀坑(50～170μm)是由平行晶体C轴的各单形晶面生长层从晶体中心向外叠堆组成,层生长机理制约晶体生长全过程,平行双面(c)的生长层较薄(5～8μm)且较稳定,六方双锥(s)生长层较厚且圆滑,六方柱(m)生长层由晶体中心往外其厚度由厚逐渐变薄(12～20μm),台间隔由窄变宽。晶体溶蚀是从面网密度最大的c{0001}开始,溶蚀面积依平行双面(c)、六方双锥(s)单形晶面由大变小,六方柱(a)、复六方双锥(v、n)单形晶面因面网密度小而未受到溶蚀。据矿物共生组合、流体包裹体均一法测量与拉曼光谱(LRS)分析表明板状绿柱石是在中–高温(303℃)、过饱和度较大、成矿介质的钠长石化发育且热动力环境相对稳定的条件下形成,气液包裹体主要由H2O、CO2及微量CH4、N2组成,CO2及微量CH4、N2可能与碳酸盐围岩有关。 相似文献
18.
The aim of this paper is to demonstrate that atomistic simulations can be used to evaluate the structure of mineral surfaces and to provide reliable data for forsterite surfaces up to a plane index of 2 using the code METADISE. The methods used to calculate the surface structure and energy which have more commonly been used to study ceramics are briefly explained as is a comparison with experimental data, most notable the crystal morphology. The predicted morphologies show that all the methods (Donnay-Harker, Attachment energies and equilibrium) show most of the surfaces that are expressed in observed crystals. The equilibrium morphology calculated from the relaxed surface energies is the only method which expresses the {201} surfaces and the {101} surfaces, which appear only upon relaxation. The more stable surfaces are shown to be those which have the highest surface density and more closely resemble close packed structures with highly coordinated surface ions and silicon as far from the surface as possible. The most stable surfaces the {100} which has alternating layers of MgO and SiO2 terminating with an MgO layer. The structure is similar to the MgO {100} surfaces and has a similar energy (1.28?Jm?2 compared to 1.20). The second most stable are the {201} which have a stepped surface topology, but is also compact with a relaxed surface energy of 1.56?Jm?2. The results indicate that atomistic simulation is well suited to the prediction of surface structure and morphology although care must be taken in choosing potentials which model the structure and elastic properties accurately. 相似文献
19.
Summary A simple quartz-arsenopyrite vein assemblage formed at temperatures of 280–200°C occurs in the Madan lead-zinc ore district, Bulgaria. Spear-shaped arsenopyrite is bounded by {101}, {010} and {210}; compositionally it is pure FeAs1-xS1+X of variable x. The BSE image of an oriented (001) section reveals a pronounced sector zoning defined solely by differences in the S/As ratio. The boundary between the (210) and (010) growth sectors reflects the changes of their growth rates V010 and V210 in perfect correlation with zonal S/As oscillations. The V°10/V210 ratios determined in 20 zones range from 0.1 to 2.4, and correspond to S variations from 39 to 34 at.% derived from 7 microprobe point analyses. The S content in the (010) sector is on average 1.5 at.% lower than in the (210) one, the difference decreasing from about 2.5 to less than 0.33 at.% with the decreasing V010/V210. The rate/composition relationship obeys the following rules: the higher the S/As the lower the V010/V210, and the slower the (010) growth. The phenomenon is interpreted in terms of crystal growth theory of a solid solution analyzing the arsenopyrite growth history on hand specimen, microscopic and structural scales. Sector zoning in this particular case is explained by processes taking place very close to the growing crystal surface which are superimposed on those produced by larger-scale variations in the hydrothermal environment.
Kinetische Aspekte des sektoriellen Zonenbaues von Arsenkies: Untersuchung eines Wachstumsphdnomens
Zusammenfassung Eine gewöhnliche Quarz-Arsenkies Paragenese im Blei-Zink Erzgebiet von Madan, Bulgarien, bildete sich im Temperatur-Intervall von 280-200°C. Speerförmiger Arsenkies ist begrenzt von den Flächenformen 11011, {010} und {210}, wobei das Mineral der Zusammensetzung FeAs1-xS1+X mit variierendem x entspricht. Die Rückstreuelektronenabbildung eines (001)-orientierten Querschnittes zeigt einen deutlichen Sektorzon-enbau, der ausschließlich durch Unterschiede im S/As-Verhaltnis bestimmt wird. Die Grenze zwischen den Wachstumssektoren (210) und (010) spiegelt Veränderungen in ihren Wachstumsgeschwindigkeiten V010 und V210 wieder, die mit den zonenkorrelierten S/As Schwankungen variieren. Die Verhdltnisse V010/V21O wurden in 20 Zonen des Kristalls bestimmt; she variieren zwischen 0.1 und 2.4 und entsprechen einer Variation von 39 bis 34 At.% S, die durch 7 Elektronenstrahlmikrosondenanalysen bestimmt wurden. Der S-Gehalt im Sektor (010) ist durchschnittlich mit 1.5 At.% niedriger als im Sektor (210); der entsprechende Unterschied nimmt von ungefdhr 2.5 bis 0.33 At.% mit der Verringerung von V010/V2 ab. Die Beziehung Wachstumsgeschwindigkeit/ Element unterliegt der Regel: je hoher das Verhaltnis S/As; desto niedriger ist V010/V210 und desto langsamer ist das Wachstum von (010). Diese Erscheinung wird im Rahmen der Kristallwachstumstheorie einer festen Lösung gedeutet, wobei die Wachstumsgeschichte des Arsenkieses makroskopisch, mikroskopisch und strukturell analysiert wurde. Auf these Weise wird der Sektorzonenbau als ein Resultat von Prozessen interpretiert, die ganz nah der wachsenden Kristalloberfldche ablaufen und die die Veränderungen des hydrothermalen Milieus überlagern.相似文献
20.
In situ dissolution experiments on a set of pure, optical quality Iceland spar calcite samples from four different localities showed etch pit step retreat rates to be inversely proportional to total inherent trace cation composition. Atomic absorption spectroscopy (AAS) revealed Fe2+, Mg2+, Mn2+ and Sr2+ in amounts varying from a few to hundreds of ppm. We used a very simple experimental set-up, with an Atomic Force Microscope (AFM) fluid cell and a droplet of MilliQ water. As the calcite dissolved and approached equilibrium with the solution, trace cations were released, which were then present for interaction with the dissolving surface. We monitored continuous free-drift dissolution, in situ, on fresh cleavage surfaces for up to 40 min. Dissolution produced one-layer-deep, rhombic etch pits that continually expanded as we collected images. The rhombohedral symmetry of calcite defines two obtuse and two acute edges on the cleavage surface of etch pits and these, as expected from previous work, had different dissolution rates. Despite identical experimental conditions for all samples, we observed lower step retreat rates for both obtuse and acute edges on calcite characterised by relatively high trace cation composition. Increased cation concentration, particularly Mn, was also correlated with rounding of obtuse-obtuse corners, resulting in obtuse step retreat rates similar to those for acute sides. Physcial limitations of the AFM technique were taken into account when measuring step rate retreat and results were collected only from single-layer etch pits, which represent crystalline calcite with minimal defects. Dissolution rates presented here are thus lower than previous reports for studies of deep etch pits and where the physical limitations of imaging may not have been considered. In addition to molecular-level proof that divalent cations inherent at ppm levels in the calcite affect the dissolution process, these results show that pure, optical quality Iceland spar calcite should not be considered pure in the chemical sense. The results imply that dissolution rates determined for ideal systems with pure, synthetic or natural, materials may be considered as the boundary condition for dissolution in real systems in nature, where cations are always present both in the solution and in the initial solid. 相似文献