Alteration Features in Natural Zirconolite from Carbonatites

2000 ◽  
Vol 663 ◽  
Author(s):  
C.T. Williams ◽  
A.G. Bulakh ◽  
R. Gieré ◽  
G.R. Lumpkin ◽  
A.N. Mariano
Keyword(s):  
Rare Earth ◽  
Accessory Mineral ◽  
Chemical Information ◽  
Secondary Phases ◽  
Complete Replacement ◽  
Rock Types ◽  
Geological Age ◽  
Physico Chemical ◽  
Backscattered Electron ◽  
Textural Changes

ABSTRACTIn nature, zirconolite occurs as an accessory mineral in many different rock types, but the majority of reported occurrences are from carbonatites (magmatic carbonates) of geological age varying from a few million years to 2 billion years old. Within these 19 carbonatite occurrences, of which 15 have been studied in some detail, zirconolite displays varying degrees of alteration in six samples. This alteration ranges from incipient minor effects to major corrosion, recrystallization and complete replacement by secondary phases. The degree of alteration broadly correlates with either the age, or actinide content of the zirconolite (or both), and thus the extent and degree of metamictization. Changes in zirconolite composition with alteration include an increase in hydration (H2O), Si, Ba and Pb (possibly radiogenic in origin), and a decrease primarily in Ca and Fe. Th can be remobilized, and of the rare earth elements (REE), there is evidence that the heavy-REE are mobilized more readily than the light-REE. Using backscattered electron images and electron microprobe analyses, this study documents and illustrates the range of alteration features observed in zirconolite from several carbonatites, in terms of both compositional and textural changes, and provides some physico-chemical information on the fluids responsible for the alteration.

Physico-chemical properties of the rare-earth metals, scandium, and yttrium

1963 ◽  
Vol 79 (2) ◽  
pp. 263-293 ◽  
Author(s):  
E.M. Savitskii ◽  
V.F. Terekhova ◽  
O.P. Naumkin
Keyword(s):  
Rare Earth ◽  
Rare Earth Metals ◽  
Chemical Properties ◽  
Physico Chemical ◽  
The Rare Earth

scholarly journals The Valance Determination of Cerium Ions in α-SiAlON by Electron Energy Loss Spectroscopy Analysis

2008 ◽  
Vol 14 (S3) ◽  
pp. 19-22 ◽  
Author(s):  
H. Yurdakul ◽  
S. Turan
Keyword(s):  
Rare Earth ◽  
Ionic Radius ◽  
High Hardness ◽  
Engineering Properties ◽  
Secondary Phases ◽  
Sintering Additive ◽  
Valence States ◽  
Domain Boundaries ◽  
Spark Plasma

SiAlON ceramics have found applications in many different areas due to their excellent engineering properties such as high hardness, fracture toughness, good thermal shock and oxidation resistance. SiAlON exist mainly in two different polymorphs: a (MxSi12-(m+n)Al(m+n)OnN16-n; M: metal and rare earth cations, x≈0,35 and n≤1,35) and β (β-Si6-zAlzOzN8-z; 0≤z≤4). In general, stable alpha and beta phases separately as well as in combination of α and β are obtained by incorporation of metal and rare earth cations as sintering additives. The metal cations such as Li, Mg, Ca, Y, and most lanthanide cations with the exception of La, Ce, Pr and Eu are able to stabilise α-SiAlON structure. Ekstrom et al. 1991 found that cerium can not occupy interstitial sites in α-SiAlON structure due to the fact that ionic radius of Ce3+ (0.103 nm) is too large, whereas ionic radius of Ce4+ (0.080 nm) is too small to stabilise α-SiAlON structure. After this work, several studies carried out to incorporate cerium cations into α-SiAlON structure. It was shown that cerium cation alone can be incorporated into α-SiAlON if the samples are either fast cooled after sintering, or when the samples are spark plasma sintered. On the other hand, cerium can also be incorporated into the α-SiAlON structure when it is used as a sintering additive together with a smaller α-SiAlON stabiliser cation such as Yb or Ca. Similar results were observed in other multi-cation doped SiAlONS that non α-SiAlON stabiliser cations like Sr2+ (0.112 nm) and La3+ (0.106 nm) are able to stabilise α-SiAlON when used together with α-SiAlON stabiliser cations such as Ca or Yb. Although it was shown that cerium existed in mixed valance state at domain boundaries in Ce-doped and spark plasma sintered α-SiAlON, there is no work on the valance determination of cerium in sintered α-SiAlON which has no domain boundaries. Therefore, in this study; it was aimed to incorporate cerium into α-SiAlON structure by combining with Yb3+ and the determination of possible cerium valence states (Ce3+/Ce4+) in both α-SiAlON grains and secondary phases.

Seeded growth of rare-earth orthoferrites from B2O3-BaF2-BaO solvent I. Study of conditions and physico-chemical crystallization parameters

1991 ◽  
Vol 108 (1-2) ◽  
pp. 309-313 ◽  
Author(s):  
S.N. Barilo ◽  
A.P. Ges ◽  
S.A. Guretskii ◽  
D.I. Zhigunov ◽  
A.A. Ignatenko ◽  
...  
Keyword(s):  
Rare Earth ◽  
Seeded Growth ◽  
Physico Chemical

Backscattered Electron Imaging and Microanalysis of Iron-Titanium Oxide Minerals in Fine-Grained Igneous Rocks

2000 ◽  
Vol 6 (S2) ◽  
pp. 428-429
Author(s):  
J. S. Lowther ◽  
K. A. Brunstad
Keyword(s):  
Igneous Rocks ◽  
Accessory Mineral ◽  
Fine Grained ◽  
Reflected Light ◽  
Mineral Components ◽  
Carbon Coated ◽  
Backscattered Electron ◽  
Oxide Minerals ◽  
Electron Imaging ◽  
Eds Microanalysis

Oxides of iron, titanium, and iron & titanium occur as accessory mineral components of most igneous rocks and generally comprise 1-2% of the total rock volume. In the fine-grained rocks the crystals are usually equidimensional and less than 1 mm across. Because most of them are opaque they cannot be examined using polarized transmitted light in the standard petrographic microscope and must be identified by reflected light (1). We have chosen to study these minerals in the SEM using BSE imaging and EDS microanalysis of carbon-coated polished sections of the rocks which contain them. Alhough this does not permit precise identification of the minerals the technique does reveal textural details which cannot be seen with light microscopy and also allows chemical analysis of the grains or parts of the grains. Furthermore, all these the oxide minerals are very easy to see in the BSE images because they are a higher atomic number (Z) than the silicates that form the bulk of the rock.

scholarly journals Thermodynamic Constraints on REE Mineral Paragenesis in the Bayan Obo REE-Nb-Fe Deposit, China

Minerals ◽  
2020 ◽  
Vol 10 (6) ◽  
pp. 495
Author(s):  
Shang Liu ◽  
Lin Ding ◽  
Hong-Rui Fan
Keyword(s):  
Rare Earth ◽  
Aqueous System ◽  
Hydrothermal Fluids ◽  
Fluid Composition ◽  
Physico Chemical ◽  
Composition Variation ◽  
Bayan Obo ◽  
Chemical Conditions ◽  
The Cost ◽  
Hydrothermal Activities

Hydrothermal processes have played a significant role in rare earth element (REE) precipitation in the Bayan Obo REE-Nb-Fe deposit. The poor preservation of primary fluid inclusions and superposition or modification by multiphase hydrothermal activities have made identification of physico-chemical conditions of ore-forming fluids extremely difficult. Fortunately, with more and more reliable thermodynamic properties of aqueous REE species and REE minerals reported in recent years, a series of thermodynamic calculations are conducted in this study to provide constraints on REE precipitation in hydrothermal solutions, and provide an explanation of typical paragenesis of REE and gangue minerals at Bayan Obo. During the competition between fluocerite and monazite for LREE in the modelled solution (0.1 M HCl, 0.1 M HF and 0.1 M trichloride of light rare earth elements (LREE) from La to Sm), all LREE would eventually be hosted by monazite at a temperature over 300 °C, with continuous introduction of H3PO4. Additionally, monazite of heavier LREE would precipitate earlier, indicating that the Ce- and La-enriched monazite at Bayan Obo was crystallized from Ce and La pre-enriched hydrothermal fluids. The fractionation among LREE occurred before the ore-forming fluids infiltrating ore-hosting dolomite. When CO2 (aq) was introduced to the aqueous system (model 1), bastnaesite would eventually and completely replace monazite-(Ce). Cooling of hot hydrothermal fluids (>400 °C) would significantly promote this replacement, with only about one third the cost of CO2 for the entire replacement when temperature dropped from 430 °C to 400 °C. Sole dolomite addition (model 2) would make bastnaesite replace monazite and then be replaced by parisite. The monazite-(Ce) replaced by associated bastnaesite and apatite is an indicator of very hot hydrothermal fluids (>400 °C) and specific dolomite/fluid ratios (e.g., initial dolomite at 1 kbar: 0.049–0.068 M and 0.083–0.105 M at 400 °C and 430 °C). In hot solution (>430 °C) that continuously interacts with dolomite, apatite precipitates predating the bastnaesite, but it behaves oppositely at <400 °C. The former paragenesis is in accord with petrography observed in this study. Some mineral pairs, such as monazite-(Ce)-fluorite and monazite-(Ce)-parisite would never co-precipitate at any calculated temperature or pressure. Therefore, their association implies multiphase hydrothermal activities. Pressure variation would have rather limited influence on the paragenesis of REE minerals. However, temperature and fluid composition variation (e.g., CO2 (aq), dolomite, H3PO4) would cause significantly different associations between REE and gangue minerals.

scholarly journals Niobium and rare earth minerals from the Virulundo carbonatite, Namibe, Angola

2012 ◽  
Vol 76 (2) ◽  
pp. 393-409 ◽  
Author(s):  
L. Torró ◽  
C. Villanova ◽  
M. Castillo ◽  
M. Campeny ◽  
A. O. Gonçalves ◽  
...  
Keyword(s):  
Rare Earth ◽  
Low Temperature ◽  
Second Generation ◽  
Accessory Mineral ◽  
Vein Formation ◽  
Rare Earth Minerals ◽  
Late Generation ◽  
Alteration Processes ◽  
Hydrothermal Replacement ◽  
First And Second Generation

AbstractThe Virulundo carbonatite in Angola is one of the largest in the world and contains pyrochlore as an accessory mineral in all of the carbonatite units (calciocarbonatites, ferrocarbonatites, carbonatite breccias and trachytoids). The primary magmatic pyrochlore is fluorine dominant and typically contains about equal molar quantities of Ca and Na at the A site. High-temperature hydrothermal processes have resulted in the pseudomorphic replacement of the primary pyrochlore by a second generation of pyrochlore with less F and Na. Low-temperature hydrothermal replacement of the first and second generation pyrochlore, associated with quartz-carbonate-fluorite vein formation in the carbonatite, has produced a third generation of pyrochlore, with a high Sr content. The Sr appears to have been released by low-temperature hydrothermal replacement of the primary magmatic carbonates. Finally, supergene alteration processes have produced late-stage carbonates, goethite, hollandite and rare earth element (REE) minerals (mainly synchysite-(Ce), britholite-(Ce), britholite-(La), cerite-(Ce)). Cerium separated from the other REE s in oxidizing conditions and Ce4+ was incorporated into a late generation of supergene pyrochlore, which is strongly enriched in Ba and strongly depleted in Ca and Na.

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