scholarly journals Micro-structural and compositional variations of hydrothermal epidote-group minerals from a peralkaline granite, Corupá Pluton, Graciosa Province, South Brazil, and their petrological implications

2012 ◽  
Vol 84 (2) ◽  
pp. 407-426 ◽  
Author(s):  
Silvio R.F. Vlach
Keyword(s):  
Alkali Feldspar ◽  
X Ray ◽  
External Zone ◽  
Icp Ms ◽  
Internal Zone ◽  
Compositional Variations ◽  
Host Rocks ◽  
Ree Patterns ◽  
Miarolitic Cavities ◽  
South Brazil

Epidote-group minerals, together with albite, quartz, fluorite, Al-poor and Fe-rich phyllosilicates, zircon, and minor oxides and sulphides, are typical hydrothermal phases in peralkaline alkali-feldspar granites from the Corupá Pluton, Graciosa Province, South Brazil. The epidote-group minerals occur as single crystals and as aggregates filling in rock interstices and miarolitic cavities. They display complex recurrent zoning patterns with an internal zone of ferriallanite-(Ce), followed by allanite-(Ce), then epidote-ferriepidote, and an external zone with allanite-(Ce), with sharp limits, as shown in BSE and X-ray images. REE patterns show decreasing fractionation degrees of LREE over HREE from ferriallanite to epidote. The most external allanite is enriched in MREE. LA-ICP-MS data indicate that ferriallanite is enriched (>10-fold) in Ti, Sr and Ga, and depleted in Mg, Rb, Th and Zr relative to the host granite. Allanite has lower Ga and Mn and higher Zr, Nb and U contents as compared to ferriallanite, while epidote is enriched in Sr, U and depleted in Pb, Zr, Hf, Ti and Ga. The formation of these minerals is related to the variable concentrations of HFSE, Ca, Al, Fe and F in fluids remaining from magmatic crystallization, in an oxidizing environment, close to the HM buffer. L-MREE were in part released by the alteration of chevkinite, their main primary repository in the host rocks.

Major- and trace-element composition of REE-rich turkestanite from peralkaline granites of the Morro Redondo Complex, Graciosa Province, south Brazil

2010 ◽  
Vol 74 (4) ◽  
pp. 645-658 ◽  
Author(s):  
F. C. J. Vilalva ◽  
S. R. F. Vlach
Keyword(s):  
Trace Element ◽  
Inductively Coupled Plasma ◽  
Alkali Feldspar ◽  
Trace Element Composition ◽  
Inductively Coupled ◽  
Coupled Substitution ◽  
Plasma Mass Spectrometry ◽  
Compositional Variations ◽  
End Members ◽  
South Brazil

AbstractTurkestanite, a rare Th- and REE-bearing cyclosilicate in the ekanite–steacyite group was found in evolved peralkaline granitesfrom the Morro Redondo Complex, south Brazil. It occurswith quartz, alkali feldspar and an unnamed Y-bearing silicate. Electron microprobe analysis indicates relatively homogeneous compositions with maximum ThO2, Na2O and K2O contentsof 22.4%, 2.93% and 3.15 wt.%, respectively, and significant REE2O3 abundances(5.21 to 11.04 wt.%). The REE patterns show enrichment of LREE over HREE, a strong negative Eu anomaly and positive Ce anomaly, the latter in the most transformed crystals. Laser ablation inductively coupled plasma mass spectrometry trace element patterns display considerable depletions in Nb, Zr, Hf, Ti and Li relative to whole-rock sample compositions. Observed compositional variations suggest the influence of coupled substitution mechanisms involving steacyite, a Na-dominant analogue of turkestanite, iraqite, a REE-bearing end-member in the ekanite–steacyite group, ekanite and some theoretical end-members. Turkestanite crystals were interpreted as having precipitated during post-magmatic stages in the presence of residual HFSE-rich fluidscarrying Ca, the circulation of which wasenhanced by deformational events.

scholarly journals GEOCHIMIE ET CONDITIONS DE CRISTALLISATION DES MINERAUX DU GROUPE DE L’HELVITE

1994 ◽  
Author(s):  
Essaïd Bilal
Keyword(s):  
Essential Role ◽  
Structural Formula ◽  
The Other ◽  
Crystal Chemical ◽  
Limited Mobility ◽  
External Zone ◽  
Internal Zone ◽  
Compositional Variations ◽  
The Stability ◽  
Highly Evolved

Minerais de berílio (berilo, fenaquita e helvita) são constituintes de veios a quartzo-muscovita e veios de siderofilita. Noalbitito, somente a fenaquita e a helvita estão presentes. Este trabalho refere-se à ocorrência do maciço de Sucuri (Goiás, Brasil),às helvitas dos skarns de Costabonne, França e às helvitas de veios a quartzo-wolframita de Dajishan, China.O estudo paragenético, geoquímico e experimental de helvitas (Mn, Zn, Fe)8 Be6 Si6 O24 S2 analisadas leva à uma novadefinição da fórmula estrutural deste mineral que, por analogia àquela da hauyna é (Mn, Zn, Fe)8-x Nax Be6 Si6 O24 S2-x Clx. Àssubstituições clássicas Zn-Fe-Mn, se ajuntam as substituições do tipo: Cl-S e Be-Li. O Li substitui o Be na estrutura da helvita, odéficit de carga elétrica resultante é compensado pela introdução de Na na estrutura. A substituição Cl-S foi confirmada porestudo experimental. Restrições de ordem geoquímica desempenham um importante papel na introdução de terras raras na estruturada helvita, de modo que num ambiente francamente alcalino, a helvita concentra mais facilmente as terras raras pesadas.A danalita (pólo ferrífero da helvita) cristaliza-se em um domínio de fugacidade do enxofre e do oxigênio muito limitado eque corresponde ao domínio de estabilidade da pirrotita. A genthelvita (pólo zincífero da helvita) é estável em um extensodomínio de fugacidade do enxofre e oxigênio. Ao contrário, a helvita (pólo manganesífero da série) se desenvolve em umdomínio de fugacidade de oxigênio moderado e de fugacidade do enxofre suficientemente extenso e comparável àquele dagenthelvita.A formação de mineralização de berílio de composição variável nos skarns resultaria, seja da sucessão de soluções tardiasricas em sílica (se a razão Si/Al é alta, a fenaquita ou a helvita se cristalizam) atuando sobre a paragênese aluminosa (musgravita,crisoberilo) formada precocemente, seja da fraca migração de Al em relação a Si. Neste último caso, a paragênese aluminosa(musgravita, crisoberilo) terá uma distribuição limitada. A atividade da alumina desempenha um papel essencial na variação dacomposição dos minerais de berílio. Se ela é fraca, a fenaquita e/ou helvita são estáveis e se ela é forte o berilo e o crisoberilo oua musgravita são estáveis.Entretanto, o aumento da atividade de alguns elementos (Ca, Mn, Fe, Zn ou alcalinos) tendendo a incorporarem alumina nasgranadas ou nos feldspatos será um fator desfavorável para a formação do berilo. Assim, a helvita é um mineral tipicamenteestável nos skarns cálcicos e se associa à fenaquita nas rochas submetidas ao metassomatismo alcalino. Por outro lado, o beriloaparece normalmente em condições onde um fluido ácido se separa de líquidos graníticos muito evoluidos (aplopegmatito).A genthelvita é estável sob condições alcalinas e oxidantes. ABSTRACT: Beryllium-bearing minerals (beryl, phenacite, helvite) are present in the quartz-muscovite and siderophyllite veins. In thealbitites, only phenacite and helvite (Fe,Zn,Mn)8Be6(SiO4)6S2 are found. Besides the Sucuri deposits, we studied also helvitesfrom the northern skarn of Costabonne (France) and from quartz-wolframite veins of Dajishan (China). Paragenetic and geochemicaldata on helvites from these three occurences lead to propose a new structural formula:(Mn,Zn,Fe)8-xNax Be6 Si6 O24 S2-xClx, analogous to that of haüyne.To the standard substitutions Zn-Fe-Mn in helvite, we may add substitutions like Cl-S and Be-Li. Lithium probably replacesberyllium in the structure of helvite, the charge deficit being balanced by the entry of sodium. Crystal chemical constraints clearlyplay an important role for the incorporation of REE in helvite, but, in an alkaline context, helvite shows preference for heavyREE.Danalite (the Fe end-member of helvite) is stable in a very limited domain of fS2 and fO2, which coincides with that ofpyrrhotite. Genthelvite (Zn end-member) is stable in a large domain of fS2 and fO2. On the other hand, helvite (Mn end-member)is stable in conditions of moderate fO2 and for a range of fS2 relatively large compared to that of genthelvite. In Sucuri,Goiás,Brazil, the ilmenites of the internal zone (where genthelvite is found) contain 16% of hematite, whereas those of the external zonecontain no hematite. The zonations observed within the helvite crystals (genthelvite core and danalite border) reflect the variationsof fS2 and fO2.The compositional variations of the beryllium minerals observed in skarns may be explained by the following processes:- either a late arrival of silica-rich solutions (high Si/Al ratio, presence of phenacite or helvite) within early formed aluminarichparageneses (musgravite, chrysoberyl),- or a limited mobility of Al relatively to Si, which would induce a limited extension of the aluminous parageneses (musgravite,chrysoberyl).The activity of aluminium plays an essential role in the stability of the beryllium minerals: phenacite and/or helvite are stablewhen it is low, and beryl, chrysoberyl or musgravite when it is high. However, the increase of activity of other elements (Ca, Mn,Fe, Zn, alkalis) that would induce the formation of minerals incorporating alumina (garnet or felspars) would be unfavourable tothe formation of beryl. Helvite is thus typically stable in calcic skarns, and it is associated with phenacite in the rocks submittedto alkaline metasomatism. Beryl, on the contrary, normally appears when acidic fluids separates from highly evolved graniticmagmas (aplopegmatitic stage).The zonation of helvite crystals in albitites (genthelvite core, danalite border) reflects the decrease of alkalinity of the fluids.This zonation on the crystal scale is also observed on the outcrop scale: helvite composition varies from Ge64Da10He26 in theinternal zone to Ge23Da50He26 in the external zone. The same variation is observed in an albitized siderophyllite vein, fromGe25Da42He33 in a slightly albitized zone to Ge55Da39He22 in a highly albitized one. The formation of genthelvite is favoured, ascompared with the other members of the helvite group, by alkaline conditions.The compositional variations of helvite in the Sucuri massif would result from variations in fS2 and fO2 in the fluids. Genthelviteis stable under alkaline and oxydizing conditions.

Microdomains in BaFeO3-y (0.35 < y < 0.50)

1990 ◽  
Vol 48 (4) ◽  
pp. 780-781
Author(s):  
M. Vallet-Regí ◽  
M. Parras ◽  
J.M. González-Calbet ◽  
J.C. Grenier
Keyword(s):  
Electron Diffraction ◽  
Chemical Analysis ◽  
Monoclinic Phase ◽  
Orthorhombic Phase ◽  
Total Iron ◽  
Zone Axis ◽  
Electron Micrograph ◽  
X Ray Diffraction ◽  
X Ray ◽  
Compositional Variations

BaFeO3-y compositions (0.35<y<0.50) have been investigated by means of electron diffraction and microscopy to resolve contradictory results from powder X-ray diffraction data.The samples were obtained by annealing BaFeO2.56 for 48 h. in the temperature range from 980°C to 1050°C . Total iron and barium in the samples were determined using chemical analysis and gravimetric methods, respectively.In the BaFeO3-y system, according to the electron diffraction and microscopy results, the nonstoichiometry is accommodated in different ways as a function of the composition (y):In the domain between BaFeO2.5+δBaFeO2.54, compositional variations are accommodated through the formation of microdomains. Fig. la shows the ED pattern of the BaFeO2.52 material along thezone axis. The corresponding electron micrograph is seen in Fig. 1b. Several domains corresponding to the monoclinic BaFeO2.50 phase, intergrow with domains of the orthorhombic phase. According to that, the ED pattern of Fig. 1a, can be interpreted as formed by the superposition of three types of diffraction maxima : Very strong spots corresponding to a cubic perovskite, a set of maxima due to the superposition of three domains of the monoclinic phase along [100]m and a series of maxima corresponding to three domains corresponding to the orthorhombic phase along the [100]o.

Hingganite-(Nd), Nd2□Be2Si2O8(OH)2, a new gadolinite-supergroup mineral from Zagi Mountain, Pakistan

2020 ◽  
Vol 58 (5) ◽  
pp. 549-562
Author(s):  
Anatoly V. Kasatkin ◽  
Fabrizio Nestola ◽  
Radek Škoda ◽  
Nikita V. Chukanov ◽  
Atali A. Agakhanov ◽  
...  
Keyword(s):  
New Mineral ◽  
Monoclinic Space Group ◽  
X Ray Diffraction ◽  
Average Chemical Composition ◽  
X Ray ◽  
Conchoidal Fracture ◽  
Icp Ms ◽  
Optical Axes ◽  
Mohs Hardness ◽  
Khyber Pakhtunkhwa Province

ABSTRACT Hingganite-(Nd), ideally Nd2□Be2Si2O8(OH)2, is a new gadolinite group, gadolinite supergroup mineral discovered at Zagi Mountain, near Kafoor Dheri, about 4 km S of Warsak and 30 km NW of Peshawar, Khyber Pakhtunkhwa Province, Pakistan. The new mineral forms zones measuring up to 1 × 1 mm2 in loose prismatic crystals up to 0.7 cm long, where it is intergrown with hingganite-(Y). Other associated minerals include aegirine, microcline, fergusonite-(Y), and zircon. Hingganite-(Nd) is dark greenish-brown, transparent, has vitreous luster and a white streak. It is brittle and has a conchoidal fracture. No cleavage or parting are observed. Mohs hardness is 5½–6. Dcalc. = 4.690 g/cm3. Hingganite-(Nd) is non-pleochroic, optically biaxial (+), α = 1.746(5), β = 1.766(5), γ = 1.792(6) (589 nm). 2Vmeas. = 80(7)°; 2Vcalc. = 84°. Dispersion of optical axes was not observed. The average chemical composition of hingganite-(Nd) is as follows (wt.%; electron microprobe, BeO, B2O3, and Lu2O3 content measured by LA-ICP-MS; H2O calculated by stoichiometry): BeO 9.64, CaO 0.45, MnO 0.10, FeO 3.03, B2O3 0.42, Y2O3 8.75, La2O3 1.63, Ce2O3 12.89, Pr2O3 3.09, Nd2O3 16.90, Sm2O3 5.97, Eu2O3 1.08, Gd2O3 5.15, Tb2O3 0.50, Dy2O3 2.50, Ho2O3 0.33, Er2O3 0.84, Tm2O3 0.10, Yb2O3 0.44, Lu2O3 0.04, ThO2 0.13, SiO2 23.55, H2O 2.72, total 100.25. The empirical formula calculated on the basis of 2 Si apfu is (Nd0.513Ce0.401Y0.395Sm0.175Gd0.145Pr0.096Dy0.068La0.051Ca0.041Eu0.031Er0.022Tb0.014Yb0.011Ho0.009Tm0.003Th0.003Lu0.001)Σ1.979(□0.778Fe2+0.215Mn0.007)Σ1.000(Be1.967B0.062)Σ2.029Si2O8.46(OH)1.54. Hingganite-(Nd) is monoclinic, space group P21/c with a = 4.77193(15), b = 7.6422(2), c = 9.9299(2) Å, β = 89.851(2)°, V = 362.123(14) Å3, and Z = 2. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 6.105 (95) (011), 4.959 (56) (002), 4.773 (100) (100), 3.462 (58) (102), 3.122 , 3.028 (61) (013), 2.864 (87) (121), 2.573 (89) (113). The crystal structure of hingganite-(Nd) was refined from single-crystal X-ray diffraction data to R = 0.034 for 2007 unique reflections with I &gt; 2σ(I). The new mineral is named as an analogue of hingganite-(Y), hingganite-(Yb), and hingganite-(Ce), but with Nd dominant among the rare earth elements.

scholarly journals Geochemical signatures of mineralizing events in the Juomasuo Au–Co deposit, Kuusamo belt, northeastern Finland

2021 ◽  
Author(s):  
Mikael Vasilopoulos ◽  
Ferenc Molnár ◽  
Hugh O’Brien ◽  
Yann Lahaye ◽  
Marie Lefèbvre ◽  
...  
Keyword(s):  
Trace Element ◽  
Sulfur Isotope ◽  
Mineral Chemistry ◽  
Chemical Compositions ◽  
Metasedimentary Rocks ◽  
Metavolcanic Rocks ◽  
Icp Ms ◽  
Stage 1 ◽  
Host Rocks ◽  
Relationship Of

AbstractThe Juomasuo Au–Co deposit, currently classified as an orogenic gold deposit with atypical metal association, is located in the Paleoproterozoic Kuusamo belt in northeastern Finland. The volcano-sedimentary sequence that hosts the deposit was intensely altered, deformed, and metamorphosed to greenschist facies during the 1.93–1.76 Ga Svecofennian orogeny. In this study, we investigate the temporal relationship between Co and Au deposition and the relationship of metal enrichment with protolith composition and alteration mineralogy by utilizing lithogeochemical data and petrographic observations. We also investigate the nature of fluids involved in deposit formation based on sulfide trace element and sulfur isotope LA-ICP-MS data together with tourmaline mineral chemistry and boron isotopes. Classification of original protoliths was made on the basis of geochemically immobile elements; recognized lithologies are metasedimentary rocks, mafic, intermediate-composition, and felsic metavolcanic rocks, and an ultramafic sill. The composition of the host rocks does not control the type or intensity of mineralization. Sulfur isotope values (δ34S − 2.6 to + 7.1‰) and trace element data obtained for pyrite, chalcopyrite, and pyrrhotite indicate that the two geochemically distinct Au–Co and Co ore types formed from fluids of different compositions and origins. A reduced, metamorphic fluid was responsible for deposition of the pyrrhotite-dominant, Co-rich ore, whereas a relatively oxidized fluid deposited the pyrite-dominant Au–Co ore. The main alteration and mineralization stages at Juomasuo are as follows: (1) widespread albitization that predates both types of mineralization; (2) stage 1, Co-rich mineralization associated with chlorite (± biotite ± amphibole) alteration; (3) stage 2, Au–Co mineralization related to sericitization. Crystal-chemical compositions for tourmaline suggest the involvement of evaporite-related fluids in formation of the deposit; boron isotope data also allow for this conclusion. Results of our research indicate that the metal association in the Juomasuo Au–Co deposit was formed by spatially coincident and multiple hydrothermal processes.

Petrogenesis of Silicic Rocks from the Phan Si Pan - Tu Le region of the Emeishan Large Igneous Province, Northwestern Vietnam

2021 ◽  
pp. SP518-2020-253
Author(s):  
Thuy Thanh Pham ◽  
J. Gregory Shellnutt ◽  
Tuan-Anh Tran ◽  
Steven W. Denyszyn ◽  
Yoshiyuki Iizuka
Keyword(s):  
Basaltic Magma ◽  
Red River ◽  
Granitic Rocks ◽  
Large Igneous Province ◽  
Weighted Mean ◽  
Emeishan Large Igneous Province ◽  
Icp Ms ◽  
Igneous Province ◽  
Single Grain ◽  
Host Rocks

AbstractThe Permian silicic rocks in the Phan Si Pan (PSP) uplift area and Tu Le (TL) basin of NW Vietnam (collectively the PSP-TL region) are associated with the Emeishan Large Igneous Province (ELIP). The Permian Muong Hum, Phu Sa Phin, and Nam Xe - Tam Duong granites, and Tu Le rhyolites are alkali ferroan A1-type granitic rocks, which likely formed by fractional crystallization of high-Ti basaltic magma that was contaminated by melts derived from the Neoproterozoic host rocks. Zircon U-Pb LA-ICP-MS geochronology yielded weighted-mean 206Pb/238U ages of 246 ± 3 Ma to 259 ± 3 Ma for granites, and 249 ± 3 Ma and 254 ± 2 Ma for rhyolites. This is contrasted with previously-published high precision U-Pb ages, obtained using CA-ID-TIMS method applied on the same zircon grains, which suggest that the calculated LA-ICP-MS U-Pb ages are variably inaccurate by up to 10 Ma, though at the single-grain level dates generally agree within uncertainty. The similarity of rock texture, whole-rock geochemistry, emplacement ages, and fractionation phases between the PSP-TL region and silicic rocks in the Inner Zone ELIP (i.e., Panzhihua, Binchuan) suggests they were spatially proximal before being sinistrally displaced along the Ailao Shan-Red River shear zone.

Twin memory and twin amnesia in anorthoclase

2000 ◽  
Vol 64 (2) ◽  
pp. 195-200 ◽  
Author(s):  
S. A. Hayward ◽  
E. K. H. Salje
Keyword(s):  
Phase Transitions ◽  
Transition Temperature ◽  
Alkali Feldspar ◽  
Spatial Distributions ◽  
X Ray Diffraction ◽  
Alkali Cations ◽  
X Ray ◽  
Natural Minerals ◽  
Synthetic Materials ◽  
Before And After

AbstractMany natural minerals and synthetic materials display twin microstructures resulting from displacive phase transitions. These microstructures may be removed temporarily from the sample by heating above the relevant transition temperature, though the twinning generally returns on subsequent cooling.In anorthoclase, the spatial distributions of twins before and after brief annealing above TC are often identical. This property appears to be a common feature in many materials which undergo ferroelastic phase transitions, and is known as ‘twin memory’. The atomic mechanisms responsible for this twin memory may be investigated by studying the annealing regimes required to remove the memory effect; how long must a sample be annealed, and at what temperature, to induce ‘twin amnesia’.High-resolution X-ray diffraction (XRD) has been used to investigate twin memory and twin amnesia in anorthoclase. In anorthoclase, the primary constraint on twin amnesia is thermodynamic, rather than kinetic. The critical temperature to induce amnesia correlates well with the top of the (Na, K) solvus in disordered alkali feldspar. For this reason, the proposed mechanism for twin memory involves the segregation of alkali cations in thin lamellae at the twin boundaries.

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