A Single Step Acid Assisted Microwave Digestion Method for the Complete Dissolution of Bauxite and Quantitation of its Composition (Al2O3, Fe2O3, SiO2, TiO2, Cr2O3, MgO,MnO and V2O5) by ICP-AES

An acid assisted microwave-based method for the complete dissolution of bauxite using mixture of H₂SO₄, H₃PO₄ and HF acids in a single step was developed for the determination of various analytes (Al₂O₃, Fe₂O₃, SiO₂, TiO₂, Cr₂O₃, MgO, MnO and V₂O₅) using ICP-AES. The method was validated with respect to ruggedness, linearity, trueness, precision, limit of detection (LOD), limit of quantification (LOQ), working range and measurement uncertainties by analysing a bauxite reference material (Alcan BXT-12) and four certified reference materials (IPT-131, BXBA-4, NIST SRM 600, NIST SRM 697). The expanded uncertainties obtained for Al₂O₃ (40.0%), Fe₂O₃ (17.0%), SiO₂ (20.3%), TiO₂ (1.31%), Cr₂O₃ (0.024%), MgO (0.05), MnO (0.013), and V₂O₅ (0.60%), were 0.80, 0.40, 0.50, 0.033, 0.0008, 0.002, 0.0007 and 0.002 respectively, which are fit for the intended use to characterise bauxite. The developed method was also evaluated through participation in an interlaboratory comparison exercise organised by the Jawaharlal Nehru Aluminium Research Development and Design Centre (JNARDDC), Nagpur, India, using bauxite sample (BXT-JNA), with satisfactory z-scores achieved.     

黔北务川瓦厂坪铝土矿床元素迁移规律研究

黔北务正道地区铝土矿矿床属古风化壳沉积型,成矿母岩具多源性,但主要来源于下覆的中下志留统韩家店组。本文依据Grant提出的质量平衡方程和图解法,对瓦厂坪矿床在成矿过程中元素迁移的富集、贫化（亏损）规律进行了定量研究。结果表明,主要成矿母岩韩家店组砂、页岩一中间产物铝土质页岩、粘土岩是主要元素A12O3、TiO2显著富集...     

The stability of sapphirine + quartz: calculated phase equilibria in FeO–MgO–Al2O3–SiO2–TiO2–O

The presence in rocks of coexisting sapphirine + quartz has been widely used to diagnose conditions of ultra‐high‐temperature (UHT) metamorphism (>900 °C), an inference based on the restriction of this assemblage to temperatures >980 °C in the conventionally considered FeO–MgO–Al₂O₃–SiO₂ (FMAS) chemical system. With a new thermodynamic model for sapphirine that includes Fe₂O₃, phase equilibra modelling using thermocalc software has been undertaken in the FeO–MgO–Al₂O₃–SiO₂–O (FMASO) and FeO–MgO–Al₂O₃–SiO₂– TiO₂–O (FMASTO) chemical systems. Using a variety of calculated phase diagrams for quartz‐saturated systems, the effects of Fe₂O₃ and TiO₂ on FMAS phase relations are shown to be considerable. Importantly, the stability field of sapphirine + quartz assemblages extends down temperature to 850 °C in oxidized systems and thus out of the UHT range.     

Mineralogy, Chemical Characteristics and Upgrading of Beach Ilmenite of the Top Meter of Black Sand Deposits of the Kafr Al-Sheikh Governorate, Northern Egypt

Egyptian beach ilmenite occurs in a relatively high content in the naturally highly concentrated superficial black sand deposits at specific beach zones in the northern parts of the Nile Delta at Rosetta. Microscopic study shows that the ilmenite occurs as fresh homogeneous black or heterogeneous multicoloured altered grains and exhibits three types (homogeneous, exsolved and altered) of ilmenite varieties. XRD data of ilmenite indicates their association with minor hematite and quartz, whereas leucoxene shows its association with Nb‐rutile, pseudorutile and hematite. Grain size distribution suggests a very fine sand size of >89% and 80% and a fine sand size of 10.5% and 18.3% for fresh and altered ilmenites, respectively. The density of fresh, altered ilmenite and leucoxene concentrates varies from 2.70, 2.50 to 2.40 ton/m³, suggesting a gradual decrease from high grade fresh to leucoxene and consistent with variation in magnetic susceptibility as a consequence of the leaching of iron. Mass magnetic susceptibility reveals 97.6% of ilmenite and 92% of the altered form are obtained at 0.20 and 0.48 ampere. Fresh ilmenite exhibits variable TiO₂ (47.18%) and Fe₂O₃^T (46.10%) with minor MnO, MgO and Cr₂O₃ (1.22, 1.10 and 0.51%). The altered ilmenite is higher in TiO₂ (76.16%) and SiO₂ (4.68%) and lower in Fe₂O₃^T (14.45%), MnO, MgO and Cr₂O₃ (0.39, 0.52 and 0.11%) compared with the fresh form. Three concentrates of ilmenites (G1, G2 and G3) were prepared from crude ore using a Reading cross belt magnetic separator under different conditions, revealing a gradual increase of TiO₂, SiO₂, Al₂O₃ and CaO accompanied by a decrease of Fe₂O₃^T, MgO and Cr₂O₃ with repetition of the separation processes. Several ore dressing techniques were carried out to upgrade the ilmenite concentrate.     

Mineralogy,geochemistry, and ore formation of Nuwaifa bauxite deposit,western desert of Iraq

Nuwaifa Formation is a part of sequence stratigraphy that belongs to the Jurassic system exposed in the western desert of Iraq. The Jurassic system consists of Ubaid, Hussainiyat, Amij, Muhaiwir, and Najmah formations. Each formation is composed of basal clastic unit overlain by upper carbonate unit. Nuwaifa karst bauxite was developed in fossil karsts within the Ubaid Formation in areas where maximum intersection of fractures and faults exist. This bauxitization process affected the upper surface of the Ubaid limestone formation, which directly underlies the Nuwaifa bauxite Formation. Nuwaifa Formation represents karst-filling deposit that consists of a mixture of allochthonous (sandstone, claystone, and mudstone) and autochthonous lithofacies (bauxite kaolinite, kaolinitic bauxite, iron-rich bauxite, and flint clay). Most bauxite bodies occur within the autochthonous lithofacies and are lenticular in shape with maximum thickness ranges from few meters to 35 m and in some place up to 100 m. Petrographically, the bauxite deposit exhibits collomorphic-fluidal, pisolitic, oolitic, nodular, brecciated, and skeletal textures indicative of authigenic origin. Mineralogy boehmite and gibbsite are the only bauxite minerals; the former is dominant in the upper parts of the bauxite profiles, whereas the latter is dominant throughout the lower and middle part of the bauxite. Kaolinite, hematite, goethite, calcite, and anatase occur to a lesser extent. The study bauxites are mainly composed of Al₂O₃ (33–69.6 wt.%), SiO₂ (8.4–42 wt.%), Fe₂O₃ (0.5–15.9 wt.%), and TiO₂ (0.7–6.1 wt.%) with LOI ranging from 13.5 to 19.1 wt.%. Geochemical investigations indicate that the immobile elements like Al₂O₃, TiO₂, Cr, Zr, and Ni were obviously enriched, while SiO₂, Fe₂O₃, CaO, MgO, Zn, Co, Ba, Mn, Cu, and Sr were depleted during bauxitization process. The results of this study strongly suggest that the bauxite deposits of the Nuwaifa Formation are derived from the kaolinite of the Lower Hussainiyat Formation.     

REE Tetrad Effect as a Powerful Indicator of Formation Conditions of Karst Bauxites: A Case Study of the Shahindezh Deposit,NW Iran

Study of the concentration of major, trace, and rare earth elements (REE) in the Shahindezh karst bauxite deposit, northwestern Iran clarifies the relationship of the tetrad effect with geochemical parameters in the bauxite ores. The existence of irregular curves in the chondrite-normalized REE patterns as well as non-CHARAC behavior of geochemically isovalent pairs (Y/Ho) are related to the tetrad effect. The meaningful positive correlation between the sizes of the calculated T3 tetrad effect and some geochemical factors such as Y/Ho, ΣREE, La/Y, (La/Yb)N, and (LREE/HREE)N as well as some major oxides-based parameters like Al2O3 + LOI/SiO2 + Fe2O3, Al2O3/Fe2O3, Al2O3 + LOI, IOL, and SiO2 + Fe2O3 indicate that the studied bauxite horizon was likely deposited by different (acidic and/or alkalic) solutions at different stages. The lower part of the studied horizon with a thickness of ~4.7 m displays alkali characteristics whereas the upper parts of the horizon with a thickness of ~5.3 m are characterized by more acidic conditions. These results are fully supported by the co-occurrence of convex-concave tetrad effect curves in the chondrite-normalized REE patterns. Therefore, the tetrad effect phenomenon used in this study has proved to be a good and reliable geochemical proxy to assess the conditions of the depositional environment in the Shahindezh bauxite ores.     

Minor element abundances in olivines of the Sharps (H-3) chondrite

Olivine crystals in 21 chondrules from the Sharps (H-3) chondrite were analyzed for CaO, Al₂O₃, Cr₂O₃, MnO, TiO₂, NiO, and Na₂O. The chondrules studied include representatives of all major types found in Sharps, and the mean fayalite contents of their olivine range from 1 to 28 %. Those olivines which contain less than 18 mol.% fayalite typically contain or occur with metallic nickel-iron; the others are metal-free.Na₂O is below detectability (0.01 wt.%) in all cases, and the abundances of Al₂O₃, NiO and TiO₂ are also typically very low. MnO varies simply and directly with FeO.Cr₂O₃ varies widely (0.03–0.21%) and several lines of evidence suggest that Cr is dominantly trivalent. It is concluded that F_{O 2} was rarely less than 10^–11 atm. during the crystallization of the chondrules in Sharps.     

安徽栏杆含金刚石基性岩中石榴子石矿物学特征

在中国东部皖北地区分布着新元古代镁铁质岩,其中一些碱性基性岩为金刚石的赋矿岩石。为了确定安徽栏杆金刚石矿区的石榴子石种类,对矿区内不同类型的石榴子石进行系统采样,测定了62件石榴子石微区化学成分。结果显示,安徽栏杆石榴子石矿物化学式A_3~(2+)B_2~(3+)(SiO_4)_3中的A组阳离子由Mg~(2+)、Fe~(2+)和Ca~(2+)离子占位,B主要由Al~(3+)、Fe~(3+)、Mn~(3+)和Cr~(3+)离子占位,三价阳离子主要为Al~(3+),二价阳离子主要为Ca~(2+),表明研究区石榴子石主要为钙铝-钙铁-镁铝石榴子石系列。在62个样品中,发现了超硅石榴子石。经过计算其形成的压力范围为12.1～12.8GPa,深度可达300km。     

Geochemistry of major and trace elements and Pb–Sr isotopes of a weathering profile developed on the Lower Cambrian black shales in central Hunan,China

This paper reports a geochemical study on the major and trace elements and Pb–Sr isotopes of a weathering profile developed in the Lower Cambrian black shales in central Hunan (China). Six weathering horizons were identified and sampled vertically throughout the profile. The chemical composition of the profile consists of variable concentrations of the major elements Fe₂O₃, FeO, MnO, MgO, CaO, Na₂O, and P₂O₅ and of less variable concentrations of SiO₂, TiO₂, Al₂O₃, and K₂O. The chemical change caused by weathering is estimated by mass-balance calculations, and the results show that the element mobility is characterised by substantial loss of SiO₂, FeO, CaO, K₂O, Na₂O, LOI, Cr, V, Ba, Cs, Rb, Sr, U, and Th, and moderate loss of Al₂O₃, MgO, Fe₂O₃, Ni, Cu, Pb, Tl, Sn, Sc, Ge and REE (Y). The high field strength elements TiO₂, Sn, Sc, U, Ga, Ge, Zr, Hf, Nb, and Ta were immobile during weathering. The chemical changes and the Pb–Sr isotopic data suggest that four types of chemical reactions occurred: the oxidation of sulphide minerals (e.g., pyrite) and organic carbon (OS), the dissolution of less resistant clinochlore-Ia, calcite, and P-bearing minerals (DL), the dissolution of detrital albite and microcline (DA), and the transformation of clay (TC) minerals (e.g., muscovite and illite–smectite). These chemical reactions then led to two stages of geochemical processes, an early stage of chemical differentiation and a later stage of chemical homogenisation. The chemical differentiation dominated by the OS, DL, and DA reactions, led to the leaching of mobile elements (e.g., MgO, Na₂O, K₂O, P₂O₅, Sr, and REE) and the redistribution of some less mobile elements (e.g., SiO₂ and Al₂O₃). In contrast, the chemical homogenisation, which was caused by TC reactions, led to the leaching of both mobile and less mobile elements from the system and ultimately transformed the weathered black shales into soil. Soils derived from black shales in South China might result from the above two geochemical processes.     

Perrierite-(La), (La,Ce,Ca)4(Fe2+,Mn)(Ti,Fe3+,Al)4(Si2O7)2O8, a new mineral species from the Eifel volcanic district, Germany

Non-metamict perrierite-(La) discovered in the Dellen pumice quarry, near Mendig, in the Eifel volcanic district, Rheinland-Pfalz, Germany has been approved as a new mineral species (IMA no. 2010-089). The mineral was found in the late assemblage of sanidine, phlogopite, pyrophanite, zirconolite, members of the jacobsite-magnetite series, fluorcalciopyrochlore, and zircon. Perrierite-(La) occurs as isolated prismatic crystals up to 0.5 × 1 mm in size within cavities in sanidinite. The new mineral is black with brown streak; it is brittle, with the Mohs hardness of 6 and distinct cleavage parallel to (001). The calculated density is 4.791 g/cm³. The IR spectrum does not contain absorption bands that correspond to H₂O and OH groups. Perrierite-(La) is biaxial (-), α = 1.94(1), β = 2.020(15), γ = 2.040(15), 2V _meas = 50(10)°, 2V _calc = 51°. The chemical composition (electron microprobe, average of seven point analyses, the Fe²⁺/Fe³⁺ ratio determined from the X-ray structural data, wt %) is as follows: 3.26 CaO, 22.92 La₂O₃, 19.64 Ce₂O₃, 0.83 Pr₂O₂, 2.09 Nd₂O₃, 0.25 MgO, 2.25 MnO, 3.16 FeO, 5.28 Fe₂O₃, 2.59 Al₂O₃, 16.13 TiO₂, 0.75 Nb₂O₅, and 20.06 SiO₂, total is 99.21. The empirical formula is (La_1.70Ce_1.45Nd_0.15Pr_0.06Ca_0.70)_Σ4.06(Fe _0.53 ²⁺ Mn_0.38Mg_0.08)_Σ0.99(Ti_2.44Fe _0.80 ³⁺ Al_0.62Nb_0.07)_Σ3.93Si_4.04O₂₂. The simplified formula is (La,Ce,Ca)₄(Fe²⁺,Mn)(Ti,Fe³⁺,Al)₄(Si₂O₇)₂O₈. The crystal structure was determined by a single crystal. Perrierite-(La) is monoclinic, space group P2₁/a, and the unit-cell dimensions are as follows: a =13.668(1), b = 5.6601(6), c = 11.743(1) Å, β = 113.64(1)°; V = 832.2(2) ų, Z = 2. The strong reflections in the X-ray powder diffraction pattern are [d, Å (I, %) (hkl)]: 5.19 (40) (110), 3.53 (40) ( $\overline 3 $ 11), 2.96 (100) ( $\overline 3 $ 13, 311), 2.80 (50) (020), 2.14 (50) ( $\overline 4 $ 22, $\overline 3 $ 15, 313), 1.947 (50) (024, 223), 1.657 (40) ( $\overline 4 $ 07, $\overline 4 $ 33, 331). The holotype specimen of perrierite-(La) is deposited at the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow, Russia, with the registration number 4059/1.     

A comparison of automatic and manual wet chemical analysis of rocks

Many modern geochemical studies require large numbers of analyses. An automatic scheme for the analysis of SiO₂, TiO₂, Al₂O₃, Fe₂O₃, Na₂O, K₂O, P₂O₅, CaO, MgO and MnO is described using a commercially available autoanalyser and an atomic absorption spectrophotometer. Data on the precision is compared to that obtained using conventional rapid methods of analysis. The results show that the automated procedures offer at least a threefold increase in production with little or no loss in precision.     

云南会泽大黑山铝土矿地球化学特征及其物源分析

大黑山铝土矿赋存于上二叠统宣威组底部,下伏地层为峨眉山玄武岩。矿石结构主要以泥晶结构为主,具有少量鲕状、粒屑结构。矿石构造以致密块状构造为主,局部呈层状、似层状。铝土矿石中Al2O3与Fe2O3、Si O2呈现较好的负相关关系,Ti O2与Zr、Hf、Nb、Ta相关性较好,Zr-Hf、Nb-Ta的相关性拟合度很高。铝土矿与峨眉山玄武岩样品具有相似的稀土元素配分模式。综合稳定元素相关性、微量元素及稀土元素标准化图解、lg Ni和lg Cr二元图解等分析方法对大黑山铝土矿的成矿物质来源进行探讨,研究结果表明铝土矿的成矿物质来源主要来自峨眉山玄武岩。     

The effect of TiO2 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies conditions: mineral equilibria calculations in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3

Mineral equilibria calculations in the system K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (KFMASHTO) using thermocalc and its internally consistent thermodynamic dataset constrain the effect of TiO₂ and Fe₂O₃ on greenschist and amphibolite facies mineral equilibria in metapelites. The end‐member data and activity–composition relationships for biotite and chloritoid, calibrated with natural rock data, and activity–composition data for garnet, calibrated using experimental data, provide new constraints on the effects of TiO₂ and Fe₂O₃ on the stability of these minerals. Thermodynamic models for ilmenite–hematite and magnetite–ulvospinel solid solutions accounting for order–disorder in these phases allow the distribution of TiO₂ and Fe₂O₃ between oxide minerals and silicate minerals to be calculated. The calculations indicate that small to moderate amounts of TiO₂ and Fe₂O₃ in typical metapelitic bulk compositions have little effect on silicate mineral equilibria in metapelites at greenschist to amphibolite facies, compared with those calculated in KFMASH. The addition of large amounts of TiO₂ to typical pelitic bulk compositions has little effect on the stability of silicate assemblages; in contrast, rocks rich in Fe₂O₃ develop a markedly different metamorphic succession from that of common Barrovian sequences. In particular, Fe₂O₃‐rich metapelites show a marked reduction in the stability fields of staurolite and garnet to higher pressures, in comparison to those predicted by KFMASH grids.     

The chemistry of gabbro/amphibolite transitions in South Norway

Sixteen gabbro (hyperite)/amphibolite pairs were collected from geological situations where it seems clear that the amphibolite formed from material similar in composition to that of the adjacent hyperite.Major element analyses show that the hyperite to amphibolite transition was not an isochemical one. K₂O, H₂O, P₂O₅ and Fe₂O₃ were increased strongly. CaO and FeO were diminished; SiO₂ may have been also. TiO₂, total iron, MgO, MnO and Na₂O were static; so probably was Al₁O₃.     

Biomineral nanostructures of manganese oxides in oceanic ferromanganese nodules

Manganese oxides, which are widespread and of great practical importance, are formed and transformed by the active role of microorganisms. Manganese aggregates occur as both crystallized varieties and disordered fine-grained phases with significant ore grade and up to 50–60 vol % of X-ray amorphous components. X-ray amorphous nanosized Mn oxides in Fe-Mn nodules from the Pacific Ocean floor were examined from the standpoint of their biogenic origin. SEM examination showed abundant mineralized biofilms on the studied samples. The chemical composition of bacterial mass is as follows (wt %): 28.34 MnO, 17.14 Fe₂O₃, 7.11 SiO₂, 2.41 CaO, 17.90 TiO₂, 1.74 Na₂O, 1.73 Al₂O₃, 1.30 MgO, 1.25 P₂O₅, 1.25 SO₃, 0.68 CoO, 0.54 CuO, 0.53 NiO, and 0.50 K₂O. The chemical composition of fossilized cyanobacterial mats within the interlayer space of nodulesis as follows (wt %): 48.35 MnO, 6.23 Fe₂O₃, 8.76 MgO, 5.05 Al₂O₃, 4.45 SiO₂, 3.63 NiO, 2.30 Na₂O, 2.19 CuO, 1.31 CaO, and 0.68 K₂O is direct evidence for participation of bacteria in Mn oxide formation. This phase consists of mineralized glycocalix consisting of nanosized flakes of todorokite. Native metals (Cu, Fe, and Zn) as inclusions 10–20 μm in size were identified in ferromanganese nodules as well. The formation of nativemetals can be explained by their crystallization at highly reducing conditions maintained by organic matter.     

Atypical,High-Fe2O3, Alpine-Type Chromite Mineralization at Guneyocak,Sivas Province,East-Central Turkey

The Güneyocak chromite mineralization is hosted by the Upper Cretaceous Divrigi ophiolitic melange, which consists of serpentinite, serpentinized harzburgite and dunite, gabbro, diabase dikes, pyroxenite, blocks of limestone, and radiolarite. Serpentinites were intensely listwaenitized near the mineralization and in other locations in the study area. The Guneyocak chromite mineralization is of interest because of its internal structure and abundant, repeated chromitite bands, as well as for its chemistry. These features are unusual for ophiolite-hosted chromite. Major-element chemistry shows that the chromites have very high Fe₂O₃ and MgO and very low FeO. The Guneyocak chromites are classified as of Alpine type on the basis of host-rock lithology and Cr₂O₃, Al₂O₃, FeO(T), and Cr/Fe values. However, the very high Fe₂O₃ and MgO and very low FeO compositions of the chromites do not correspond to those of an Alpine-type chromite deposit. Repeated chromite banding and high Fe₂O₃ content of the chromite strongly suggest repeated oxygen fugacityf(O₂) fluctuations and that the Guneyocak mineralization formed at relatively shallow depths. The Güneyocak chromite is characterized by a slightly boninitic character, which represents high partial melting under conditions of high oxygen fugacity. We conclude that the Guneyocak chromite mineralization formed in the uppermost part of the ultramafic rock series of the Divrigi ophiolitic melange.     

The Effect of Cr2O3 on the Partial Melting of Spinel Lherzolite in the System CaO-MgO-Al2O3-SiO2-Cr2O3 at 1{middle dot}1 GPa

Chromium as Cr³⁺ substitutes for octahedrally coordinated Alin upper-mantle minerals, thereby reducing the activity of Al₂O₃in the system and hence the concentration of Al₂O₃ in partialmelts. The effect of Cr₂O₃ on melt compositions multiply saturatedwith the spinel lherzolite phase assemblage has been quantifiedin the system CaO–MgO–Al₂O₃–SiO₂–Cr₂O₃at 1·1 GPa as a function of 100 Cr/(Cr + Al) in the spinel(Cr#_sp). The decrease of Al₂O₃ in the melt with increasing Cr#_spis accompanied by increasing MgO and SiO₂, whereas CaO remainsalmost constant. Consequently, the CaO/Al₂O₃ ratio of the meltincreases with Cr#_sp, and the melt becomes richer in normativediopside, hypersthene and quartz. The effect may explain certainmantle melts with unusually high CaO/Al₂O₃ ratios. The concentrationof Cr₂O₃ in the melt remains low even at high Cr#_sp, which meansthat the strong effect of Cr₂O₃ on partial melting equilibriais not readily apparent from its concentration in the melt itself.The existence of a highly refractory major component such asCr₂O₃ nullifies simplified conclusions from the ‘inverseapproach’ in the experimental study of basalt petrogenesis,as there is insufficient information in the composition of thepartial melt to reconstruct the conditions of melting. KEY WORDS: basalt petrogenesis; partial melting; reversal experiment; spinel lherzolite; system CMAS–Cr₂O₃; CaO/Al₂O₃ of melt; effect of Cr₂O₃     

Magnesioneptunite, KNa2Li(Mg,Fe)2Ti2Si8O24, a new mineral species of the neptunite group

A new mineral of the neptunite group, magnesioneptunite KNa₂Li(Mg,Fe)₂Ti2Si₈O₂₄, a Mg-dominant analogue of neptunite and manganoneptunite, has been found in the Upper Chegem caldera near Mount Lakargi, Kabardino-Balkaria, the North Caucasus, Russia in a xenolith of altered sandstone located between skarnified carbonate xenoliths and ignimbrite. Magnesioneptunite occurs as nearly isometric grains and aggregates up to 0.1 mm in size in the cores of some grains of a Mg-rich variety of neptunite with Mg/(Fe + Mn) = 0.7?1.0. The chemical composition of magnesioneptunite with a maximum Mg content is as follows, wt %: 3.63 K₂O, 8.21 Na₂O, 1.73 Li₂O, 6.47 MgO, 0.04 MnO, 5.87 FeO, 0.07 Al₂O₃, 18.73 TiO₂, 56.88 SiO₂, 99.62 in total. The empirical formula is (K_0.67Na_0.32Ca_0.01)_Σ1.00Na_2.06Li_1.00 · (Mg_1.39Fe _0.71 ²⁺ )_Σ2.10(Si_7.90Al_0.01)_Σ7.91O₂₄. Grains of magnesioneptunite are dark brown to red-brown, translucent, with vitreous luster. D _calc = 3.15 g/cm³, and the Mohs hardness is 5–6. Cleavage parallel to the (110) is perfect. The new mineral is optically biaxial, positive, α = 1.697(2), β = 1.708 (3), γ = 1.725(3), 2V _meas = 45(15)°. The mineral is associated with quartz, alkali feldspar, rutile, aegirine, and neptunite. Magnesioneptunite and the Mg-rich variety of neptunite were formed as products of ilmenite alteration. Magnesioneptunite is monoclinic, C2/c; unit-cell parameters: a = 16.327(7), b = 12.4788(4), c = 9.9666(4) Å, β = 115.6519(5)°, V = 1830.5(1) ų, Z = 4. The type specimen is deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow.     

青藏米巴勒地区中新世钠质方沸石(霞石)响岩中单斜辉石环带研究:对岩浆-构造演化的启示

在拉萨地块中部米巴勒地区产出的中新世钠质方沸石(霞石)响岩中的单斜辉石发育大量环带结构,环带结构包括正环带、反环带和韵律环带。其中正环带结构从核部到边部Mg^#、MgO、Cr₂O₃呈逐渐降低趋势,而FeO、Na₂O、Al₂O₃、TiO₂呈逐渐升高的趋势,它反映了岩浆正常结晶的历史。反环带结构中Mg^#、MgO、Cr₂O₃从核部到边部呈升高的趋势,FeO、Na₂O、Al₂O₃的含量总体上呈降低趋势,本文认为响岩中单斜辉石的这种环带结构是受到了和它共存的钾质-超钾质火山岩岩浆混合或混染所致,两种岩石应是各自独立的母岩浆的结晶产物。具韵律环带结构的单斜辉石从核部到边部出现成分规律性波动,Na₂O、Al₂O₃、TiO₂、Cr₂O₃含量的变化在正反环带的范围内,Mg^#值的变化范围较小,在正环带斑晶和基质的范围内,它是岩浆多次混合作用的结果。该地区钠质过碱性岩的出现可能指示青藏高原在这套火山岩喷发时(13~12Ma),构造体制发生了显著的改变,进入到一个典型的陆内伸展阶段。     

Statistical analysis and source rock of the Minim-Martap plateau bauxite,Cameroon

The Minim-Martap plateau bauxite deposit, located between the Minim and the Martap villages, is one of the 11 plateaus within the Minim-Martap bauxite region. The plateau has an elevation of 1294 m above sea level, with three to more 30 m thickness of bauxite horizon. These plateaus were formed as result of supergene weathering of volcanic rocks occurring as dissected flow basalt landscapes that form relatively flat plateau rising steeply from the surrounding granites. The bauxite deposit of the plateau is lateritic, with the surface of the plateau been completely covered by indurated caps. Seventeen bauxite samples were collected from the plateau and prepared for geochemical analysis. Whole rock analysis was carried out using the X-ray Fluorescence technique and ICP-MS was used for trace elements investigation. Statistical analysis reveals that average values of Al₂O₃ (54.87%), Fe₂O₃ (7.17%), SiO₂ (2.44%), and TiO₂ (4.54%) indicate the plateau bauxite deposit is an of a world class standard with very little impurities compared to the standard major element contents of bauxite (>?40% A1₂O₃, less than <?20% Fe₂O₃, and less than <?8% combined SiO₂). Abundant trace elements include Zr, Ce, Sr, V, Ba, La, Nd, Ga, and Nb. Weathering due to chemical alteration indices using the Ruxton ratio and CIA approaches revealed the plateau have undergone intense weathering process that formed the bauxite deposit. Three different classification systems indicate it as a low iron-rich bauxite deposit. Precursor rock investigation indicates the origin of the bauxite is mafic, basaltic andesite igneous rocks with intermediate pH (basic–acidic characteristic).