The relative criticality of mineral commodities is evaluated using a wide range of parameters and in different contexts (e.g., from the standpoint of their importance to national security, or to a specific industrial application), which explains the multiplicity of classification schemes and variations in terminology applied to these commodities in the literature, media and government reports. The core group of critical metals, listed alphabetically, includes: antimony, beryllium, chromium, cobalt, gallium, germanium, indium, lithium, niobium, platinoids, rare-earth elements (REE, including yttrium), tantalum and tungsten. The present retrospect briefly describes the emergence of critical metals as a distinct resource type and the evolution of society's perception of these commodities over the past 100 years.
Carbonatites and alkaline-silicate rocks are the most important sources of rare earth elements (REE) and niobium (Nb), both of which are metals imperative to technological advancement and associated with high risks of supply interruption. Cooling and crystallizing carbonatitic and alkaline melts expel multiple pulses of alkali-rich aqueous fluids which metasomatize the surrounding country rocks, forming fenites during a process called fenitization. These alkalis and volatiles are original constituents of the magma that are not recorded in the carbonatite rock, and therefore fenites should not be dismissed during the description of a carbonatite system. This paper reviews the existing literature, focusing on 17 worldwide carbonatite complexes whose attributes are used to discuss the main features and processes of fenitization. Although many attempts have been made in the literature to categorize and name fenites, it is recommended that the IUGS metamorphic nomenclature be used to describe predominant mineralogy and textures. Complexing anions greatly enhance the solubility of REE and Nb in these fenitizing fluids, mobilizing them into the surrounding country rock, and precipitating REE- and Nb-enriched micro-mineral assemblages. As such, fenites have significant potential to be used as an exploration tool to find mineralized intrusions in a similar way alteration patterns are used in other ore systems, such as porphyry copper deposits. Strong trends have been identified between the presence of more complex veining textures, mineralogy and brecciation in fenites with intermediate stage Nb-enriched and later stage REE-enriched magmas. However, compiling this evidence has also highlighted large gaps in the literature relating to fenitization. These need to be addressed before fenite can be used as a comprehensive and effective exploration tool.
Abstract Th-rich (up to 18.4 wt% ThO 2 ) loparite occurs as an accessory phase in foyaite pegmatites at Mt. Eveslogchorr, Khibina complex, Russia. It is associated with aegirine, astrophyllite, eudialyte, lorenzenite, lamprophyllite, magnesio-arfvedsonite and gerasimovskite. Loparite crystals are zoned from niobian loparite (core) to niobian thorian and thorian niobian loparite (rim). Th-enrichment is accompanied by a decrease in Na, LREE , Sr and increase in A-site vacancies. The most Th-rich composition approaches (Na 0.39 LREE 0.19 Th 0.12 Ca 0.05 Sr 0.02 ) Σ0.77 (Ti 0.76 Nb 0.27 ) Σ1.03 O 3 . The mineral is partly or completely metamict and after annealing gives an X-ray diffraction powder pattern similar to that of synthetic NaLaTi 2 O 6 and naturally occurring loparite of different composition. For the Th-rich rim sample, the five strongest diffraction lines (Å) are: 2.72 (100) 110, 1.575 (60) 211, 1.925 (40), 1.368 (30) 220, 1.222 (20) 310; a = 3.867(2) Å. The X-ray diffraction patterns do not exhibit peak splitting or other diffraction lines typical of low-symmetry and ordered perovskite-type structures. Composition determinations, infrared transmission spectroscopy and X-ray diffractometry show that thorian loparite is partly replaced by betafite with LREE and Th as dominant A -site cations (‘ceriobetafite’). Some loparite samples also exhibit thin replacement mantles of belyankinite with high LREE 2 O 3 and ThO 2 contents. Both ‘ceriobetafite’ and belyankinite were formed due to metasomatic alteration of loparite.
Titanite is a relatively rare Ti silicate in carbonatitic rocks. It is a primary phase in alkali rich carbonat ites, and may also occur in silicocarbonatit es whose composition was modified by assimilation of wallrock silicate material. More typical is late-stage titanite that forms by reaction of a precursor Ti mineral with deuteric fluids. Both genetic types show significant variations in chemical composition arising mostly from the substitution of Ti with AI, Fe, Nb and Zr. Cationic substitutions at the Ca site are limited to several atomic per cent. Zoning in primary titanite typically involves a decrease in the proportion of Nb and Zr toward the rim, whereas deuteric crystals show the reverse zoning pattern.
Summary A swarm of subparallel steeply dipping carbonate dikes is exposed on numerous small islands in the central part of Paint Lake in the Superior Boundary Zone in central Manitoba. The swarm has been traced over a distance of 21 km and is generally conformable to the regional tectonic structure and gneissosity. The principal constituent of all dikes is calcite enriched in Sr, Y and rare-earth elements (REE) and showing evidence of plastic deformation and cataclasis, but the modal composition and texture of individual bodies vary from anchimonomineralic zones of coarse-grained calcite to fine-grained saccharoidal rocks with phlogopite-rich stringers to inequigranular foliated varieties containing a large proportion of calcic amphiboles, apatite, diopside, scapolite and xenocrysts. Regardless of these textural variations, the rocks are consistently enriched in Sr, light REE and show 18 OSMOW, 13 CPDB, Y/Ho, Zr/Hf, Th/U and Nb/Ta ratios similar to the primitive-mantle values. The contents of chalcophile and high-field-strength elements are systematically low. On the basis of the available structural, petrographic and geochemical data, the examined rocks are interpreted as calcite carbonatites of postorogenic affinity. The Paint Lake carbonatites host a variety of REE minerals, including (in order of decreasing abundance): allanite, titanite, monazite and bastnasite.
Marianoite, a new member of the cuspidine group of minerals, occurs in phlogopite–calcite silicocarbonatite in the western part of the Mesoproterozoic Prairie Lake intrusive complex, in northwestern Ontario, Canada. The mineral forms flattened prismatic crystals with resorbed faces, up to 0.3 mm in length, and is associated with uranoan pyrochlore, titanite and natrolite–muscovite pseudomorphs after xenocrystic nepheline. The crystals are translucent, very pale yellow macroscopically and colorless in plane-polarized light. The mineral is biaxial negative (α 1.700, β 1.715, γ 1.725, 2 V meas 80°, 2 V calc 78°) and shows a weak optic-axis dispersion ( r v ). Marianoite is relatively homogeneous in composition; its average empirical formula is Na 1.93 (Ca 4.00 Mn 0.04 ) ∑4.04 (Nb 0.97 Zr 0.90 Ti 0.09 Fe 0.08 Mg 0.03 Hf 0.01 ) ∑2.08 (Si 3.97 O 14 )O 2.93 F 1.07 . By analogy with wohlerite, the simplified formula of marianoite should be written Na 2 Ca 4 (Nb,Zr) 2 (Si 2 O 7 ) 2 (O,F) 4 . The structure of the new mineral species, refined by single-crystal methods to an R 1 of 4.65% (for | F o | > 4σ F ), is monoclinic, space group P 2 1 ; the unit-cell parameters are: a 10.8459(15) A, b 10.2260(14) A, c 7.2727(10) A, β 109.332(3)°, V 761.1(3) A 3 ( D calc 3.45 g/cm 3 ). The mineral is isostructural with wohlerite [Na 2 Ca 4 (Zr,Nb) 2 (Si 2 O 7 ) 2 (O,F) 4 ], but shows the preponderance of Nb in the smallest octahedrally coordinated cation sites in its crystal structure. The occupancy of these sites cannot be determined accurately because of the similar X-ray scattering characteristics and ionic radii of Nb 5+ and Zr 4+ . The mineral is named in honor of Anthony Nicola Mariano (b. 1930), in recognition of his contributions to the study of alkaline rocks and carbonatites.
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