Katedra mineralógie, petrológie a ložiskovej geológie

Scientists from the Comenius University in Bratislava have discovered a rare new gadolinium mineral in Slovakia

Monazite-(Gd), thenew mineral from Slovakia, found on Prakovce-Zimná voda locality was approved by Commission on New Minerals, Nomenclature and Classification (CNMNC) of International Mineralogical Association (IMA). Although there are ca. 100 new minerals described per year, it is undoubtedly a significant milestone in the field of mineralogy, crystal chemistry, and geochemistry. This new mineral belongs to the group of nominally anhydrous monoclinic REE phosphates. Among them, monazite-(Ce) is the most common end-member, which, however, with zircon is one of the most studied minerals in geosciences, widely used in geochronology, geochemistry, environmental mineralogy and material science. Similarly, it is a principal economic source of rare earth elements together with REE carbonates. In addition to monazite-(Ce); monazite-(La), monazite-(Nd), and monazite-(Sm) occur as rare minerals in nature, and their names follow the Levinson suffix for rare earth minerals (lanthanides Ln and yttrium Y = REE). Newly described monazite-(Gd) is unique due to Gd-dominance on the crystallographic cationic site and shows the highest published Gd content in natural minerals (<23 wt.% Gd₂O₃). It is only the second Gd-dominant mineral worldwide after lepersonnite-(Gd), which, however, contains relatively low absolute concentration of essential Gd (2.1 wt% Gd₂O₃).

Gadolinium-dominant minerals are generally quite rare in nature due to the low absolute abundance of Gd, which is relative to other, more abundant REE (La-Sm, Y) in the Earth’s lithosphere. Selective enrichment of Gd or other individual MREE and HREE in terms of absolute abundances is a rare geochemical process which occurred as a rather, localized anomaly under specific conditions. The monazite-(Gd) is formed by geochemical processes that lead to selective enrichment of hydrothermal fluid by medium rare earth elements (MREE) and with a conspicuous maximum at Gd. Here, the newly approved monazite-(Gd) occurs in association with other REE phosphates and silicates enriched in Gd. The research of these geochemical processes and formation of monazite-(Gd) and other Gd-rich minerals during hydrothermal remobilization of REE and alteration of uranium minerals such as uraninite and brannerite was provided on top analytical instruments by an international collective of scientists from Slovakia, Germany and Czech Republic. The scientific report of Ondrejka M., Uher P., Ferenc Š., Milovská S., Mikuš T., Molnárová A., Škoda R., Kopáčik R a Bačík P.:  Gadolinium-dominant monazite and xenotime: selective hydrothermal enrichment of middle REE during low-temperature alteration of uraninite, brannerite and fluorapatite (the Zimná Voda REE-U-Au quartz vein, Western Carpathians, Slovakia) has been accepted for a peer-reviewed publication in a prestige mineralogical journal American Mineralogist (doi: 10.2138/am-2022-8418). The approved proposal IMA 2022-055 of monazite-(Gd) has been submitted by Ondrejka M., Uher P., Ferenc Š., Majzlan J., Pollok K., Milovská S., Mikuš T., Molnárová A., Škoda R., Kopáčik R., Orovčík Ľ. a Bačík P.

Ferrous hydroxychlorides hibbingite (γ-Fe2(OH)3Cl) and parahibbingite (β-Fe2(OH)3Cl) as a 1 concealed sink of Cl and H2O in ultrabasic and granitic systems

Peter Koděra, Juraj Majzlan, Kilian Pollok, Stefan Kiefer, František Šimko, Eva Scholtzová, Jarmila Luptáková a Grant Cawthorn

American Mineralogist 2022, Vol. 7, 826-841 – Link

Abstract

Ferrous hydroxychlorides are geochemically important but less recognized mineral species due to their extreme sensitivity to oxidation and hydration in contact with air {typically they convert to akaganéite [Fe³⁺(O,OH,Cl)]}. Only the γ-form was previously known as the orthorhombic mineral hibbingite, associated with altered mafic intrusive rocks. In this study, we describe the β-polymorph of Fe₂(OH)₃Cl as a new mineral parahibbingite that was found in pyroxenite from the Karee platinum mine in the Bushveld Complex, South Africa. The two minerals were distinguished by a combination of Raman spectroscopy and FIB-SEM-TEM analytical techniques (TEM-EDX and TEM-SAED). They can be easily recognized by their distinct Raman spectra. Parahibbingite has two very strong vibration bands at ~3550 and 3560 cm^–1, accompanied by much weaker bands at ~124 and 160 cm⁻¹, while the Raman spectrum of hibbingite has a sharp, strong band at 3450 cm⁻¹ and two moderate bands at 199 and 385 cm⁻¹.

Parahibbingite was found as fine-grained reaction rims at the contact of orthopyroxene phenocrysts and talc inside a drill core. It has a trigonal space group [R3m, a = 6.94(5) Å; c = 14.5(2) Å], with an empirical formula (⁠Fe2+1.98Mn2+0.01Fe1.982+Mn0.012+Ca_0.01)(OH)_3.08Cl_0.92. The origin of this mineral in the Bushveld Complex is most likely related to a late hydrothermal alteration of pyroxenite. Hibbingite forms as an abundant daughter mineral hosted by fluid inclusions and salt melt inclusions in hydrothermal quartz associated with granitic systems during cooling under reducing conditions. Such inclusions are common in Au-porphyry mineralization worldwide, such as the Biely Vrch (Slovakia) deposit studied in detail in this work. The lattice parameters obtained by TEM-SAED are a = 6.30 Å, b = 7.12 Å, and c = 9.89 Å.

Hibbingite was recognized as the only phase that carries “water” (as a hydroxyl group) in otherwise water-free, salt melt inclusions. Furthermore, both hibbingite and parahibbingite should be considered as reservoirs for Cl and H₂O in large volumes of altered basic and ultrabasic rocks. They can transport volatiles to shallow levels of subduction zones. Alternatively, their dissolution can fuel remobilization, transport, and deposition of sulfidic ores in saline fluids. Their detection, however, is difficult because of their sensitivity to oxidizing atmospheres. For example, in natural outcrops exposed to air, they may vanish, thus distorting estimates of their abundance and role in many processes that involve mineral-derived volatiles.

The Department of Mineralogy, Petrology and Economic Geology express support and solidarity to the people of Ukraine. We hope that peace will be restored as soon as possible.

A shot from the peaceful candlelight march, that took place on February 24, 2022 from 6 PM in Bratislava and was attended by thousands of people.