Experimental studies increasingly often report low-temperature (200–800 °C) and low-pressure (0.05–3 kbar) hydrosilicate fluids with >40 wt.% of SiO2 and >10 wt.% of H2O. Compositionally similar fluids were long suggested to potentially exist in natural systems such as pegmatites and hydrothermal veins. However, they are rarely invoked in recent petrogenetic models, perhaps because of the scarcity of direct evidence for their natural occurrence. Here we review such evidence from previous works and add to this by documenting inclusions of hydrosilicate fluids in quartz and feldspar from Itrongay. The latter comprise opal-A, opal-CT, moganite and quartz inclusions that frequently contain H2O and have negative crystal shapes. They coexist with inclusions of CO2- and H2O-rich fluids and complex polycrystalline inclusions containing chlorides, sulphates, carbonates, arsenates, oxides, hydroxides and silicates, which we interpret as remnants of saline liquids. Collectively, previous studies and our new results indicate that hydrosilicate fluids may be common in the Earth’s crust, although their tendency to transform into quartz upon cooling and exhumation renders them difficult to recognise. These data warrant more comprehensive research into the nature of such hydrosilicate fluids and their distribution across a wide range of pressure and temperature conditions and geological systems.
The Datian uranium deposit is a migmatite-hosted, high temperature, hydrothermal deposit in the Kangdian region. Detailed information on the chemical composition and formation age of the uraninite remains lacking, which impedes our understanding of uraninite genesis. Two phases of uraninite have been identified according to their relationships with other minerals and their field relationships. The phase 1 (Ur1) uraninite is characterized by local development of microfractures and pores in the crystal of uraninite, a scattered distribution, and irregular crystal shapes, and it is associated with ilmenite, biotite, and rare earth element (REE) minerals (monazite and xenotime). The phase 2 uraninite (Ur2) has anhedral crystal shapes with well-developed microfractures and pores and is associated with pyrite, albite, pyrrhotite, molybdenite, zircon, and chlorite. X-ray element mapping revealed that the distributions of U, Th, and Pb in the Ur1 uraninite are homogeneous, whereas those in the Ur2 uraninite are heterogeneous. The results of the electron microprobe analysis (EMPA) show that the UO2 and PbO contents of the Ur1 and Ur2 uraninite do not vary significantly. The high ThO2 contents of the Ur1 (1.08–1.68 wt %) and Ur2 uraninite (3.41–4.83 wt %) indicate that they formed at different temperatures. The laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis results show that the ∑REE of the Ur1 uraninite (3747.5–7032.3 ppm) is lower than that of the Ur2 uraninite (8369.2–11,484.3 ppm), and the REE patterns of the Ur1 and Ur2 uraninite are sickle-shaped with large negative Eu anomalies. The LA-ICP-MS U–Pb dating results revealed that the ages of the Ur1 (841.4 ± 4.0 Ma) and Ur2 (834.5 ± 4.1 Ma–837.2 ± 4.5 Ma) uraninite are in consistent with that of the migmatite. Thus, the Datian uranium deposit underwent at least two hydrothermal events, and the uraninite was formed due to the migmatization.
AbstractUnlike in the bulk, at the nanoscale shape dictates properties. The imperative to understand and predict nanocrystal shape led to the development, over several decades, of a large number of mathematical models and, later, their software implementations. In this review, the various mathematical approaches used to model crystal shapes are first overviewed, from the century-old Wulff construction to the year-old (2020) approach to describe supported twinned nanocrystals, together with a discussion and disambiguation of the terminology. Then, the multitude of published software implementations of these Wulff-based shape models are described in detail, describing their technical aspects, advantages and limitations. Finally, a discussion of the scientific applications of shape models to either predict shape or use shape to deduce thermodynamic and/or kinetic parameters is offered, followed by a conclusion. This review provides a guide for scientists looking to model crystal shape in a field where ever-increasingly complex crystal shapes and compositions are required to fulfil the exciting promises of nanotechnology.
Framboids are constituted by microcrystals with approximately log-normal size-frequency distributions, and 95% of framboidal microcrystals are between 0.1 and 3.1 μm. Nanocrystals are not generally observed in framboids. Packing efficiencies vary between close-packings in which the microcrystals occupy up to 74% of the framboid volume and random packings with a 56% volume of microcrystals. The ratios between framboid diameter and microcrystal size show a clear bimodal distribution which reflects the populations of close-packed ordered framboids and randomly organized framboids. Framboids may contain up to 500,000 microcrystals. The average numbers of microcrystals in both disordered and ordered framboids are similar, which suggests that the organization of microcrystals is the result of an additional process. Minerals that do not commonly produce equant crystals forms are unlikely to display the framboidal texture. Framboid microcrystals are essentially limited to isometric minerals like pyrite which produce equant crystals. Pyrite displays the greatest variety of crystal shapes among the common minerals. This means that pyrite is able to approximate forbidden fivefold symmetries such as the pentagonal dodecahedron, but with asymmetric pentagonal faces, and the icosahedron, again with different-sized triangular faces, as a combination of the octahedron and pyritohedron.
Abstract. For over a century, the anomalous shapes of Michigan
copper crystals from the Michigan Copper Country have been acknowledged.
They are well known by mineral collectors and curated in museums from all
around the world; still, their particular habits remain enigmatic. These
natural crystals do not seem to follow crystal shape theories, based on the
internal three-dimensional crystal structure. In this article, we offer a
unique perspective on the formation of Michigan copper crystals. Firstly, we review the most common theories of crystal shapes. Then, taking
into account the surface reconstructions induced by adsorbed oxygen,
detected by ultra-high vacuum techniques, we present evidence of a strong
correlation between these oxygen-induced surface reconstructions and the
anomalous shapes. Finally, in order to understand why these shapes are not
found in copper at other localities, oxygen dosing was performed using
NanoSIMS on different natural copper crystals as a preliminary
investigation. The higher oxygen content found in the Michigan copper
crystal studied compared to others supports the influence of adsorbed oxygen
on the anomalous crystal shapes. This result shows which mechanisms could
modify crystal shapes and allow the development of strategies to monitor
them, due to the presence of oxygen impurities. This new find is of great
importance in shape-dependent catalysis, sensor characteristics, or other
properties of material such as nanocrystals.