The ikaite to calcite transformation: A key to understanding the palaeoclimatic significance of glendonites?

<p><strong>Keywords: Ikaite; Glendonite; Palaeoclimate; Powder X-Ray Diffraction; Clumped isotope thermometry</strong></p><p>Marine sedimentary ikaite is the parent mineral to glendonites, stellate pseudomorphs found throughout the geological record. Glendonites are a controversy in palaeoclimatic studies as there is an ongoing debate as to whether their presence in sedimentary successions may be used as cold-climate indicators. Glendonites are typically found associated with glacial sediments, and for a long time ikaite was believed to only nucleate and grow at temperatures < 7 °C. However, with the successful laboratory synthesis of ikaite at higher temperatures, the climatic significance of glendonites was brought into question. This study uses a combination of physical and inorganic chemistry techniques to demonstrate the variable stability of natural marine sedimentary ikaites over short (minutes to hours) and longer (weeks to months) timescales, both between ikaite samples, and in a given ikaite. We examine the nucleation of calcite from the destabilized ikaite, observing that this process is much more complex than previous studies suggest. We demonstrate that over much longer (e.g. months or more) timescales, natural marine sedimentary ikaites are not stable above 5 °C, and thus glendonite presence in sedimentary successions may be considered cold climate indicators.</p>

Metrology of convex-shaped nanoparticles via soft classification machine learning of TEM images

The shape of nanoparticles is a key performance parameter for many applications, ranging from nanophotonics to nanomedicines. However, the unavoidable shape variations, which occur even in precision-controlled laboratory synthesis, can...

Laboratory synthesis and characterization of Knasibfite K3Na4[SiF6]3[BF4] and the homologous Ge compound K3Na4[GeF6]3[BF4]

Herein we present the synthesis, crystal structure, and the infrared and Raman spectra of K3Na4[SiF6]3[BF4]. The compound also occurs in nature as the mineral Knasibfite. We obtained it from the reaction of stoichiometric amounts of SiO2, Na[BF4], K2CO3, and Na2CO3 in hydrofluoric acid at room temperature. Hydrofluorothermal synthesis at 200 °C lead to a product of higher purity. Knasibfite is colorless and crystallizes in space group Im 2m with a = 5.546(2), b = 9.261(2), c = 17.184(4) Å, V = 882.5(4) Å3, Z = 2, at T = 400(2) K. At 342 K a phase transition to a monoclinic structure was observed (I 2, a = 5.5003(6), b = 9.0890(9), c = 17.0048(2) Å, β = 90.041(9)°, V = 852.2(2) Å3, Z = 2, T = 100(2) K). By replacing SiO2 with GeO2 as a starting material K3Na4[GeF6]3[BF4] was obtained. This compound also crystallizes in form of two polymorphs, both of which are isotypic to the respective ones of K3Na4[SiF6]3[BF4].

A review on graphene quantum dots and their nanocomposites: from laboratory synthesis towards agricultural and environmental applications

This review highlights recent and important advances related to the synthesis, characterization and application of graphene quantum dot (GQD)-based nanocomposites for the agriculture and environmental sectors.