Petrogenesis of the Neoarchean zincian chromite within ultramafic xenoliths,Bastar Craton,India

The present study reports and discusses the genesis of zincian chromite in the ultramafic xenoliths from the Dongripali area,Bastar craton,Central India.The zincian chromite is in the ultramafic xenoliths of Bengpal supracrustal rock hosted by Neoarchaean Bundeli gneisses.Compositionally zincian chromite shows a range of Cr_(2)O_(3)(39.69 to 51.66 wt%),Al_(2)O_(3)(05.30 wt%to 08.71 wt%),FeO(21.74 wt%to 27.51 wt%),Fe_(2)O_(3)(10.19 wt%to 19.36wt%)with higher ZnO content ranging from 1.73 wt%to 4.08 wt%.Accordingly,their Cr#[Cr/(Cr+Al)]varies in a narrow range from 0.83 to 0.85.Its calculated melt composition supports metamorphic or post-magmatic nature rather than common occurrences such as inclusion in diamonds,meteorites,and association with any sulfide-rich mineralised belt.This reveals that the post-magmatic processes play a vital role in transforming chromite to zincian chromite.The empirical thermometric calculation from chromite,amphibole,and pyroxene support their metamorphic origin and formed during low-P and high-T amphibolite grade facies of metamorphism(~700℃).The Neoarchaean granitic magmatism has a significant role in generating and transferring the heat during contact metamorphism with hydration of ultramafic xenoliths and further alteration,i.e.,serpentinisation.The olivine is a major repository for Mn,Zn,and Co in peridotite/ultramafic;these elements get mobilised during the metamorphism and serpentinisation.This is a possible reason for the mobilisation of zinc and incorporation in the chromite within altered ultramafic.As a result,chromiterich ultramafic xenolith subjected to metamorphic process gets enrichment of Zn and Fe due to elemental exchange.It converts common chromite into zincian chromite,as reported in altered ultramafics elsewhere.

Formation of Laser-Induced Periodic Surface Structures on Reaction-Bonded Silicon Carbide by Femtosecond Pulsed Laser Irradiation

Reaction-bonded silicon carbide(RB-SiC)is an excellent engineering material with high hardness,stiffness,and resistance to chemical wear.However,its widespread use is hindered due to the properties mentioned above,making it difficult to machine functional surface structures through mechanical and chemical methods.This study investigated the fundamental characteristics of laser-induced periodic surface structures(LIPSSs)on RB-SiC via femtosecond pulsed laser irradiation at a wavelength of 1028 nm.Low-spatial-frequency LIPSS(LSFL)and high-spatial-frequency LIPSS(HSFL)formed on the surface along directions perpendicular to the laser polarization.SiC grains surrounded by a large amount of Si show a reduced threshold for LIPSS formation.By varying laser fluence and scanning speed,HSFL-LSFL hybrid structures were generated on the SiC grains.Transmission electron microscopy observations and Raman spectroscopy were carried out to understand the formation mechanism of the hybrid LIPSS.A possible mechanism based on the generation of multiple surface electromagnetic waves due to the nonlinear response of SiC was proposed to explain the hybrid structure formation.Furthermore,the direction of laser scanning with respect to laser polarization affects the uniformity of the generated LIPSS.