Room, low, and high temperature dehydration and phase transitions of kernite in vacuum and in air

Abstract

Kernite Na 2 B 4 O 6 (OH) 2 ·3H 2 O dehydration in air at high temperature and in vacuum at room temperature has been studied. It was found that kernite easily dehydrates forming a new phase-I both on heating and in vacuum. The chemical formula Na 2 B 4 O 6 (OH) 2 ·1.5H 2 O of the new phase-I has been estimated on the basis of thermogravity analysis. It is triclinic with the unit cell parameters a = 7.047(8), b = 8.76(1), c = 13.08(2) Å, α = 93.40(9), β = 95.32(9), γ=90.28 (9) o changing slightly on pressure reduction. Due to the relatively low temperature (353 K) and reversibility of the kernite ↔ phase-I transition an anion of the new phase-I likely consists of the same chains [B 4 O 6 (OH) 2 ] 2-like in kernite structure. The high anisotropy of kernite thermal expansion was explained by approaching of NaO chains due to the initial removing of water molecules from kernite crystal structure. The behaviour of the new phase-I at low temperatures in vacuum was also investigated. A formation of an additional new phase II has been detected at the temperature of 93 K.

An Examination of Kernite (Na2B4O6(OH)2·3H2O) Using X-Ray and Electron Spectroscopies: Quantitative Microanalysis of a Hydrated Low-Z Mineral

“…Only a few weakly expressed low-angle diffraction peaks are still faintly distinguishable, and these are shifted to slightly higher angle (and thus smaller atomic-plane spacing) than the apparently analogous features in the as-received specimen. Although previous authors noted the appearance of an intermediate, “phase-I” (Sennova et al, 2005), under pressures lower than atmospheric, no such structure was observed in our measurements. One difference in specimen conditions between the two studies is the orders-of-magnitude lower pressures used in the present work relative to those described in Sennova et al (2005).…”

“…In the course of these studies, it became apparent that kernite was susceptible to both vacuum damage (Sennova et al, 2005) and electron-beam damage, complicating the analysis. To better understand the nature of the structural and compositional changes occurring under analysis conditions, X-ray diffraction (XRD) measurements were collected on untreated powdered kernite specimens, vacuum-treated (10 −8 Pa for over 48 h) powdered specimens, and electron-beam (5 keV) irradiated powders using a Bruker AXS D8 (Bruker AXS, Ewing, NJ, USA) equipped with a Cu X-ray source.…”

“…This extremely fissile mineral cleaves readily on two crystal planes (Giese, 1966) and is highly soluble in water. It slowly absorbs water vapor from the atmosphere to eventually form natural borax (Na 2 B 4 O 7 ·10H 2 O) (Garrett, 1998) but is also known to lose structural water under vacuum conditions (Sennova et al, 2005). Thus, improper handling of kernite specimens could result in measurements either too low or too high in water content.…”

High‐pressure behavior and phase stability of Na2B4O6(OH)2·3H2O (kernite)

“…A recent study by Gatta et al 10 shed new light on the H-disorder in the structure of kernite, on the basis of single-crystal neutron diffraction data. A few studies were devoted to investigate the stability of kernite under ambient pressure conditions [11][12][13] . When heated in air, above ~353 K, kernite dehydrates to Na2B4O6(OH)2•1.5H2O; at T >388 K it becomes amorphous and, at temperature above ~800 K, an anhydrous phase with composition Na2B4O7 crystallizes 12 .…”

“…A few studies were devoted to investigate the stability of kernite under ambient pressure conditions [11][12][13] . When heated in air, above ~353 K, kernite dehydrates to Na2B4O6(OH)2•1.5H2O; at T >388 K it becomes amorphous and, at temperature above ~800 K, an anhydrous phase with composition Na2B4O7 crystallizes 12 . However, only one high-pressure study explored the P-induced behavior of kernite via Raman and IR spectroscopy by in-situ experiments 14 .…”