Absorption Spectra of Ni2+ in Zaratite and Takovite

Abstract

The EPR and optical absorption studies are reported for the nickel-bearing minerals, zaratite and takovite.The optical absorption spectrum of zaratite at 300 K exhibits three intense bands and a weak band and also four other vibrational bands. The strong bands at 8195, 14 285 and 23 805 cm-1 are assigned to 3T2g(F), 3T1g(F) and 3T1g(P) and the weak band at 21 735 cm-1 assigned to 1T2g(D) of Ni2+ in an octahedral symmetry. The crystal field parameters are evaluated to be Dq = 820, B = 899 cm-1 and C = 4.1B. The four vibrational bands are attributed to water and hydroxyl stretching. The EPR spectrum of zaratite shows no transition due to Ni2+ but includes signals due to Mn2+, Fe3+ and Co2+. It is derived from the absorption studies that Ni2+ in zaratite is sited in an octahedral symmetry in a normal oxide/hydroxide structure.The optical absorption spectrum of takovite at 300 K consists of three strong bands at 9100, 15 380 and 26 665 cm-1 which could be assigned to the three spin-allowed transitions and two weak bands assigned to 1Eg(D) and 1T2g(D) of Ni2+ in an octahedral symmetry. The two components at 8200 and 10 000 cm-1 might be due to mild tetragonal distortion to split the octahedral band at 9100 cm-1. The crystal field parameters which gave a good fit to the observed energies at Dq = 910, B = 940 cm-1, C = 4.25 B. The EPR spectrum and the near infrared spectrum of takovite have close resemblance with those of zaratite. It is concluded that Ni2+ in takovite might be sited in a slightly tetragonally distorted octahedron in a normal oxide and or hydroxide structure.