Spectral and Chemical Composition Characteristics of Black Spinel from Muling, Heilongjiang Province

There are a large number of ruby and sapphire deposits related to basalt in Muling area, Heilongjiang Province, in which black spinel is usually mined as an accessory mineral. In recent years, high-quality black spinel has also attracted people's attention because of its large particle size and strong lustre. However, there are few reports on black spinel in Muling area. In this paper, the gemmological characteristics of Muling black spinel were studied by conventional gemmological tests, spectroscopic tests and chemical composition tests. The results indicated that the black spinel samples from Muling predominantly exhibit euhedral to subhedral octahedral crystal shapes, opaque, and displays surface abrasions. Melt inclusions, sulfide inclusions, and octahedral inclusions are visible under the microscope.Muling black spinel belongs to magnesia alumina spinel in a strict sense, and contains some magnesioferrite and magnetite. The high content of iron (14.97-26.37 wt.%) causes mullite spinel to be black and completely impermeable. Mossbauer spectroscopic results showed that iron exists in spinel in two valence states of Fe²⁺ and Fe³⁺, in which Fe³⁺ occupies two cation positions (quaternary coordination and hexagonal coordination) in the crystal structure. The tetrahedral coordination of Fe³⁺ makes the spinel show some anti spinel structure. Therefore, the Raman spectrum of Muling black spinel is different from that of gem grade transparent magnesium aluminum spinel, but similar to that of other spinel minerals(such as chromite, magnetite). It is dominated by strong wave numbers at 749 cm^-1 and 538 cm^-1, with smaller peaks or shoulders occurring at about 343, 402, 645 cm^-1 and 705 cm^-1.