Origins and occurrences of Ti-nanominerals in a superhigh-organic-sulfur coal

Abstract

Two Ti-nanomineral types seldom seen in a superhigh-organic-sulfur coal (Ganhe coal) are identified through the integrated techniques of high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray spectrometer (EDS), and fast Fourier transformation (FFT). The first Ti-nanomineral type is that of irregular-shaped Ti-nanominerals (Rutile, Ti4O7, and Ti6O11) associated with minor proportions of Cr and V. This is unusual, because Ti4O7 and Ti6O11, which are referred to as Ti magnéli phases (very high temperature products), are rarely found in coal. In addition, other integrated evidence of acid volcanic-ash components in Ganhe coal are also confirmed, including sanidine associated with Tl, flocculated texture illite, La-monazite associated with minor proportions of V, and rare-earth elements (REEs) distribution patterns (Eu negative anomalies). It is thus implied that the origin of the irregular-shaped Ti-nanominerals is the transformation of rutile to Ti4O7 and Ti6O11 under the influence of acid magma followed by transportation and deposition in the Ganhe peat swamp. The second type is that of spherical Ti-nanominerals that exist individually within the organic matrix; they have minimum areas of 3.79–56.10 nm2 and contain large proportions of Cr and V. It is considered that owing to the euxinic, anoxic, and reducing marine deposition environment, Ti, Cr, and V could easily have been originally precipitated or adsorbed by the organic macromolecular structure within the peat and low-rank coal. With elevation of the coal rank, they could then have been expelled by dehydration, decarboxylation, and demetallation via closed nano-size pores (nano pores) prior to finally entering mineralization under natural hydrothermal conditions. Although this result is promising, the hydrothermal formation of nano TiO2 particles requires further demonstration using an artificial coalification experiment.