Case study of the igneous intrusion effect on the mineralogical composition of the Carboniferous coal from Jingxi Coalfield, North China

Igneous intrusions significantly affect the mineralogical composition of coal. The Jingxi Coalfield, North China was subjected to the intrusion of a Mesozoic aplite; however, the resulting coal mineralogy has not been well investigated. This paper reports on a study of the mineralogical composition of five coal bench samples collected from the No. M3 coal in the Jingxi Coalfield, which was intruded by an aplite sill along the roof. The minerals present in the highest proportions in the No. M3 coal are margarite–paragonite group mineral (21.4%) and ammonian illite (72.7%), followed by ankerite (4.2%), anatase (0.7%), and rutile (0.9%). The formation of margarite, paragonite and ammonian illite is attributed to the intrusion of the aplite sill. The margarite and paragonite were formed prior to the formation of the ammonian illite. The proportion of the margarite–paragonite group mineral decreases, and the proportion of ammonian illite increases from the sill to the roof. The Ca²⁺ and K⁺ are more easily incorporated into the aluminosilicate mineral lattices than Na⁺ and NH₄⁺, respectively. Thus, the Ca²⁺/Na⁺ ratio in the margarite–paragonite group mineral decreases, and the NH⁴⁺/K⁺ ratio in the ammonian illite increases from the sill to the roof. The formation of ankerite is also attributed to the igneous intrusion, but this mineral was formed later than the aluminosilicate minerals. The proportion of ankerite in the coal samples increases from the sill to the roof.