Carbonate minerals in Bulgarian coals with different degrees of coalification

Abstract

Coals of various rank (ranging from lignite to anthracite), and originating from seven coal basins in Bulgaria, were investigated for the occurrence and types of carbonate minerals. The following carbonates are present in their characteristic forms: Calcite (clastic, organogenic, cryptograined, xenomorphic, massive and infiltrational); siderite (cryptograined, xenomorphic both infilling and replacing, massive, euhedral, framboidal and infiltrational both fracture-filling and cleat-filling); dolomite (cryptograined, xenomorphic, massive and infiltrational) and magnesite. Carbonate mineralization depends on the penetration of ready mineral forms and mineralized solutions during peat genesis and the infiltration of mineralized solutions during coalification. The mineral composition of the carbonates in the coals studied and the occurrences of the various carbonate minerals depend on the combination of a series of factors such as: (1) the presence of carbonate rocks in the province feeding the peat bogs; (2) the way and capacity of feeding; (3) acidity (pH) and reducing conditions (Eh) of the environment, (4) tectonic situation during the peat genesis; and (5) availability of infiltrational mineral solutions during coalifications. No infiltrational carbonate mineralization was found in the lignites studied. Due to the combination of the above five factors, bituminous coals from the Balkan basin and the anthracite from Svoge are characterized by the greatest variety of carbonate minerals and their forms.