The mineralogy of the Middle Ordovician Jacksonburg limestone formation in eastern Pennsylvania and western New Jersey is simple and almost monotonously uniform.
Calcite is the predominant mineral. There is a small amount of dolomite. Quartz and mica minerals 1M, 2M muscovite and possibly biotite) occur throughout the formation. Each of them constitutes 30-40 per cent of the acid-insoluble residue, which in turn makes up 15-40 per cent of the rock. The insoluble residue also contains well crystallized, ferriferous chlorite minerals ranging from a trace to about 15-20 per cent; montmorillonite minerals occurring sporadically up to about 10 per cent; 5-10 per cent of sodic feldspar; pyrite in amounts generally less than 5 per cent; and non-graphitic carbonaceous material constituting a maximum of 5 per cent.
The bulk of the Jacksonburg is characterized by the presence of chlorite. A chlorite-poor unit 50-80 feet thick overlies the Jacksonburg-Beekmantown contact. This unit occurs within whatever stratigraphic subdivision or facies happens to be present at this position. There may be a chlorite-poor unit at the top of the Jacksonburg formation.
The chlorite-poor units of the Jacksonburg formation may represent shallow marine deposits with the bulk of the formation having been deposited in deeper water which provided more opportunity for diagenetic alterations of detrital illite and montmorillonite to chlorite. The presence of chlorite in the Jacksonburg in small areas isolated from the main belt suggests that the formation was deposited in an extensive sea rather than in isolated small basins and that the present outcrop pattern is the result of erosion.
The high degree of crystallinity of the clay minerals is a result of post-depositional stress deformation. Montmorillonite is attributed to minor admixtures of volcanic ash. The absence of kaolinite from the Jacksonburg formation may indicate a lack of kaolinite from the source area since the diagenetic alteration of kaolinite requires much slower deposition than seems probable, at least for the chlorite-poor units.