Bulletin of the Geological Society of Malaysia, Volume 26, April 1990, pp. 201 – 223
Geological Survey of Malaysia, Ipoh
Abstract: The three types of cave rock phosphate namely encrusted phosphate, replacement phosphate and sedimentary phosphate are described and discussed in terms of their relative importance. The primary source of phosphate for all the three types is the bat guano deposited in certain limestone caves. The encrusted phosphate was formed by the deposition of phosphate cement from phosphate-rich solution derived by rain water seepage through the guano. The phosphate cement which was deposited on the limestone surfaces on which the phosphate-rich solution flowed down, is made up of a variety of texture and colour, reflecting its subaerial origin. The replacement phosphate was formed as a result of prolonged contact of wet/moist guano with the receptive limestone wallrock that is thinly bedded, jointed or fractured. The sedimentary phosphate was formed by authigenic and allogenic sedimentation from phosphate-rich solution collected in depressions at the base of the guano cave.
The encrusted phosphate exhibits mainly laminated and colloform textures, consisting of cryptocrystalline to opaque cement and/or weakly birefringent, fine-grained apatite in radiating fibres or spherular masses. Besides the cryptocrystalline phosphate, four phosphate minerals have been identified by X-ray diffraction. The most common is apatite, which by comparison to the chemical analyses, is probably carbonate-apatite and/or hydroxylapatite. The other three minerals are crandallite, whitlockite and mentgomeryite. Crandallite occurs in small amounts in some samples, whereas whitlockite and montgomeryite are rare.
The common non-phosphate minerals are calcite, rancieite and pyrolusite, depositing together with the phosphate cement. Calcite and quartz also occur as crystals lining the vugs. In the sedimentary phosphate, goethite is the common authigenic mineral occurring as ooids, whilst quartz is the common allogenic mineral.