Apatite varieties in Recent and fossil linguloid brachiopod shells

Apatite varieties in Recent and fossil linguloid brachiopod shells 

Sample sets of 10 or more specimens of Recent, subfossil and fossil linguloid brachiopod valves were studied by X-ray diffraction analysis (XRD). One task of these studies was to advance data sets of apatite lattice parameters that could serve as a map for further XRD studies of skeletal apatite. 

The study of a collection of ten valves of subfossil Discinisca tenuis from a coquina on the coast of Namibia confirmed that during the post-mortem alteration processes, shell apatite can be rapidly partially replaced with apatite varieties of lower hydroxyl content. 

In this XRD study, we use lattice parameters as a tool for distinguishing the varieties of apatite, occurring in Recent, subfossil and fossil linguloid shells. Our tasks include 

Pure fluorapatite, usually considered as the prototype of the apatite family (e.g. Nathan, 1984), belongs to hexagonal syngony. The F-apatite lattice can be described by a unit cell, having a shape of a prism with rhomboidal base. This unit cell can be characterised by two lattice parameters, a (margin of the base) and c (height). In the case of F-apatite, approximate values are a = 9.37 A and c = 6.8SA (McConnell, 1973). Pure F-apatite can be obtained synthetically, but it occurs seldom in nature, and is not known to be formed biogenically (Carter, 1990). 

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