Calcite morphology, texture and hardness in the distinct layers of rhynchonelliform brachiopod shells
European Journal of Mineralogy 21(2): 303-315
We have investigated the texture and shell microstructure together with the individual hardness distribution patterns of recent calcitic brachiopods of the species Kakanuiella chathamensis, Liothyrella uva and Liothyrella neozelanica. One of the most distinctive features of the studied species is the number of layers that compose the shell. Kakanuiella chathamensis is built entirely of nano- to microcrystalline primary layer calcite. Liothyrella uva contains a nanocrystalline outer primary layer and an inner fibrous secondary layer. Liothyrella neozelanica is composed of three layers, a nanocrystalline Outer primary layer, a columnar secondary and an innermost fibrous tertiary shell layer. Even though Kakanuiella chathamensis consists only of primary layer material we observe some textural features and a pattern in the distribution of hardness within the shell. The texture of Liothyrella uva and of Liothyrella neozelanica is significantly more defined than that of Kakanuiella chathamensis. Within the valves calcite crystal c-axes are perpendicular to and rotate accordingly with the shell vault. In contrast to the valves, a multimodal c-axis distribution pattern is present within the hinge region. The hardness distribution in Liothyrella neozelanica and Liothyrella uva is such that the outermost part of the shell is hard while the innermost shell portion is soft. In general, Liothyrella uva is significantly harder than Liothyrella neozelanica and Kakanuiella chathamensis, even though Kakanuiella chathamensis contains only primary layer calcite.