Goetz, Andreas J.; Griesshaber, Erika; Neuser, Rolf D.; Lueter, Carsten; Huehner, Manfred; Harper, Elisabeth; Schmahl, Wolfgang W.
Calcite morphology, texture and hardness in the distinct layers of rhynchonelliform brachiopod shells.
In: European Journal of Mineralogy, Vol. 21, No. 2: pp. 303-315
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We have investigated the texture and shell microstructure together withthe individual hardness distribution patterns of recent calciticbrachiopods of the species Kakanuiella chathamensis, Liothyrella uva andLiothyrella neozelanica. One of the most distinctive features of thestudied species is the number of layers that compose the shell.Kakanuiella chathamensis is built entirely of nano- to microcrystallineprimary layer calcite. Liothyrella uva contains a nanocrystalline outerprimary layer and an inner fibrous secondary layer. Liothyrellaneozelanica is composed of three layers, a nanocrystalline Outer primarylayer, a columnar secondary and an innermost fibrous tertiary shelllayer. Even though Kakanuiella chathamensis consists only of primarylayer material we observe some textural features and a pattern in thedistribution of hardness within the shell. The texture of Liothyrellauva and of Liothyrella neozelanica is significantly more defined thanthat of Kakanuiella chathamensis. Within the valves calcite crystalc-axes are perpendicular to and rotate accordingly with the shell vault.In contrast to the valves, a multimodal c-axis distribution pattern ispresent within the hinge region. The hardness distribution inLiothyrella neozelanica and Liothyrella uva is such that the outermostpart of the shell is hard while the innermost shell portion is soft. Ingeneral, Liothyrella uva is significantly harder than Liothyrellaneozelanica and Kakanuiella chathamensis, even though Kakanuiellachathamensis contains only primary layer calcite.