# Calcite Tailoring by Limpets: The Prismatic and Acicular-Foliated Microstructures of Members of The Nacellidae

## TL;DR

This paper studies how limpets in the Nacellidae family build unique shell structures with specific crystal arrangements.

## Contribution

The paper reveals a novel prismatic-acicular-foliated shell microstructure in Nacellidae limpets with distinct crystallographic orientation.

## Key findings

- Nacellidae limpet shells have a prismatic outer layer transitioning into an acicular-foliated layer.
- Prisms in the outer layer have c-axes oriented at high angles and aligned toward the shell margin.
- The crystallographic arrangement is unique among molluscs and likely results from organic templating and crystal competition.

## Abstract

Limpets of the family Nacellidae construct their shells
with an
outer prismatic layer, underlain by an acicular-foliated layer, and
occasionally an internal crossed-lamellar layer. The outer layer consists
of prisms oriented approximately perpendicular to the shell growth
surface. At the transition to the acicular-foliated layer, these prisms
subdivide into segments that become incorporated into the underlying
folia. The folia are extensive, radially oriented, and further subdivided
into transverse acicles, hence the term “acicular-foliated”.
Crystallographically, the outer prisms are remarkable in having their c-axes oriented at a high angle to their elongation and
aligned toward the shell margin, with one a-axis
typically perpendicular to the growth surface. The prisms exhibit
a high degree of co-orientation, with textures ranging from diffuse
to well-organized sheet textures. When considering rhombohedral face
orientations, the texture may appear either single- or double-crystal-like.
This crystallographic arrangement is inherited by the acicular-foliated
layer, where the texture invariably becomes stronger. Such an unusual
orientation likely results from oriented nucleation on an organic
template, possibly the mantle surface, combined with crystal competition
in the direction toward the shell margin. From both morphological
and crystallographic perspectives, the prismatic-acicular-foliated
microstructural complex of nacellids is unparalleled among molluscs.