Page 44 - Atlas of Sedimentary Rocks Under The Microscope

Carbonate rocks

Skeletal particles (Bioclasts)

Introduction

Skeletal particles, or bioclasts, are the remains, complete or frag­
mented. of the hard parts of carbonate-secreting organisms. There is
such a variety in the mineralogy. structure and shape of skeletal
material that several books could be written on this subject alone.
When trying to identify bioclasts, the f o llowing f e atures should be
considered:
I. The overall shape and size or the particle.
2. The internal wall structure of the particle. Many structures are
more easily visible with polars crossed than in plane polarized light.
It is important to distinguish those bioclasts which were originally
calcite and have well-preserved wall structures from those which
were originally aragonite and have had their wall structure
modified or replaced during the alteration to . calcite.
In this section we have attempted to show the diversity of skeletal
structures present in ancient limestones, concentrating on examples
from groups which arc particularly common or occur over a wide
stratigraphic range. For more detailed descriptions and illustrations of
skeletal particles readers are referred to Majewske (1969), Horowitz
(
(
and Potter 1 9 71), Bathurst 1 9 75) and Scholle 1 9 78).
(

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