An Introduction to Environmental Chemistry

202 Chapter Six

Box 6.5 Abiological precipitation of calcium carbonate

Where a skeletal source cannot be identified,

calcium carbonate (CaCO 3 ) grains and fine-

grained muds may be of abiological origin.

The most famous occurrences occur in

shallow, warm, saline waters of the Bahamas

and the Arabian Gulf. In these areas two

distinctive morphologies are present, ooids

and needle muds (Fig. 1).

Ooids are formed by aggregation of

aragonite* crystals around a nucleus, usually

a shell fragment or pellet. Successive layers of

aragonite precipitation build up a concentric

structure, which may vary in size from about

0.2 to 2.0 mm in diameter. Needle muds are

also aragonitic; typically each needle is a few

micrometres wide and tens of micrometres

long.

It has long been thought that the warm,

(a)

(b)

Fig. 1(a) Ooid-rich sediment from the Great Bahama Bank. Individual grains are typically 1 mm in diameter

(photograph courtesy of J. Andrews). (b) Scanning electron microscope photograph of aragonitic needles

from the Great Bahama Bank. Scale bar= 1 mm (photograph courtesy of I.G. Macintyre, from Macintyre and

Reid 1992).

(continued)