The image below shows an ooid from our Sample 300 from northern Andros Island in the Bahamas. The ooids have been mounted in epoxy and ground and polished so thin that light passes through them. Hence these are transmitted-light images, rather than reflected-light images like the lower-magnification views shown in most of the other pages of this website.         The image shows that, like most ooids, this one consists of an inner nucleus and an outer laminated cortex (red arrows point to the boundary between the two). The nucleus is some pre-existing carbonate grain around which more CaCO3 was precipitated. The cortex is aragonite, a CaCO3 mineral, that was precipitated by seawater onto the nucleus. The layers or laminae of the cortex presumably reflect increments in the addition of aragonite during the growth of the ooid. This ooid is about 0.5 mm in diameter.

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