This is my third contribution to the current Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “C is for (Fossil) Coral”.
“C is for Calcite (Calcium Carbonate) in a Ward Beach fossil oyster stone”
I found this stone in early October 2024. It’s not the type I normally collect – I like nice smooth stones that are colourful or have interesting patterns. What caught my eye with this (very rough and un-colourful) stone were patches of tiny elongated features. I wondered if they were crystals or maybe fossils like sponge spicules. A month later, John Taylor, a rockhound and fossil enthusiast who lives in Scotland, visited me and I brought out the stone to show him. John looked closely at it under a magnifying glass and with a blacklight torch. The latter revealed some calcite present. Calcite will fizz when dilute hydrochloric acid is placed on it, but we didn’t do that test. John thought the stone was most likely a shell fossil, probably of an oyster of some kind. The long thin features that had caught my eye are maybe part of the shell material remaining after some erosion. Calcite is found in many shells and during the process of fossilisation is often replaced with silica. In this case, perhaps a little calcite remained. Calcite is the only common non-silicate rock-forming mineral, and is a significant component of limestone – see University of Auckland Geology.
The next entry is “D is for Diopside (with Chromium)”. An Index for this Alphabetical Series is here.
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