Dr Steve Hood
So here we have a lovely boulder of hard well cemented Ōtorohanga limestone of latest Oligocene, earliest Miocene age. I just want to point out some of the key characteristics.
Clearly, this is dominated by burial diagenesis and burial cementation. It’s extremely hard, well indurated. There’s no visible porosity in the rock. That’s all been plugged up during pressure dissolution – burial of skeletons, skeletons pressure dissolving into each other and then that material going to produce or precipitate a cement.
A key characteristic of these limestones is the flaggy appearance, and the flags here are separated by thinner less carbonate-pure seams. Now we envisage that the amount of time that’s encapsulated in one of these flags is perhaps as much as you know 500, 1000 years.
So one of the key characteristics of the carbonate rocks or the carbonate minerals, so that’s limestones in particular, is they fizz in association with the application of acid. If I take a piece of Ōtorohanga limestone, hard indurated non-porous limestone, what I’m going to do is pop it in a glass beaker here, and if I take some 2 molar hydrochloric acid and pour that on the rock, we will see that we get quite vigorous fizzing there.
So we see with the addition of the hydrochloric acid that the calcium carbonate – the bonds are broken – we’re liberating CO2 to the atmosphere producing water and calcium chloride.
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