Different grades of limestone

On site in a King Country limestone quarry, Professor Cam Nelson draws attention to three types of limestone – Ōtorohanga A, B and C – each with a different calcium carbonate composition. Knowledge of this composition is of economic interest to the quarry owners. The B layer is a high-grade limestone with over 98% calcium carbonate. It is a valuable resource with a multitude of industrial uses.

Transcript

PROFESSOR CAM NELSON The Ōtorohanga Limestone, as I’ve said, is a pretty pure limestone – certainly better than 85, commonly 90 and even up to 95, 98% – but sometimes this expresses itself in the way the limestone weathers.

And here I could recognise three types of limestone. There’s a relatively thin zone of sort of horizontal beds of limestone in the uppermost part. You then go into a central part here, which is rather more rubbly, broken up – there is some of the horizontal layering, but it’s nothing like as persistent and consistent as above. And then right near the bottom, we go into horizontal layered limestone we call flaggy limestone once again.

So here there are three limestone types, I have called these in the past Ōtorohanga Limestone A, B and C for the lower flaggy, the knobbly and the upper flaggy limestones. And the importance of that one asks the question, why do they look like this? What’s really the difference?

And you need to go to the laboratory to analyse the limestones and under a microscope to analyse them. And when you do that, we find that his middle rubbly limestone is one of the highest grade limestones in New Zealand. It consistently runs at 98% calcium carbonate or thereabouts, so it’s a very, very high-grade quality limestone. And this quarry recognises that, and they take that lime and use it for special purposes compared to maybe the limestone above it and below it. The limestone below is a little less in calcium carbonate, is the dominant one, and that’s the main one they’re quarrying for agricultural lime purposes.

So I think that the limestones we’re seeing here do have economic ramifications which tie back to their calcium carbonate percent. And just to make comment, this very high-grade limestone, very pure, it’s got almost no iron in it, which is a little unusual for limestones, and that makes it a good limestone to get into the manufacture of crystal glass that you want to be clear glass, not green or brown as in beer bottles, which also have lime, but they can have limestone with an iron content. Here, if you’re wanting crystal glass, that would be the limestone to go for. So yes, economically, a knowledge of the make-up of these limestones can be, you know, very, very useful for quarry people.

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