Structured Interview 2

If you'd like to start by telling me, when you examine a rock, what sort of things do you look for, what sort of things are you trying to observe?

Well, the most obvious thing is the colour of the rock, different rocks have got different colours, it's not diagnostic but it could have a certain possibility. And you've got to look at the grains, you don't actually identify what they are, just looking at them, how they relate to each other, and that's the texture of the rock, grain size, shape, how well developed they are. That's just an overview of the rock. And then next you go into the structures inside the rock, that will give you some sort of general idea of the environment that the rock formed in. Then with those you can start going into identifying the minerals themselves. We have three types of minerals, we've got so-called essential minerals, and accessory minerals, and secondary. The essential are the ones which make up a large proportion of the rock, for that rock to be called by that name, those minerals have to be there, and the accessories can occur as well, but they are not necessary, and the secondary, those are from secondary things. And you start identifying essential minerals, beginning with the most abundant one and then moving to the next, and doing all the rest in that order, moving from the accessories to the secondaries. And from that mineral assemblage you can actually tell this rock is this, and you can actually give it a name.

And are there any other things that you would look at at the same time? Or are these the essential things that you'd need to ask?

I think that these are essential, these are the things you have to have in mind.

Right, if we could go back then, and work through each of these in turn, with the colours, what sort of colours would you be looking for, and what would each colour tell you?

Again, it depends what rock type you're dealing with.

We're dealing with the igneous rocks.

With igneous rocks we've got two main colours, light coloured and dark coloured. The light coloured ones tend to be acidic, and also the inermediates, those with high over 52% silica, well, even with these there are a few cases where they are otherwise. There is one rock called pitchblende, it's pitch black, like it's acidic, it's got the same composition as a ??granite??, a very very light rock.

Is that about the only exception to that rule?

No, I can't think of any other examples. Then you have the basic rocks, which are generally very dark. Even in those as well, you could in some cases have fairly light coloured basic rocks, like one called leucogabbro, I think.

Could you explain a bit about the distinction between basic and ultrabasic? Just how you decide where one starts and the other one stops?

Silica percentage less than 42%, that's your ultrabasic, and from 42 to 52 is basic, and 52 to 66 is intermediate, and greater than 66% is acidic rock.

That's handy, because if at all possible we need numbers.... So what else is there to say about colour?

Not a lot really, except like I said, it's not very very diagnostic. You could have secondary effects.

Could you tell me a bit about secondary effects?

When you get rocks they can be stained because of weathering.

Is there any tendency to be stained to one particular colour?

That depends on actual elements. Take iron, iron will typically give a reddish stain.

And what are the other elements which are likely to produce secondary staining effects? Are there just a few that you're likely to come across, or could it be more or less anything?

They tend to be more common in other kinds of rock than the igneous, and again, if you're looking at igneous, you tend to go for the fresh rock.

How would you decide which a fresh rock was?...

Fresh rocks, what you mean by that is they are less weathered,

And how would you identify weathering?

They tend to be very very crumbly.

So if you've got a hard rock, and the rock isn't stained reddish or anything, then the rock's likely to be fresh.

If you can actually separate the individual grains then it is likely to be fresh, if the grains are still intact.

That's very handy. That leads us on to grain size, which you mentioned. Could you tell me a bit about grain size, and shape, and texture and so on?

You've got the coarse grained, medium grained, and the fine grained. And in igneous rocks those are related to the environment in which that rock formed. They are related to different levels of the crust. Fine grained rocks and glassy rocks, which is where the grains are not there, which is mainly the result of very very rapid cooling, they tend to be found on the surface. And medium grained rocks are at moderate depth, and coarse grained rocks are generally formed at very very great depth.

Could you give me some figures, just roughly, for what would constitute a moderate depth or a great depth?

Great depth maybe five to ten kilometres, well, greater than that, (ten) with intermediate between ???five and ten (PARTLY INAUDIBLE)

And one of the things we've been having trouble with is a definition of grain sized. Could you give me an idea in terms of millimetres about where one stops and the other starts?

Coarse grains are usually anything greater than two millimetres, medium between two and one, less than that is the fine grained, the sort of thing that you can't actually see.

Could you tell me a bit more about grains and what you look for?

The size, the shape, how well developed they are, that will give you an indication of the sort of environment they formed in. Where the magma is surrounded by cold rocks you tend to get very rapid cooling...very very fine mass which results from rapid cooling.

You mentioned structures as well within the rock. Is that directly related to grain size and shape and so on?

In a way it is, yes. In igneous rocks you tend to get structures like layering, which you tend to associate with sedimentary rocks, but you do get some layering with igneous. You get already formed crystals within a magma, they tend to settle as they become heavier and heavier, ??normal sedimentation.

Does this mean that the same rock will have different composition at different depths?

The same rock will have different composition at (inaudible)

So if it was the same rock and it was lower down the crystals would be larger and better formed. Is that the way round that it works?

Yes, they tend to be larger, they have spent more time within the heavy ?? mantle in which they grow

And the minerals themselves, could you tell me a bit about the minerals and how you diagnose the minerals and which ones are most important for the igneous rocks?

The minerals,you have to go through a format of describing a mineral.

Could you tell me about that, then?

Its properties, its colour, this is just on hand specimen, right? I don't have to use any microscope or anything?

If you'd tell me about what you'd look for with the microscope if you'd use that as a normal part of identifying it,

In hand specimen you'd look at its colour, its shape of crystal, what sort of fracture they have, what sort of cleavage they have, its lustre, whether it's got any twinning, different planes of orientation within the crystal, the single crystal

Somebody else mentioned twinning. If you could explain in basic elementary terms what's meant by twinning

What happens in a ??grade, is you could get a number of different crystals and those crystals could be at different orientations and that's what's meant by twinning. You could have one grain developing that way in that orientation and the next one developing in a different orientation to that.

So is it just any situation in which the crystals are forming at different orientations to each other or is it just one specific combination of angles? Would you like to draw a diagram of it?

If, say, you had an orientation, and that's one crystal, and in the next crystal you have something like this...

If it's too tricky a thing to explain in two minutes from scratch then don't worry about it, but I thought that since you'd mentioned it, it might be worth clearing that point up.

It is the orientation of different crystals within a grain.

Right. I'll follow that one up in more detail with someone else, then. So you've gone through the colour and the shape, is there anything else about minerals that you'd be looking at?

??It's less seen with the naked eye than with the microscope, except for one or two minerals like plagioclase you can actually see the grain. (Inaudible)

What would you do with a microscope, then, if that's what you'd look for in a hand specimen? Could you give me a rough idea of what you'd be looking for under the microscope?

The colour again, okay, let's start with this. There are two types of microscopy, they can be under what is called plane polarised light or cross polarised light. Under plane polarised light you look for transparency and colour. Fractures, cleavage, and how it's related to other minerals, how does it stand out in relation to them. Then you go into cross polarised light. What you have is, depending on the orientation of the crystal, you can get different colours. Orientation of the crystal (inaudible) mean you can get different colours with size and orientation. That's what we call birefringence. And that would give you orders of interference. Interference colours that go with increasing thickness of the ??grain. You also look for twinning, ... in some cases you get zonation, you get one grain and you have compositional dimensions within that grain. And you can work out the actual speed of light that's being transmitted in the mineral, the speed of light is related to the refractive index of the mineral.

And is that actually much used as a way of identifying minerals?

It's very very diagnostic but the problem is you need ideal crystals, which are very very rare.

And is the equipment expensive?

No, you just use a normal microscope. But what you need is an ideal crystal, or something approaching an ideal shape. .... (inaudible). What happens is you have a crystal under here, and you rotate the thing around, the crystal's going to go dark, ??four times it's rotated?? ... oh, extinction it's called, you can measure the angle at which it becomes extinct, when it completely goes black (inaudible) is related to the orientation of the crystal (inaudible) which plane you're dealing with.

So with the same mineral, if you turned it round, would all of the crystals of the same mineral as you were turning the slide round

They could be different orientation; that's one way of working out twinning. Like if you get a mineral like plagioclase, a twinned plagioclase crystal, you turn it round one part of it will become extinct and the other part will be illuminated (inaudible)

That sounds fairly comprehensive. Could we work through one or two examples of rocks then with you telling me what you'd expect to find in the rock. So if we were looking at granite, for example, what sort of colour would you expect granite to be?

A granite would tend to be light coloured...they are coarse grained.. they could be medium grained, or microgranite...

So you actually get fine grained granites as well?

Not fine, but smaller grained.

So they're all coarse, but it's a matter of how coarse they are? And what sort of minerals would you find in granite?

Orthoclase feldspars and quartz. Those are the essentials. You could have some albite. Accessories are biotite, hornblende. Secondaries could be like some clay minerals

You mentioned albite. What does that look like?

It's a plagioclase, dull, dull grey, it's got good cleavage,

Would that be enough to identify granite uniquely, or are there any other things that would separate it off from any other similar rocks?

I think that would do it.

And what sort of percentage of silica do you expect to find in granite?

It's actually got free quartz as an essential mineral. Very very high.

Could you do the same sort of thing for some of the other igneous rocks? So that we've got a fairly complete set of rules?

Basalt. It's dark, greenish grey, fine grained, can have quite a few textures, what is called porphyritic, that's where you have well developed crystals,very large crystals, within a very fine grained matrix. It could be amygdaloidal, where you have voids that were later infilled by other minerals.

How can you distinguish porphyritic from amygdaloidal? If both consist of one thing surrounded by other things..

Well, amygdaloids are usually infills, they're not crystals as such, you get a mineral which.. which could be a whole lot of other minerals whereas with the other you're dealing with crystals of a single mineral.

So if it's a single well defined crystal then it's porphyritic, if it's just a sort of roundish mass...

In some cases you don't, the voids are not infilled, that's vesicular

...You were telling me about basalt. We got as far as texture.

There could be glassy as well.

Do you get that in basalt at all?

Yes, you do get that in basalt.

And what sort of minerals would you expect to find in basalt?

The essential minerals are plagioclase and pyroxene

And are there any others which might occur in basalt?

Accessory minerals like olivines, amphibole, some micas,

Could you give me a few more igneous rocks?

Ultrabasics. Those tend to be coarse grained. Peridotite. The main minerals are things like pyroxenes and olivine.

Could you go through peridotite in a bit of detail explaining just what would mark it off as being definitely peridotite and not something else?

Peridotites are coarse grained,??ultrabasics?? they are dark coloured, greenish olive, they have, this is essential minerals, pyroxenes, and olivines, and most particularly, no plagioclase. And that's one distinction between a peridotite and maybe a coarse grained basic rock. There is no plagioclase.

Is there anything else about peridotite that's worth mentioning? Or would that be enough to identify it uniquely?

They're very very rarely found fresh. They tend to be serpentinised. Especially the olivine (inaudible) serpentine.

Well, I think we'll leave that to one side. Could you do a few more igneous rocks?

In intermediate examples like andesite, it's got essential hornblende and ?andesine plagioclase, it's light coloured; just like a basalt, it could be porphyritic, it could be amygdaloidal, it could be vesicular. Mainly because it's ?? an intrusive rock (inaudible)

And do you..

(inaudible), lavas, things like that

And do you get regular features occurring with intrusive rocks?

Yes, but not a great deal. (inaudible) The volatile component of the rock as they escape will lead to a more vesicular rock or if you have (INTERRUPTION)

structures in (inaudible) are related to the viscosity of the magma, let's call it lava because it's outside, and the volatile content. (NEXT BIT OMITTED BECAUSE OF SUBJECT BEING UNSURE WHICH WAY ROUND THE RULE WENT). The more runny ones will have more large scale structures, tha gases are able to escape, while the sticky ones do not cover that great distances, and the gases are unable to escape, (inaudible) and then you will have the amygdaloidal texture. And it could also be a later thing, you could have a void, an empty void, which is later infilled with minerals percolating through.

Could we work on through a few more examples of igneous rocks?

We covered examples of basic, intermediate, and acidic. Other than that,

Because we're trying to build up a system which will identify individual rocks, so if you could give us a few examples of igneous rocks and how you would identify them

Gabbro. A coarse grained equivalent of basalt. Same chemical composition, same mineralogy, it's just different texture, different because it's coarse grained, mainly due to the fact that it cooled in the (inaudible) and the crystals had enough time to develop.

Have you got anything medium grained between gabbro and basalt?

Yes, you have dolerite.

So are the three completly equivalent except for grain size, then? Are there any differences at all in the minerals?

(inaudible)

What about the colours? Do the colours change?

No, not at all. And you get the same thing with granite as well. Rhyolite is the fine grained equivalent of granite.

And do you get a medium grained version there as well?

Yes, like microgranite.

And again, are the minerals and the colour and so on the same for all three?

I think they remain the same, yes

Could you give me any other examples of that kind of thing, where they're exactly the same apart from the grain size?

Syenite, andesite. Syenite is the coarse grained equivalent of andesite.

And do you have a medium grained example there as well?

(inaudible)

That's all right. What we need is rules where people say "If this mineral is present and that mineral is present and the grain size is such and such then it's such and such a rock.

That kind of breakdown is quite crude, it's not very very diagnostic. There have been quite a few cases where a rock has not been able to fit in that sort of a classification (inaudible)

Is it common for that sort of thing to happen? Or do most rocks fit fairly clearly into one category?

I think it all depends on the details of the classification system.

Do you have any borderline cases occurring because of the things you mentioned, like intrusions. Are things like intrusions responsible for rocks becoming borderline? Or is it not as simple as that? Say, if you've got a rock which is intrusive, will you then get rocks around the edge of it which aren't quite the same as the rock in the middle?

Oh yes, you do get that. That's what they call a chilled margin. Some do, some don't have it. You have a rim around the intrusion of fine grained rock made still of the same composition and same mineral association. The grains are just a bit smaller, and that's related to the cold, the minerals didn't have enough time to crystallise, whereas the large well formed crystals are the result of slow cooling.

So if it's next to a different type of rock and the crystals are small then..

That's related to temperature, the size of crystal, the rock it's in contact with, very cold rock related to its temperature which could be well over a thousand.

Could we chuck in a few more examples of any more igneous rocks that you could think of, or are you starting to get exhausted yet?

I think that under basic, intermediate, acidic, those are the main rock types. But you could go on to alkali rocks, which as a rule have no free quartz and high amounts of sodic and potassium calcide.

Can you tell me a bit about those?

An example being nepheline.

So what would a nepheline look like? Or if you could think of any other rocks from that same category?