Laddered Grid 4

So, if we start of with dacite, could you tell me what the most important differences between dacite and granite...

It's smaller grained.

Dacite is?

Dacite's smaller grained than granite.

Right, erm, what grain size is erm dacite?

Erm, medium grained.

And what grain size is granite?

Large

Large, yeah I know it's all <something>, but we've got to be thorough and meticulous. Could you tell me if there are any other grain sizes apart from medium and large?

Small? <chuckle>

Right... Can you give me some examples of small grain... grain size <unintelligible>

Basalt... ... Micro-granite... ... ... ... ... Don't know if peridotite is or not because you can't see any grains in it, only little specs. Err... ... ... Apart from that, I don't know.

Right, don't be ashamed to <unintelligible>

Trachyte I think is small grained too, I'm not sure.

Give me some examples of medium grained from that list?

Andesite... Diorite... I think, no diorite's large grained, dolerite... ... granodiorite... mmm... ... have I said rhyolite?

Erm, no..

Right.

And can you give me some examples of large grained?

Diorite. Gabbro... granite... romsenite (?)... syenite... have you got trachyte down with them?

You've put it down as possibly small grained

Fine.

OK. Erm...so can you tell me what the main difference is between basalt and micro-granite?

Basalt's a basic rock, micro-granite is acidic.

And how can you tell if a rock's basic?

It's usually darker in colour, this is in hand specimen is it?

Right, you can tell me in hand specimen or whatever.

Yeah, it's usually darker... and... it weathers darker, when you see it in <something> exposure. And it's a very informal-looking composition because it's *all* dark, whereas with the acidic rock it tends to be speckled and you can see it better `cos it's light and dark freckles, the acidic.

And could you say something about how you'd be able to tell them apart in any other way than in hand specimen, <something> hand specimen

Yeah, under a microscope erm, in plain polarised light, you'd probably see, in the basalt, you'd see biotite which would be yellow to brown pelochroic. Whereas in the acidic rock you tend to see muscovite, which is also mica, and that looks lighter.

Erm.

It's not lighter under the microscope, it's just, it's just sort of, it'll grey-white under plain polarized whereas the other is yellow-brown, but muscovite's white mica, biotite's brown mica, so in hand specimen it's lighter.

Right

But there's, you might get both, you can't really say for definite. Erm... the basic, the basalt might have an <ansabel> in it.

Right then, let's have a thorough session on basalt then. So, could you tell me how you'd be able to tell that a rock was basalt then? Give me all its erm all the properties and I'll get them all down together.

What from the start?

Erm

As in not going through anything we've already gone through like being small grained?

Erm... could you go through things like that, I want to ask you to go back on things like how you'd be able to tell that one of the minerals was erm biotite, see.

Mmm.

It's just so that I've got them altogether rather than sprawled out through several different sets of rules and things...

Basalt's small grained. That means that if you look at it in hand specimen, you probably won't be able to actually see individual grains in it.

Uh-huh. Right. And what else about basalt?

You might get porphyritic crystals in it. Namygdales (?) `cos it's a lava and sometimes cavities getting filled, so that's one, I think it is yeah, and that's only a guideline (?) porphyrine means you have a large crystal in a small groundmass, depending on how the basalt's cooled.

Right, and could you say something else about basalt and how you'd be able to identify it, or would that be enough to... say something about the sort of minerals that you'd get in ...

Erm... you mean in hand specimen? If you look at it in hand specimen, you might occassionally see tiny green crystals of olivine, but you can't say for sure. The main thing about it is that it is dark, it sometimes it's got a greenish tinge to it, but it's primarily blackish really, blackish with a greenish tinge, and it's small grained, and that's how you tell basalt.

Right. Erm, could you say something about what it would look like under the err microscope.

Erm... in plain polarized light, you'd probably have ansabels (?) and they would be pleichroic in green to brown, and you might also see biotite, which is pleiochroic in the outer browns instead, and the other minerals... ... there might be pyroxines of some sort, but you can't really tell that so much under plain polarized, you might see pink to white if you've got orthopyroxine. I think that's wrong actually.

OK, erm what would it erm pyroxines look like under cross polarized light?

Clinopyroxines have, they'd have high second order colours, lots and lots of different types I think, no that's all green, no they'd just have second order colours, clinopyroxines do.

And are there any other sorts of phyroxines that you're likely to find?

Other pyroxines... which... both of them sometimes show twinning, both clinopyroxine and orthopyroxine. And the orthopyroxine under the microscope, their colours are first order as opposed to second order, so they might be greys to yellow, but that's all, not the bright colours, the second order.

Right...

The, sort of every second order colour you can get, but also under plain polarized they sometimes look green, and you can see red in them sometimes, and the grain shape, they're usually roundish, like rounded grains when you see them.

Right, is that about the lot for basalt then?

Think so <chuckle>

It sounds pretty comprehensive, are you an igneous specialist by any chance?

No.

Oh, oh well, erm... could we work through the rest of the list then, since this seems to be going...

<laughs> I only did basalt `cos that's an easy one

Ah, well that's encouraging, I thought I'd be totally exhausted with a stiff wrist by the end of this, otherwise I'd.

No.

So if we could just start at the top and work through, if you could tell me how you'd be able to tell that each of those was that, if you had a specimen of it.

Well, I've never seen adamelite, not that I know of. I think it's acid. Therefore it'd be lighter. And, what kind of rock have I got it down as? I don't want to contradict myself.

Um... I don't think you have put it down as anything.

Oh.

So I don't think you should worry about that. The more obscure ones have been shoved in to make people think a lot more about what they're doing.

Oh.

You'll be delighted to hear.

Oh, it might the third years think about what they're doing, but not me. Andesite... that's acid. Erm, medium grained, I think. Trying to remember now... light in colour, speckled. If I've got to go through all these, the acid rocks have pretty much the same composition, there've only one or two minerals different, but I wouldn't necessarily know.

Right...

So if you want a definitive description of an acid rock, it'd be easier if I did it for granite..

That might be... yes please, erm well if you could give a general all purpose acid rock description that would hold for all the acid ones, and then just mention any pecularities.. would that be...

Yeah

All right, ok, so the acid rocks...

OK, light in colour, usually with dark speckles. Dark speckles tend to biotite or something. Erm... they weather light with exposure, course that's just a rough guide... sometimes in hand specimen, you can see quartz, which is transparent and harder. If you, if you've got a bit of quartz you can test because it's got bits, a lot harder, so you come across them feeled. But I mean that's, you could only really do that for granite, `cos the rest wouldn't be big enough.

Right

Erm, it also has feldspars... and they're white, and either translucent or opaque, no they can be grey as well, depending.

Don't worry if I've got it all down or not

White grain pink is that. It depends, you see, because orthoclase feldspar, you can get it in pink varities. And, well quartz is mainly like I say transparent, colourless, but sometimes it's not, it just varies depending on what you've got. And then you've got the mica, which can be the white, or it can be darker, some speckles. Erm... under plain polarized light, you wouldn't see very much at all... you might see biotite, if you've got biotite, that's yellow to brown again, pleochroic.

Erm, just want to put anything else that you might see in <unintelligible>

Not really..

Right, erm, what I <unintelligible>

When they're cross polarized, quartz grains are quite distinctive. They look fairly small under cross polarized, more than you'd think, and... <sigh> well, I don't know how to explain it. As you revolve them around the microscope, bits of them go into extinction at different times, that's go dark. But, unlike when you've got twinning, it's not definite, and it looks like it's soft of oscillating, sort of waves of grey, light grey and dark grey going across, and the shape they are. They look like they're jigsaw, fitting like jigsaw pieces.

Right

They're quite easy to identify.

Sounds pretty, er.. identifiable.

Erm... feldspars... they tend to show twinning, and they're usually, also they're greys and black. `cos they're also [stardichiers], occassionally you sort of get, I think you can get yellow tinges. And they're really striped, if you've got twinning, which you usually do have... Sort of lines down it, and they can be thicker blocks or thinner blocks. I think it's the... I can't rem... what did I say the two types of feldspar were, I can't remember now. Orthoclase.

<unintelligible> You kept quiet about the other one.

Did I? Gosh. <chuckle> That's a serious lack of knowledge. Err anyway, I think orthoclase has the thicker stripes, but I couldn't be too sure. Erm... Muscovite. If you've got biotite when it's under cross-polarised, it goes reddish colour. And if you have muscovite, that, that has lots of different second-order colours in it. A bit of a mish-mash of them... And the grains in the acid rocks, this, they're a clearer shape than they tend to be in the basic ones, like... you get decent shapes growing in them quite often. What's that called? Sapeedral usually, whereas... semi-decent shapes. And you don't under plain polarised... Think that's about it.

That sounds pretty comprehensive. Erm, if you told me which of these rocks were acid.

Erm... There's acid, basic and intermediate. And some are on sort of an acid side of it, and some are towards the other. I think that Ademalite is... intermediate turning to acid... and I couldn't tell you anything else about it really, except that I think it might be medium-grained. That's about all.

Right.

Andesite you get forming lavas, and that's an acid, it's acid. It's about medium grained, I think. It's extrusive.

Can we just the names, and then come back and er. otherwise I'll end up with this vast chaos of, of notes and that. Can we just get the names of the rocks under the categories and then

OK

Come back and do each one with its properties.

Right. Basalt's basic. Dacite acid-intermediate. Diorite basic. Dolerite's basic. Dunite-peridotite, I think that's basic-intermediate. Gabbro's basic. Granite's acidic. Granodiorite, erm... basic-intermediate. [Rondernite]'s basic I think. Microgranite's acidic. [Cyonite]'s acidic. Peridotite's basic. Rhyolite's acidic. Trachyte, acid-intermediate.

Right, erm. Could you say a bit about, don't think we've got anything about what the baseline is when we're talking about basic rocks. Is there anything worth saying about intermediate rocks, in terms of how you'd be able to tell if a rock was intermediate?

Erm... You look for the composition, like how much quartz they have in, how much feldspar, things. It's usually the acid rocks you look for, their composition, because the chart's based on silico, which is, it can be in the form of quartz. It's usually see how much, how much feldspar or how much ac.. how much quartz there is. You won't normally find quartz in basic rocks. So it's feldspar that you look for.

Yes, erm... Could you say roughly what sorts of percentages you'd expect with each of these types of rocks?

Erm, acids' about 60% quartz. And I think basic is under 45%. And intermediate's between, it's sort of between 55 and 45 intermediate.

And what about the feldspars, what sort of, what sort of proportions of those?

Erm, acid-intermediate about ... 2/3 feldspar, I think.

And what about basic?

Can't remember.

I mean is it more or less.

Oh, it's less feldspar.

Right, So erm, we've got for each of these, basic or intermediate, I think we've got them on grain size as well, yes. Err, if you'd like to just run through briefly and say about any particular identifying characteristics of each of them that we haven't erm done yet.

Ohh...

We've got as far as basalt fairly exhaustively I think. You'd be able to tell, if you're talking about the type of rock which is dacite.

I wouldn't know.

Right

<laugh> I think it's acid, and apart from that I couldn't really identify, except by elimination perhaps.

Right, what about the diorite?

Erm... ... er, larger, white crystals with small black. But still quite a high percentage of white if you look at it. Dolerite's usually sort of, it's intermediate black and white speckles, oh, diorite's large grained. And it usually has a greenish tinge to it. And you usually find it in veins if you find it in exposure, in dykes, sorry, not veins, dykes. I don't know the difference between Dunite-peridotite and Peridotite.

Well, I think we just call it dunite full stop.

Well can't we just call it peridotite full stop?

<chuckle> Then we'd have peridotite twice.

Yeah, well I don't know the net is.

Fine, it's one less for me to handle.

Gabbro, that's large grained basic rock. And it's usually got a very high proportion of dark minerals. It also have white speck, white crystals in it, probably. Err, granite. That's large-grained acid. Quite often there's colour in it. That, it can have a high proportion of pale minerals, and they'll be large, and you can get pink versions, and sometimes it's grey. Usually it has, er, colour to it, and the dark and light minerals are usually quite big, whereas in the gabbro some are, some smaller than others.

Right.

[Mornephite or thru-pherno-gabrock], I presume it's like gabbro, only, it's only usually I mean, sort of yellow colouration to it. Microgranite, intermediate grainsize.

Sorry, medium grainsize?

Yes, sorry, that's what I mean. It's acid. And Nepheline-syenite. I don't really know, I thought nepheline was parasine anyway? Oh that's [Natsaline], nevermind. <laughter> nepheline, I don't know, if it's got syenite in it I presume it's a bit yellow. Peridotite is primarily olvine, so it's very green if you look at it, and you usually have a black-speckled. It's sort of, it looks like a green crystalline mass with tiny black speckles in it. Rhyolite... is acid-intermediate. Usually, it's usually all, pretty well grey. Because it's often found in the lava, oh it's got porphyritic here. It's got larger crystals in it. And depending on what happened to it, there can be a [fabric todee] or it can be squashed or, depending what's happened to the lava, but it'll probably be like grey crystals in a lighter grey [grammer] and then porphyritic crystals were larger. Trachyte... what have I got trachyte down as?

Erm, lessee, acid, and possibly small-grained.

Yeah, it's er usually found as a lava I think, so acid and small-grained as I said. Mmm... it might show some sort of lava formation. I think you often find it with Andesite, and... it'll be lighter grey, and you won't have clear, defined crystals much. but you might have inclusions in it... ...

Well, this is pretty intensive. Erm, there's just one thing which you've mentioned, erm, [amphiboles] could you tell me what an [amphibole] looks like, or anything which <unintelligible>

It's a mineral.

Oh.

Yeah, didn't I tell you?

Erm, I don't remember, but it might be one of those <...>

[Amphibole's] the one that's pleochroic in green to brown, that's in polarized light. Hornblende's an example of an [amphibole]. And the colours are second-order, interference colours.

That's in cross-polarized light?

Yeah.

Could you give me some other examples of [Amphiboles] apart from hornblende, well, as well as hornblende?

Erm... no.

Right. <chuckle> Thought I might get you. Well, I'll declare this session at an end, unless you can think of anything else to er.. say about any of these rocks that we've overlooked?

No, hopefully I won't have to do much more of it.