Structured Interview 5

Right, so if you'd like to start with telling me about how you'd set about identifying igneous rocks...

Igneous rocks

in general, I've got a universe of rocks there, so just a few general principals to start off with...

Right

so <unintelligible> for a minute please, start to get stuck, give me a shout.

Right, start with, you would start with erm, hand specimen identification, and look for proportions of feldspars to quartz, to any other, darker minerals within them, then you'd use a thin-section erm staining for potassium feldspars would be helpful, and under the microscope you look for proportions again of, of the feldspars, and again on that you can work out, well this'll be different, this is granite, you'll be working out what kind of granite it is, then for basalts you'll be able to tell them because they'll be quartz-free, and they'll be darker, they'll be a basic rock, you'll have, usually have plagioclase glass in them, so in, they're very, very distinct when you put them under the microscope because they're just, just dark as I say, well, little crystals in them rather than the big, `cos granite's usually made up with interlocking big crystals, but the basalt'll be dark in hand specimen, there's no crystals obvious usually.

Can you say a bit about what you'd look for if you had an unknown rock, one that you'd never seen before, and just general principles you'd apply to any rock when you were first looking at it, and then we can come back and look at individual rocks after that.

Right, OK. I'd usually, first of all look at the colour of it, if it's a light colour then it's more tec-it contains the acid, an acid rock. If it's a dark colour, then it'll be a basic rock, and if, if it's, it'll be dark green if it's an ultra-basic rock. Look for quartz to find out if it's a granite. Then... presence of olivine will probably tell you it's a basalt, and then, under the microscope it would need to be, depends ultra-basic minerals tha-that's the peritonite and the [pintonite] and erm, I dunno just look for the, the minerals that are indicative of that type of rock, but first of all I would usually go on colour <chuckle> and the presence or not presence of quartz.

Right, so tho-those are the general things that you'd need to know about any igneous rock before you could

Yeah you'd have to, yeah you'd have to know what kind of minerals you'd expect to be present.

And what are the main minerals that you'd erm be on the lookout for in any igneous rock?

Erm, in granites, it would be quartz, orthoclase and plagioclase, in basalts, it would be plagioclase, olivine, pyroxene. In the ultrabasic rocks, err... <unintelligible> <laughs> haven't done much in it.

Could you describe those minerals in a bit of detail so that you would be able to say which mineral it was, just what a mineral looks like.

Is this in hand specimen?

Hand specimen and under the microscope if you, if you will.

Like quartz in hand specimen, you'd see a vitreous crystal, usually not very much crystal shape, use, well in granite they're all inte-, they're all intergrowing out so they're all, so easily defined by the other surrounding crystals. Under the microscope, it's in, in plain polarised light it's just clear, very clear, usually not many inclusions umm, under cross polarised it's very low polarisation colours, which is the same as plagioclase, but plagioclase usually occurs in <something> prismatic crystals, which erm, often show twinning, erm, <olbite> twinning for, for plagioclase and <microfine> for orthoclase. Umm, oh yeah olivine, olivine has definitely green, tends to be green and sort of I would say it's a fruit pastille, it's kind of sugary, and under the, under the microscope it's also cracks, dark cracks, it has high, high relief err, under cross poles it's got very high birefringence colours and the same third order. The other one, pyroxene's usually <a something for that a use little> shape as well, and you have eight-sided (eight-sided?), yeah eight-sided cross sections to the crystals, and er two intersecting cleavage at erm 90 degrees to each other, and their polarisations are usually red, browns and oranges. Erm, I think that they're the main ones.

Erm, have you mentioned er the discrepances between orthoclase and plagioclase feldspars? I think you mentioned that you'd got both of those, but you haven't said how you, you'd tell them apart.

Yeah, difference between them? Well, orthoclase is, has negative relief erm relative to the balsam that you glue a big block of this like the, and you base your analysis on that.

Er, how does that show up then, negative relief? <unintelligible>

Erm, well it doesn't, it doesn't look like any-, you get a, a piece of the balsam next to a piece of the, of the orthoclase, and if your light is, is, if you turn, if you increase the, the area between the lens and the slide, then the <brecky, becky> lines, which is a light line around the mineral, will move towards the mineral with the higher refractive index, one with the high relief, and so if you move it up, then it will move away from orthoclase, `cos it usually moves into the mineral, and towards the balsam.

Right, are there any other minerals which show the same movement direction as orthoclase.

There are not many which are negative relief, no, and definitely not any more, any other feldspars, any other feldspars are positive.

Right, OK, so...

I mean that wouldn't be the way you usually, that's if you're really stuck, you would do that, if you just do it on twinning it would.

You mentioned to me about staining as well, for potassium feldspars. Could you tell me a bit about that, how that works?

Erm, I don't know, I don't know the name of the stain or anything, but it's, it turns, it's easier for, er, a thin section if you just hold it up to the light `cos that's easier for you to tell proportions of minerals, erm, then it turns the potassium feldspar, it turns it a very slight yellow colour and the plagioclase feldspar just very cloud-, cloudy white mineral, quartz would be the clean mineral.

OK, so erm, is there anything else that you can think of worth mentioning about minerals, before we go on to erm, individual rocks? You mentioned a bit about birefringence colours, can you say something about those that er?

Birefringence colours are the, are the colours showing erm, depending the difference between the highest refractive index and the lowest refractive index and that, depending on which way the crystal is orientated, so you get a colour erm of this wavelength and er, it's just very sort of, I don't know, it jus' te-, it tells you which mineral it is anyway, `cos they've all go-, all got distinctive colour, or along with o-, other things in it tell you, tell you the colour.

OK, so maybe if we could move on to some of those individual rocks, you give me a description of what that rock would look like.

<chuckles> <unintelligible>

Well, somewhat are unusual, which has been done on purpose to make people think about what they're doing.

<chuckles> Ademalite I've no, I don't, I've never heard of it.

Right

Andesite, andesite now... I always think this is a calcium, a calcium basalt, but it's not. I can't think what that one is either. Well, it's a type of basalt. Basalt on its own is one you can, you can tell that because it's got (is it 70%?), oh no, and orthoclase, it, it's plagioclase composition it depends on. Erm, basalt has an orthoclase I think it's between, greater than 30% and orthoclase, I think that is, and that's how you could tell andesite, that will depend on the plagioclase composition. Dacite, I su-, that, that's a calcium...

Erm, sorry, erm, you were on about the composition of the erm

The plagioclase

Plagioclase, what's sorts of plagioclase can you get then?

Well, you get a range from an <ausite>, which is a calcium plagioclase up to olvite, which is the sodium plagioclase, and it all depends on the percentages of calcium to sodium in the, in the mineral, and the, an orthoclase thirty would be 30% calcium relative to sodium in the mineral, so it would need to be greater than 30% to be, to be a basalt, and a dacite would be less than 30 `cos that's a calcium, calcium basalt, so erm, it would have to be less than 30%, oh no no no, it would need to be near 100% er, amorsite, calcium end, for it to be dacite.

Is that the only distinction between the other ones, or do they look identical in hand specimen and so on?

Erm.

Or are there other differences as well?

Erm, I don't know what they look like in hand specimen, don't think I've seen a dacite, but, and that's the main way, that's the main way of telling them apart.

Er, what would basalt look like in hand specimen then?

A basalt would just be, it's just black, very fine grained.

What other grain sizes do you get besides fine.

You get erm, the medium grain sizes like a gabbro would be a medium grained, and then the coarse grains like granite, depending on where they are.

Right, and could you put some sort of figures on what makes them coarse grained as opposed to a medium grained?

Erm

Sort of order of magnitude.

Er, a coarse grained would probably be anything over, I would say, anything over 2mm grainsize, and then medium grain would be one to two millimetres, and fine grain would be anything less than that, usually fine grained, you can't see the crystals at all, unless they're phenocrysts, that's the, they're larger crystals which are usually the olivines or the plagioclase. Right, what do you need more?

Yeah, sorry, take your time, and give us as detailed a description as you can, so that, that way we'll have a, a viable base.

Yeah, OK, we'll just go back to dacite then, I'm su-, as I say I can't remember if I've seen, but I'm sure it must be a light coloured rock, because of all the calcium in it. Diorite-tonalite. <sigh> I should know <chuckles> Oh, I can't remember what diorite is. Dolerite is a, a medium grained intrusive rock, it with a, so that it would have, it would have grains you could see in hand specimen.

Is that a characteristic of intrusive rocks, that they've got grains that you can see in?

Generally, but not, not totally, I just thought, when I'm doing my mapping, and I er, came across all these rocks, and I just looked at basalts, except they had large olivine phenocrysts that I, so that's an olivine basalt, but in, I'm sure that they were intrusive as well, you can get basalts that are intrusive, but they're generally extrusive, and the coarser grained ones come from deeper into the ground.

Right.

Just to do with the rate of cooling, you get that. Erm, dolerite is a medium grained intrusive rock, so it'll have larger, larger crystals of, of erm plagioclase and olivine and pyroxene, you'd be, be able to identify them in the field.

What sort of erm grainsize would you expect to go with that, I mean what sort of colour?

Erm, er probably... probably <unintellible> a green and white, green and white crystals. Erm, the grainsize probably a bit, probably about one millimetre, nearer the finer grained end of the medium grained rocks. Erm, dunite-peridotite, they're the, the ultra-basic rocks, they're very dark in colour probably. I haven't seen any of them either. Usually you'd find olivine in them, in the rocks, I think.

What sort of grainsize would they be?

They would be...

Don't worry if you don't remember

I don't know, I don't know, I should know <chuckles>. Erm, they're probably medium grained, because the only fine grained ones are the ones that are cooled quickly, which is the basalt, which is common in erupted onto the surface, so I should think that they would be medium but I'm not too sure. The gabbro is, a gabbro is the same as the basalt, and has the same feldspar composition, but it's just coarser grained, and that's, well that's the main, in fact I think that's the only distinction between the two. They all have the same plagioclase, plagioclase composition, both of them, it's just that gabbro is, is an intrusive rock and is, has coarser grains. Granite, well, I said that one before, it's a...

<unintelligible>

<laughs> <unintelligible> that's the only reason I know. Erm, granite, quartz has the greater than 10% quartz, erm, grainsize is very coarse usually, erm, and it also has feldspars, and it also has the orthoclase too, plagioclase composition determines the type of granite that is there. Erm, and sometimes it has, I think amphiboles it has in it as well.

What do amphiboles look like?

Amphiboles are, are very similar to pyroxenes in that they have erm, two cleavages that are particularly to 120 degrees to each other rather than 90 degrees, it has erm six-sided cross section rather than a eight-sided cross-section. Erm, I think that one of the two has straight extinction, I think it might be amphiboles.

And what's erm, extinction, we haven't heard that before?

Oh sorry, erm, extinction is when you have er, when you have the polariser in the microscope, the birefringence when you turn the stage, turn the stage round, erm, birefringence will tur-, go to black at four points round the stage. Erm, and the angle between the vertical and that, and the angle it's going to extinction is the extinction angle. So straight extinction is when, when erm, the mineral is black on the vertical cross hair, on the horizontal cross hair, sorry so it's four times. So I think amphiboles have straight extinction, and if it's not amphiboles it's pyroxenes <chuckles>.

What other sorts of extinctions do you get then, do you just measure it in degrees or...?

Yes, oh well that's the, that's the way of determining the plagioclase composition, is by measuring the extinction a-, extinction angle. You've just, you've got a <verny> around the side of the stage, and you measure it off from, measure the vertical and then measure the two, or the, you measure from maximum extinction round to, maximum birefringence sorry round to extinction, and that will be your extinction angle. And from that you get a little graph, and, and you, from that you can tell your plagioclase composition. So, but, granite <unintelligible> olivine.

Yes, yes er, if it's any consolation we go through it quite a lot more often than you do.

Yes, suppose so. Granodiorite, oh erm, it's a type of granite, so it's always typical of it, I think it's in, that depends on, the percentage of orthoclase to, to plagioclase feldspars, and that's the only, that's the only way you can tell that all the granites from each other, yeah. And I don't know what it looks like or anything, I don't know. The next one, ken-, kentallenite, I don't know what that is. Microgranite, I would just suppose that's just a fine grained granite. And there's not much more I can say about it. Nepheline-syenite. Nepheline was a, <not contain any> nepheline is like a... silicon ox-, silicon oxide?, that was a stre-, it's limited to quartz, it's not got any oxygen I think. This is a silicon mineral. Erm, I don't know what it, I don't know what it looks like compared to quartz but it's erm, it's usually formed in a crystal chamber where all the oxygen's been used up, and it's just got loads of silicon left, so it just, erm, it's pure silicon just about, but I've no idea what it would look like. Compared with quartz, which I'm sure would be the, the way to identify that one. Peridotite is an ultra-basic rock, and it would have erm, lots of chlorite erm, sorry olivine in it, usually also <siliconsonite>. Erm, all, it just has all the dark minerals in it I think, it would have pyroxenes and amphiboles and, but no quartz, a little plagioclase probably, they all have plagioclase, but definitely no quartz in that one. And I don't know the grainsize or, or the colour would be dark green in hand specimen. Erm, rhyolite, erm... I think that's a hydrous, that's a hydrous rock, so erm, it would have an amphibole in it, it would have amphiboles and pyroxenes. It would usually have either pyroxenes or it would have amphiboles, and er, amphiboles are a hydrous form of a pyroxene. So I think that would, that would have the amphiboles in it, plus your, your plagioclase, and, and maybe olivines as well. Trachyte, I think that's another type of basalt, so you'd have some plagioclase composition again. I think rhyolite's a type of basalt as well, but I'm not too sure. I don't, I can't think what that one looks like either. We've only looked at the hand specimen <chuckles>

<unintelligible> range of people who prefer sedimentary rocks <unintelligible>

<laughs> yeah.

You mentioned erm basic and ultra-basic as well, could you say a bit about how that system works?

Erm, that was first year work. The distrinction between basic and ultra-basic is, I think, on, oh yes it's on the, on the amount of plagioclase again, the percentages, I think it's, it's either less than two thirds calcium feldspar, or plagioclase, one of the two. It's all like, like erm, greater than less than two thirds of this one, and half of this one, things like that. But I can't, I can't remember the exact figures.

Right, what other sets of rock do you get apart from ultra-basic and basic?

You get the acid, which is the granites, which are greater than 10% quartz mmm... I think that's it, just the same, I think. And then there's the, there's the nepheline, the ones containing nepheline. I can't remember what you call them, that's just the, that they have no quartz in them, whereas the rest could, <something> high percentage of quartz.

Right, we've come to the end of the...

<unintelligible> thank goodness, we've come to the end of all of them <chuckles>

Could you give me any other thoughts that have occured to you in the course of this task in identifying igneous rocks.

Apart from I should know more about it <chuckles>.

<uninitelligible>

I'd say, if you were, if you just had a rock and didn't know, then calculate the plagioclase composition, `cos that's probably the most important identification thing. Erm, mark down what kind of other minerals you have, the olivines in the basalts can tell you what type of basalt you have, whether it's an, a foliated basalt which doesn't have an olivines in it, and it would, and it also has orthopyroxenes rather than clinopyroxene, or rather it has both, it has ortho and clino pyroxene, whereas the erm, olivine basalt's an alkali basalt will only have clinopyroxene and also olivine. So er, if you can <something> of the other ones that are there, and take plagioclase composition, then you're on your way.

Right, what's the difference between clinopyroxene and pyroxene?

Erm, clino <something> it's the crystal structure. Clinopyroxene erm... under the, under the microscope <something> this is, orthopyroxene sometimes is pleochroic, which is when you turn the stage in plain polarised light, the colour changes, and that's a very slight pink, very light pink, and very light grains sometimes. That means it's definitely orthopyroxene because clinopyroxenes just don't show that at all. Erm, the birefringence colours are different as well: the orthopyroxenes very generally just are higher than the clinopyroxenes, they always go the nice ones birefringence, you should see, so er if you see one which is a very, very light brown or orange, then it's definitely not clinopyroxene `cos you should turn the stage a bit, and you will see, `cos it's the darker end of the, of the spectrum then.

Do the orders of colour relate simply to the intensity of the colour, or the darkness of the colour or what? You mentioned first order or second order.

Oh. Does it?

Well I mean, are higher order colours in the first order of the spectrum or, could you just say a bit about how you'd know that it was a higher order rather than a first order, so?

Well, erm, when I say first and second order, the colours of the first order are totally different from the colours in the second order. The colours in the first order go from black to green, to yellow to brown, and the colours in the second order are bright blues and bright pinks, so there's no, there's no difficulty about identifying colour.

Are there any orders above two?

There's the third, it goes up to fourth order. The third order are erm, third and fourth order go to the more pastel colours, so they're pale pinks and pale yellows, but they're, they're different from the others, you have a chart in front of you, a chart that has all the colours in it, and it's erm very easy to tell what colour it is when you actually look at it.

<laughs>

<laughs> That's all that I'm going to say about it.

It sounded pretty comprehensive.