Structured Interview 4

What happens if you don't know that?

Just, well, if you want you can tell me the ones you don't know anything about up front, straight away

OK

and then just ignore them.

Right, I don't know anything on Ademelite

Right

Kental- Kentallenite?

OK

Nepheline-Syenite, and Trachyte

Right, that's the ones that nobody knows about, probably. Seems to be the consensus that er, they're not known about.

Can I write on that piece of paper?

Sure, yes, please do.

Erm... ... ... ... ... ... ... ... ... ... Do you want me to go through them one by one or whatever?

If, if possible, yeah, you know.

Right, the erm, Andesite is intermediate in composition erm, generally fine grained, light to medium coloured.

Now, is that how you'd tell that it's erm, intermediate?

Yeah

Right

Typical composition would include erm high proportion of plagioclase.

And that's plagioclase feldspar?

Feldspar, yeah. <signing> but only a minor amount of othoclase. I can't remember the rest of the composition of that.

Now, just to check, if if, if plagioclase is present, what does that look like?

Err, that is light grey elongated grains, straight extinction.

Now what about orthoclase, what does that?

Err, that is fairly similar. I don't know.

Right, if it becomes clear, if you could mention it.

Yeah, yeah. Do you want me to go on?

Yes please.

Right, the err, the basalt. Generally fine-grained, is a lot more common, it's one of the commonest rocks on earth, has a high percentage of ferro-magnesium minerals.

How do you tell if ferro-magnesium minerals are present?

Err, the rock is, that is, it's due to the rock being generally dark coloured.

Right, right.

Let's try and think of typical minerals which have iron and magnesium in.... Can I come back to that one?

Yeah sure.

Err, and can we also add Dacite to the list I didn't know about?

Oh yeah sure.

Don't know that one.

Again, that's one that a large number of people are...

Yeah, you hear about it a lot but err. Erm... Right, Diorite is, I'm referring to Diorite because Tonalite I'm not exactly sure on.

OK, that's fine.

Intermediate in nature, light to medium err, light to medium coloured, fairly course grained, and exactly the same composition as the Andesite.

Right, so you get plagioclase, and orthoclase and erm.

Should be similar, similar compositions, only just an increase in grainsize.

Right.

Erm... no, can't do anything more on that. Dolerite erm, again basic. Same composition as basalt, only medium grained rather than fine grained. Dark colour, erm. I'm not sure I can think of the minerals... Olivine, that's it. A lot of olivine.

And how do you tell that olivine's present, is there any?

Erm, you can generally see green, very small green crystals under the microscope. Generally very high relief. Err... speckled appearance, giving second-order colours.

What are erm second order?

Erm, they're generally very bright colours, when you put the analyser under the microscope.

So that's bright colours under the analyser?

Yeah. Erm... and that, that again applies to the err basalt.

Oh right, basalt contains...

Yeah, the basalt and the dolerite are exactly the same.

Right, that was the mineral you were trying to think of for basalt?

Yeah. And, pyroxene, that's it. Pyroxene also comes into the basalt and dolerite.

Right, how, how do you tell pyroxene's present?

Err, pyroxene... erm, I'm not sure on this. I think it's got very high relief and, again a light dusting effect on it. And there's two types, orthopyroxene and clinopyroxene, and they vary by, erm, when they go extinct under the polarizer.

Right, and going extinct is...

Erm, you've got cross-hairs under the microscope and, err, they're generally needed on <gape> minerals and err, when you rotate the stage, the mineral, if it goes black and it's also parallel with one of the cross-hairs then it's straight extinction, therefore it's orthopyroxene; if it's at an angle to one of the cross-hairs on the microscope, then it's inclined, then it's clinopyroxene.

Right.

Difficult explaining that one <laughs>

<laughs> Visual sort of information is difficult to...

Yeah, that's right. Erm... Right, the err Peridotite and Dunite, in terms of composition it's just mainly err olivine. Dunite's virtually 100% olivine. A very dense rock. Erm... that's all I can say, it forms the earth's mantle.

So, will that be a basic rock, then?

That will be more likely ultrabasic.

Ultrabasic. And what, what makes it ultrabasic?

Err, the composition, I think. The, the large amounts of olivine in it, and it'll also be very dark in colour.

Right, and, and...

Be dark black.

The darkness, I think, when we did the card sorting, the darkness reflects the lack of quartz, does it?

Yes, that's right.

And the...

And the increasing of the ferro-magnesium.

Right, which is olivine.

Yes, that's right.

Right

Erm, also be very dense as well. Erm, the gabbro is exactly the same as the basalt and dolerite except it's err very course grained. So again it'll have olivine and pyroxenes, and it'll be basic in composition.

But it's the very course grains that mark it out?

It's the course, yeah.

That is the coursest, is it?

Yeah, out of that, that basic group.

The basic group is?

Being basalt and dolerite and gabbro.

Right.

Right, erm... the granite is acidic, course-grained, consists of quartz, mica, orthoclase and plagioclase feldspar... I think that's about all that it does contain.

This mica, erm how do you tell?

That can be subdivided into two types, erm, biotite and muscovite.

And granite has both, does it?

Yes. But err, the type of granite depends on the, obviously the variable proportions of them.

How do you tell that biotite's present in any rock?

Biotite when you just look down the microscope, it's a pale brown mineral. A regular shape. When you put the polarizer in, it's generally first-order colours, which are generally light, not, varied colours, but bright, erm, not very bright.

Right, so palish?

Yes that's right.

And what about muscovite?

Erm, muscovite is generally clear, I think, I'm not sure on that. Erm, but when you put the polarizer in, you get very bright colours. Muscovite tends to be erm... a mosaic of very bright...

Are they what you call second-order colours or...?

Second-order yeah, you know more about it than I do <laughs>

You mentioned second-order colours, earlier, so you know.

Right, erm, the granodiorite... the only thing I know about that is it must be fairly course grained, given that granite and diorite are both course-grained rocks, and it must be a mixture of the two.

Right.

And I can't say anything more about that <laughs>

That's all right.

Right, the microgranite is exactly the same as the granite, except it's medium-grained... do you want me to run through the same things we said for the granite?

Yes please if you could, just for the tape.

Erm, acidic, consisting of quartz, mica, orthoclase and plagioclase feldspar.

Right.

And erm, the Peridotite, a fairly high, it's a basic rock, ultrabasic rock, very dense, dark coloured, erm, high proportion of olivine, err, no doubt some other ferro-magnesium mineral, but I'm not sure which. Very similar to the Dunite-peridotite I mentioned earlier. Erm... generally course-grained, I should have said from that, I think, because it's been at great depth, therefore it's had a chance to...

Only it is erm depth that is one of the sort of key...

Depth, yeah, does control crystal size.

Right

It's the erm, time taken to cool.

Right, and the longer it takes to cool...

Then the larger the crystal size.

Right, so something that's cooled very quickly...

Will have small grainsize.

Is fine-grained. Right, so the grainsize of these things tells you something about the cooling history.

That's right, yeah. So all the Ryolites, the Andesites and the Basalts cool very quickly at the surface, get things like Granites, Diorites, Gabbros and the Peridotites erm, at greater depth, cooled over a long time.

Things that are sort of intermediate grainsize...

Will be probably near the surface, but not actually at the surface.

Not on it, right.

And it's all gradation, really. It's erm...

It's a continuum rather than packets of...

Than distinct drawing of line.

Right, right.

Ryolite, err, has got porphyritic in, that's a term describing the texture of the rock, and it refers to a, a fine-grained background matrix surrounding courser-grained crystals. These err crystals will probably be erm... quartz I would have thought. It's an acidic rock anyway. Erm... normally you'd expect to have it fine-grained all the way through it, but since it's porphyritic, you've just got set of these isolated crystals, larger isolated crystals.

What's the, oh right, it's, it's acidic so it tends to be a fairly light...

Yes, it'll be light coloured. And would have quartz, mica, the two feldspars, orthoclase and plagioclase.

And quartz you can... have I checked the... now quartz itself, how do you tell quartz is... present?

Erm, under the microscope it's very very low relief, it's transparent. Sometimes it's, it's just, just those parameters. When you put the analyzer in, it becomes grey and white, alternating crystals are grey and white, it's irregular shaped masses, no distinct shapes to them.

Right

I've just remembered about err distinguishing the orthoclase and the plagioclase....

Right

The erm, the orthoclase, when you put the analyzer in, it has a distinct pattern to the crystals the erm, it's called twinning. Plagioclase tend to have lots of twins, so it's dark bright dark bright lines all the way through it, err, and that's... alvite twinning I think it's called.

What's it called, sorry could you spell that?

Alvite, A-L-V-I-T-E.

Right, and that's in the plag.

That's right, yeah. And then there's another type of twinning called carlsbad.

Could you spell that?

C-A-R-L-S-B-A-D.

Right

And that is just a line drawn down the centre of the crystal on the right hand side is kind of dark and the left hand side is white, they could be the other way round. Erm, that's about it.

Oh, Trachyte you didn't...

Trachyte, no.

Erm, I'll just tell you what you've, what sort of dimensions you've used, and then if that's OK then we'll stop, and if there's anything that you wish to add that you think is important for discrimination, then if you could mention it. Right, erm, acidity, you know acid, basic or intermediate, which is reflected in a colour continuum as well, going from light to dark, acid to basic. Erm, grainsize, which we've said is reflected to the sort of depth of placement of the rock. Erm, various inclusions of various minerals: erm, feldspars, we know how to find both types of those; erm, olivine; erm, presence of other ferro-magnesium mineral-minerals; erm, (what else have we got?) pyroxene, two types again; err, olivine. Density, you've mentioned. Biotite, muscovite being the two types of mica. Err, that's it. Then you tell me how to find out, oh yes, relief of crystals to, to get to the various minerals, use of the analyzer, use of extinction. Is there anything else you wish to add?

Erm, no I don't think so.

That's fine.