Laddered Grid 3

Right, this is subject ... with the laddered. So er, I seem to remember that you weren't too happy about dacite last time

<unintelligible><laughs>

So, if we start off with erm granite instead, can you tell me the most important difference between granite and basalt, what's the most important single thing?

Most important, er, difference, is that granite contains greater than 10% quartz and basalt doesn't contain any, usually.

Right. And can you give me some other examples of rocks which have got more than 10% quartz?

Erm

From our universal list there.

... Erm, gabbro? Gabbro I think. Granodiorite, perhaps. Microgranite. That'll be all.

Right, and can you give me some other examples of rocks with no quartz?

No quartz? Dolerite, Dunite, Peridotite, Nepheline-syenite... probably a lot more as well.

Right, so within these 10%, greater than 10% quartz, could you tell me the main difference between gabbro and granodiorite.

Erm, grainsize.

Right. Which

I think.

Which grainsize?

Which g, Gabbro will be medium to coarse grainsize, granodiorite, I should think will be fine.

Right, Could you give me some other examples of other rocks with medium grainsize?

Erm, dolerite, I think it'd be diorite as well, peridotite, maybe err, <mutters>, and granite.

Right, can you give me some examples of fine grain rocks.

Basalt, andesite, rhyolite, [what] is porphyritic though which means it isn't.

Right

Erm, trachyte, nepheline-syenite, microgranite, dacite, and that's all.

Are there any other grain sizes that aren't medium and fine.

There's a coarse grainsize.

Right, could you give me some examples of coarse grainsize rocks?

Well, erm, granite could be, erm. Hmm... that's all I think, from that list.

Right, <unintelligible>. Could you tell me the main difference then between dolerite and diorite, since they're both medium grained?

No, <laughs>

Right, howabout dolerite and peridotite?

Right, erm... peridotite's made of erm, all the [ultimatick] minerals which... erm... have a lot more of olivine in it, a lot less plagioclase. Erm, a lot more pyroxene than the dolerite.

Right, could you give me some other examples of rocks which are rich in [ultramathick] minerals?

Erm... <chuckles> [Komatiate], which is <chuckle>, that's the only <something> I know, that I can think of, can't really think of anything else.

Right, I don't think that's in our real list.

Not erm, I'll tell you the ones from here.

Yes, if you would

I'm sorry

...now and again, but er

Erm, I don't think I can <something> look, dunite-peridotite, I think that could be, oh yes, dunite.

OK, erm, could you tell me the main difference between basalt and andesite

Erm, andesite contains more [calcate]-rich minerals, so it, in a plagioclase composition, andesite will have a large percentage of [anosite] rather than [oldite]. Oldite's a sodium, plagioclase andosite is calcium plagioclase.

Right, and could you give me some other examples of calcium-rich rocks?

Dacite. Erm... Rhyolite and Trachyte.

Right, and could you give me some examples of rich which are, rocks which are poor in calcium?

Dunite-peridotite. Basalt, I think. Dolerite I think probably, gabbro. That's all that's here.

Right, erm, could you tell me the main difference between nepheline-syenite and microgranite?

Mmm, the nepheline-syenite erm, contains silicon in, well rather the granite contains silicon in an oxidised form, so that it's going to contain quartz, but the nepheline doesn't have any quartz in it, that's the distinguishing feature.

OK. Erm, you, ok. You've mentioned olivine. Could you give me some examples of rocks which contain olivine?

Right, erm. Some basalts do. Dolerite, dunite, dunite's a nearly a pure olivine rock, I said. Erm, peridotite, that'll be all.

And, what about rocks which contain plagioclase?

Basalt. Andesite, dacite, dolerite, gabbro, granodiorite, granite, microgranite, nepheline-syenite, erm peridotite might contain a bit, I think rhyolite and trachyte do.

And could you give me some examples of rocks with pyroxene in?

Basalt. Dolerite, dacite and andesite. Erm, diorite. Err, I'm not too sure about the dunite. Granite. Granodiorite, microgranite, nepheline-syenite, peridodite, rhyolite and trachyte.

Right. So, erm, you've given me olivine and plagioclase and pyroxene and quartz as minerals. Are there any other minerals which are likely to occur within this set of rocks?

Oh, there's erm potassium feldspar in some basalt. Err... do you want me to give you the names of the rocks that they're in? Or just list them?

Erm, if you give me the minerals, and I'll come back and get the rocks.

Erm, [anthebole], erm iron ores appear in every rock, in all of them. Erm... the other accessory minerals that you can find in some, I don't know if you're counting those.

Right. Could you give me some examples of rocks then that potassium feldspar's in?

Erm, probably be psollioditic basalts <chuckle> psolliodtic <chuckle>

Psolliodtic, yes, would you care to spell that?

PSO erm LL I think

Right

IOD, TIC <laughs>

Tic, right. Where the hell did I meet foliated rock? You're probably just making words up happily the whole time.

<laughs> Erm, other ones. Granite, granite and microgranite. Gabbro. Umm, dolerite could have I think. Erm, I don't know about any of the others.

Right, and [antheboles] you mentioned. Could you give me some examples of rocks with antheboles in?

Granite and microgranite again. Dunite-periodotite I think and peridotite. That's all I can think of those.

Right, and if now move on to the minerals themselves, how could you identify a mineral as being potassium feldspar?

Erm

Is there anything else.

Erm, it would have low birefringence colours. Negative relief. Er, if it's pleochroic, it will be very slightly pleochroic from pink to green. Umm... Good extinction, if you have a [line for par], a long crystal. Umm, that's all that is now.

Right, and how bout olivine, how could you identify that?

Erm, very high relief. Always cracks, and dark cracks. Rounded crystals. Second order birefringence colours, bright blues and pinks. That's usually enough to distinguish that.

Right. Yet another sheet of paper.

<laughs>

Right, erm, so we've done those two. What about plagioclase? How could you identify that?

Easily long, long prismatic crystals, first-order birefringence colours, green and, greens and whites. Twinning is common. And twinning'll tell you whether it's, is it plagioclase we're talking about?

Erm, it's plagioclase.

Erm, in twinning it'll be [olvite] twinning, which is dark and white stripes all over the crystal. Erm... Low relief, usually in thin section, erm <unintelligible> cloudy appearance, which would distinguish from quartz, which is always clear. That's all.

Right, so if we could go onto quartz itself now and erm, tell me how you could identify that.

It's always clear, never usually any cracks or <something> products. Low birefriengence colours, greys and whites again, same as plagioclase. No twinning. Sometimes, undulated extinction, changes this erm, it goes black at different parts in the crystal as you, as you twirl it round, which is a mottled extinction.

And pyroxene, could you tell me how you'd identify it?

Erm, eight-sided cross-section. Sometimes coloured, sometimes it's green if it's iron-rich. Two cleavages at right-angles to each other, so you'd see a criss-cross pattern in it, in the cross-section of the crystal. Erm, yellow to brown birefringence colours. Erm, oh if it's coloured it'll be pleochroic. And I think that's all.

Right, and to finish that lot, anpheboles.

Anpheboles are eight-sided cross-sections, so it's similar to pyroxene. Err, mainly two cleavages as well in the cross section they're at 120 degrees, that's the main way to tell them too apart. Erm, birefringence colours are a bit higher than the pyroxenes, so it's yellow too, that's all I think, I think that's all for that one.

Right, so, erm, we've done the mineral content, and I've done some sort of rich of that. Right, so how can you tell if a rock's calcium rich?

You can tell by its plagioclase composition.

Right, and which way round does that work?

You calculate from the extinction angle, erm, of the twins in the crystal. And the extinction angle you can look up on a graph, and it'll tell you the percentage of anorsite-augite, which is in the crystals, which gives you the calcium/sodium ratio.

Right, sorts that one out. Right, so if we can look at some of the actual rocks themselves, I think that would probably be <something>. Erm, we started off with ademalite, do you know anything about that?

Nothing at all <laughs>

Right, fine

Leave that one

Right, how could identify a rock as being andesite then?

It'd be fine grained. Erm, and similar to basalt, but higher calcium content, and a light colour <something> rather than dark, be a blue rock. I think that's all. I can use you the basalt contents, which is plagioclase black, the matrix of pyroxenes, plagioclase, it's usually distinctive.

Sorry, which of those is the matrix? Was it the plagioclase one?

Plagioclase, plagioclase and pyroxene in a matrix bubble, so plagioclase lasts larger through the crysting. But then that's only for basalt, just the plagioclase composition you have to work out.

Right, and then basalt itself.

Basalt is a very dark fine grained rock. With or without [phenocrysts], phenocrysts could be olivine or plagioclase. Or pyroxene, in the matrix, of all of these. Dacite <laughs> well, I <something> it's the same as andesite, it's a calcium-rich one as well, but I think one of the two is more calcium rich than the other one. It's all dependent on the plagioclase composition, and it, basalt's the er less than 30% [anorthite], andesite will be 50, 50 [dacer] or 50 to 75.

Right, can you give me those figures again, or are you just making up sort of rough...

Well, I'm just making them up, I don't know what <something>. Diorite-tonalite, I don't know.

Fair enough.

Don't know the difference

We have had to put in some skewers to get people thinking.

Dolerite, erm, medium grained, dark rock usually. Erm... I don't know what type of minerals, probably, probably the same as the basalt, except they're all a lot coarser grained, big enough to see them in hand specimen, pyroxenes and the plagioclase and the olivines.

Right, <page turning noises> rustle rustle rustle. Guess we're onto the next one are we?

It's dunite, is almost pure olivine. I think it's medium to coarse grained, it'll be very green the rock, and that's very characteristic, there isn't <unintelligible>

right

Gabbro, erm, that's a coarse-grained basalt, it's the same, same as basalt, except it's medium grained. Dolerite, I think dolerite has different feldspar compositions from basalt.

Can you remember which direction the difference was?

Erm, I think it had a higher proportion of potassium feldspars than plagioclase feldspars.

Right.

Gabbro's the same as basalt but coarser grained. I think that's all you could do with that.

Right

Granite, greater than 10% quartz. It can be medium or coarse grained, with plagioclase feldspar and potassium feldspar. Plus other <something> minerals like anpheboles, and <something> not too sure.

Do all of these erm contain [others]

Yeah, most of them will have [others] I don't know the next one, Kenta... kentallerite? I don't know that one. Granite will be granite but with dist, err, fine grained scale. The same, the same in others. Nepheline-syenite is... oh... I'm sure, it's the same as granite, but it doesn't have any silicon in, just the silicon form, without the oxygen. I don't know how you can tell nepheline from quartz though. That would be, if you did know, that's how you'd tell the difference between the two rocks. Peridotite erm, I think peridotite is a pyroxene-olivine rock.

So you mean it contains just those two?

Yes, maybe erm, a little plagioclase, but not very much, a little feldspar <something>

And did you say a little anphebole or a little feldspar.

Erm, feldspar. Rhyolite... I think, I'm sure rhyolite and trachyte are the same as well, it's like andesite and dacite, olivine's in them. Yeah they're fine grained, basaltic rocks as well, and I'm sure that all depends a bit, feldspar composition too.

OK, have you got any idea of which way it goes?

I think erm, I think that trachyte's greater than 70% anorsite, and then it goes decreasing from that, between 50 and 60 maybe percent is dacite's the next one, or rhyo... I dunno, rhyolite's the next one, then dacite then andesite.

Is rhyolite then 60 to 70, or 50 to 60% anorsite

Oh, 60 to 70.

Right

That's 60 to 70, then, what did I say? dacite? is 50 to 60. Andesite be 40 to 50. And basalt would be less than, less than 40.

Right, and could you tell me to make sure I've got it on the record, how you could tell anorsite was present, or what percentage was present, or...

Erm.

We've probably done it before, but I want to make sure I've got it down.

Yes, the extinction, the extinction angle for the twins. Of the crystals, will tell you, the ratio of anorsite to olvite and plagioclase.

Right, and how do you use the extinction angle?

Erm, with your crystal on the vertical [prosier], with, with the twins, erm, all the same colour, and then, you turn, <unintelligible> read off, read off the angle it's at, then you take the stace so it's one hal... one side of the extinction, and read off the angle, then do it the other way, and average two, both the angles, and then read that off a graph, and it'll give you the, it just gives you the ratio of the anorsite.

Right, and anorsite is a type of feldspar, is it?

Ahh, it's a type of plagioclase.

Right, so, let's put, now we've got feldspars at the top

Uh-huh, and that <unintelligible>, plagioclase is and calcium feldspar is. And the plagioclase breaks down anorsite and olvite. And the anorsite's the calcium, calcium one, and olvite is the sodium, envever. So that it between it, you can erm, so that anorsite is 100% calcium, and olvite's 100% <something>, and then you get the <something> in between.

Right, and you said this was an end method

Uh-huh

Because it's right at the end. Right... ... ... Great, I think that's that lot clear. So if we could just erm, round them off, we've got as far as rhyolite and trachyte.

That's it.

That's the end is it, oh great, oh. Is there anything else you can think of that I haven't erm dredged up in the course of that?

No, nothing I can think of.

Right, that seems pretty comprehensive, so that's `til ... right, end of interview.