Laddered Grid 1

Right, so the first rock is dacite. So how could you tell something was dacite?

Well, er, examine the er fresh surface and the weathered surfaces first. Er, looking at grainsize, the er relationship between the grains actually in it, er

Can I just stop you there. What, the grain size, what types of grain size is it?

Coarse, medium, fine grain, oh, you want me to [actually say what dacite is?

[The grain, in dacite what would it be?

Er, hmm, medium grained.

Medium grained, right. So, from that list, can you give me other examples of medium grained rocks?

Medium grained rocks

Have a pen if you want it

Thank you. Er, the dolerite... Granodiorite as well... And we'll stay with that.

Right, erm, what alternative is there to a medium grained rock?

Well, you can have a coarse grained one or a fine grained one, those are sort of the three major ones.

Right, can you give me examples of coarse grained rocks?

Er, gabbro, granite... hmm, yeah, those two.

And any examples of fine-grained rocks?

Er, basalt... er andesite, trachyte... er, yeah, microgranite as well.

Right, erm so. What about the others, there are seven you haven't mentioned in there, are they?

Well a couple of them I haven't really got a clue about.

Right, what are they?

Like the nepheline-syenite, `cos I mean some of these are sort of a metamorphic ones where you're going to get a large grains in a fine-grained sort of matrix thing. There's femocrysts in them, and that's what we call the large grains, can't exactly classify them.

Is, is there a word for that kind of texture or?

Er, a porphyritic mixture

Can you give me the examples of the porphyritics that you...

Well, I'd plump for the er, the nepheline-syenite, oh and this Ke, number 11, Ke, (Kentallenite) I can't even say the word, I've never heard of that one, but er, yeah, I think that one'd be porphyritic.

Right. Where does ademalite fit into it?

I don't think it really does.

That's ok, erm, right, how would you go about telling the difference between dolerite and granodiorite? What is the key difference

Between the dolerite

Dolerite and granodiorite.

Granodiorite, er... probably... yeah, er, I think there's going to be some compositional difference. Er, yeah, yeah, there'll be a compositional difference and probably a, a grain-size difference betw, because dolerite is essentially a, a er textual difference because basalt, dolerite and gabbro, they're all compositionally identical, but the grain size between them, I mean the basalt is the finer grained one, and the gabbro's the coarser grained one, and dolerite's meant to be an intermediate grainsize between the two.

Right, so, to switch the question in that case, what... so dolerite, gabbro and basalt are textually different. What is the compositional similarity?

Well, they're all er, a basic igneous rock, er, generally intrusive although basalt can be an extrusive one. It all, it all basically depends on the, sort of depth of emplacement, size of the intrusive body, and the rate of cooling, `cos the gabbro is a slow cooling one, so it's coarse grained, whereas the basalt is very often an extrusive, which cools rapidly, and it's fine grained, they're all er, they've all got er plagioclase feldspar in it, er, clinopyroxene, bit of olivine, the basalt will probably have some glass in it, which is sort of like, the magma when it's thoroughly chilled it forms a glass as opposed to a, a crystal or a grain.

So, ba-basalt's got a glass, but the other's are..

Yeah, generally.

Right, erm, but, the compositional similarity of dolerite, gabbro and basalt. Erm, right, so, so the compositional similarity of dolerite, gabbro and basalt is that they're got plagio-plagioclase, clinopyroxene and olivine.

Er, yeah, those are the main ones, I mean there are all sorts of little, other, minor, bits and pieces.

Accessory er

Yeah

Type things, but they're the

But those are the main, main three.

Right, so that's the key similarity, and the key difference between them is

Erm, er yeah, and they don't have quartz in them

No, oh right.

Yeah, no quartz

Right

They're er, I think they're er undersaturated with respect to [exhaust] silica. There's no excess silica in them, that's all used up in forming other silicates and, any excess if there was would be precipitated out as quartz, but there isn't.

Right, can you give me any other examples of rocks in that universe that have no quartz?

Err...

Apart from those three, are there, are there any?

... er... peridotite, you know, and the dunite-peridotite as well.

Right, right, can I ask you what is the key difference between, on the one hand dunite-peridotite, and peridotite, taken as a couple

The key difference

And, say, basalt

Say the basalt, well, generally speaking, the basalt is an extrusive rock, it's sort of like a lava, yeah

Yeah

And these, these peridotites are er, a mantle rock, which is, you know, sort of beneath the crust, I mean they're sort of within the earth like, and they very rarely are seen actually on the surface, and they're sort of, they prob, these peridotites you've sort of extract the basalt liquid, the magma sort of is plucked out then go through various differentiation processes, and becomes the basalt or classed as a basalt.

Right, erm, imagine that all I've got is a hand-sample, a thin-section, how do I tell the difference?

Between the

The key difference, yeah, the key visual sort of clues to which it might be

As to what's a basalt, and

What's a basalt, or what's a dunite-periodotite or a peridotite.

Well, you're going to look at the grainsize in the hand-specimen. The basalt's going to be really fine grained, probably, dark grey to black matrix with white flecks, [lats] of plagioclase in it.

But, so grainsize is a key indicator, then?

Yeah

And, but, the other thing is that predominant colour of basalt is dark grey.

Yeah, I think the peridotite tends to have more olivine and clinopyroxene in, so a greener colour.

Peridotite greener.

Yeah, and it's probably more coarse grained as well.

More coarse grained. Right, now, er you've mentioned that a rock can be dark grey or it can be predominantly green in colour. What alternatives among that universe are there to those two types of colours.

Well, er, granite there's various, well, two main sorts of granite, there's a [shaft] granite, which has a, er, a pink feldspar in it. Err, there's a lot of quartz in, so it's going to be sort of a whitey-grey colour. There's a lot of er white feldspar in it, and black mica's, I mean you sort of get a, a fleckaly, er <whistles> er sort of like flecks of the different bits in it, er, you'll get muscovite in there, which is a white mica, and that'll be shiny when you catch the cleavage, means it'll reflect the light on it. Erm, and like I said, a [shaft] granite's got this er pink feldspar in it.

Is that [microline] feldspar?

I'm not sure which type of feldspar it is, but it isn't plagioclase. It has both the feldspars in it.

Right, what, what rock's among the list are dark grey?

Are dark grey? er, the basalt, dolerite tends to be dark grey also, but erm, the grain size is different. Mmm, most of these lavas, when they, when you get them in a fresh surface, not the [slaggy] cooled surface, but fresh internal one like the andesite and the rhyolite, and the trachyte, they tend to be a sort of grey-y, darkish.

Right, erm, right, what tends to be green.

Green, the periodotite tends to be greener due to the more er olivines and the pyroxenes present in them, probably a few [amphiboles] in there as well.

Pyroxene tends to make them green as well, do they?

Er, yeah, yeah, they also, the olivine in it is-isn't the main sort of green colour, but these sort of also enhance the colouring as well.

Right.

Er, yeah.

Any, any others green apart from periodotite?

No, not that I could er, safely say, no.

And you've got another set which are prodominantly white, would you say, whitish?

Yeah, the gabbro tends to be er, sort of white as well, that's got a, I mean that's more coarse grained and the way the er feldspar's cooled, crystalised in that, they tend to be larger grained, things and they are white in colour, so that tends to come out more than the grey, grey-black colour of the basalt and the dolerite.

Right, so granite and gabbro tend to be whitish.

Yeah, yeah they tend to be fairly white.

You can get the kind of [shaft] granite, you say, which tends to be the

It's got pinky bits in, yeah

Yeah, but predominantly white

Er, well I wouldn't say predominantly white in it, there's white, black and pink grains in it.

And, and gabbro tends to be similar does it, lightish, but again

It tends to be whitish with black bits in it, grain, texturally, grainsize-wise, gabbro and the granite are erm, practically the same, they're both sort of coarse grain, and then again, it's the compositional differences, which can classify the two

Can-can you tell me what the compositional differences are between them?

Well, they're both got the erm feldspars in them, but the granite's got a lot of quartz in it, you see, which is, I mean if the quartz was in the gabbro, it wouldn't be a gabbro.

Right.

`Cos the gabbro uses all its silica up to form silicates like the er, the feldspars and the er pyroxenes and the er olivines. And er, if it had any spare it would precipitate those as a er, as quartz. But I mean a granite doesn't usually have a olivine or pyroxene in it.

So gabbro's got olivine.

Er, yeah.

And pyroxene.

Yeah

And granite has lots of quartz.

Yeah, yeah, basically yeah.

Anything else that you'd...

Mmm... it's got the muscovite in it, the white mica, I don't think the gabbro often has the white mica in it, if at all, I mean there's always the exception which proves the rule, but...

Right, erm, just a second. Right, can we just look for a second at, can you give me a list of rocks off that list which have olivine in?

Erm... gabbro, the dunite-peridotite, the peridotite, dolerite and the basalt. Err, it's olivine is it?

Olivine, yeah.

Hmm... the nepheline-syenite may have it in, I'm not right sure on it, but yeah.

I'll put a question mark.

Yeah, I'd leave it at that.

I think that's pretty off-the-wall example isn't it, nepheline-syenite?

Yeah, it's, I think it's a metamorphic one.

Right, erm, can you give me a similar list of rocks that contain pyroxene?

Er, yeah, well the gabbro, dolerite, basalt thing `cos they are basically compositionally the same, textually different. And the peridotite again, the dunite and the, dunite-peridotite and the peridotite.

Dunite-peridotite. Any others?

Er, I'm not sure whether it's in the dacite.

I'll put another question mark in it

Yeah, I know, those are the ones that I'm sure about.

Right, right, that's good enough. Right, can you give me a similar list of rocks with a high quartz content?

Yeah, erm, the granite, erm, the microgranite, er... the, the andesite. The andesite is a sort of like the acidic equivalent of a basalt, I mean it's a lava, that should basically be compositionally the same as a granite, but a finer grain size `cos it's extruded out as a lava. So that should have quartz in it. Err, yeah, leave it at that.

Right, erm, what alternatives are there to having high quartz content?

Well, the high quartz content are saturated with quar-well, over saturated with respect to quartz, and they precipitate it out as quartz. Saturated ones tend to have a very small amount of quartz because they've just got enough silica to form all the silicates, other silicate minerals in it, and there's not a lot left over to precipitate out as quartz, whereas the under-saturated ones haven't got enough quartz, well silica, present to er, form all the silicate minerals that er form the rock, and so they have er, a sort of deficiances within the latice, so there is no silica left to form quartz, precipitate it out as quartz.

Amongst this sort of universe, would there be dimensions which said low quartz and no quartz?

Er, you mean rocks on here which are representative of ones which saturated those...

Yeah, would you, would you call any, would you say there're any which have a low quartz content?

Yeah, the er, the basic ones, the gabbro, the basalt, dolerite, er, I think the trachyte as well is also a, er, basic lava, which er shouldn't have any quartz in it at all.

The-this should have low to none then, is that?

Yeah, er, yeah, they shouldn't have any in really at all.

Right, ok. Erm, right, what is, just a second.... Right, can you give me a list of rocks that contain plagioclase feldspar?

Er, yeah, basalt, dolerite, gabbro. Er, granite, microgranite, er, yeah the granodiorite as well. The trachyte and the rhyolite should also, I mean it's a fairly common mineral is plagioclase feldspar.

Right, can you give me a similar list of rocks that contain clinopyroxene?

Er, CPX, yeah, er, the basalt, the dolerite and the gabbros, I think it may come up in the dacite... er... yeah, the trachyte, and it may or may not be present in this nepheline-syenite, that's rather a funny rock.

And er, let's see... right, just trying to settle on a couple and ask you a couple more key differences. Erm... right... dolerite, gabbro and basalt we can do. What is the key difference, would you say, between peridotite and dunite-peridotite?

Uh-huh. Er, well, I think they'll basically be the same but the dunite-periodotite will probably have some er, some compositional difference, but I, I couldn't, couldn't pin it down.

Right.

Erm, this-this dunite part, I'm not too sure on that, but peridotite's going to be the basic sort of standard thing and the dunite's probably going to be a further classification of a peridotite.

Right.

And a peridotite tends to be a, well bucket-term isn't the right word, but er, it's a broad, sort of a peridotite, and a dunite-peridotite is going to be a, like a sub-class of the peridotite.

Right, so-so to use the term peridotite is almost a super-class in a sense.

Yeah, yeah.

Right, erm, can you give me the key difference between erm, dacite and erm dolerite?

Er, it's going to be a compositional difference, I couldn't give you the exact composition of the dacite, but er, I think they're of, they're of similar grainsizes. So erm, I said before the, what the dolerite compositionally is, and the term dolerite is basically a, a grainsize, of a standard grainsize, so er, I assume that there will be a, er, compositional difference between the two.

Right

But the grain size is fairly similar.

Right, erm, let's see what else is there... dolerite, gabbro and basalt... ... Is there a, is there, what sort of alternative minerals could a rock contain other than plagioclase feldspar?

What other minerals could it contain apart from that?

Yeah, that would, that would be sort of useful in sorting out that bunch.

Er, well I mean the quartz, whether it's got quartz or hasn't got quartz or the percentage of quartz present will define whether it's an acidic rock or a basic rock, basic not having any quartz in it at all, and then er if there's a low amount, that's going to be an intermediate rock, I mean, quartz is a fairly, you know you can bash it down into the three, the three groups.

Which, which are the intermediate?

Er, on this list?

Yeah

Er, <laughs> you got me stumped on that, but I reckon, I'd plump for the dacite.

Dacite, anything else?

Er.

You've got high quartz are granite, microgranite, and andesite, and no quartz gabbro, basalt, dolerite and trachyte, intermediate dacite.

Hmm, er, one of these, one of these lavas on here's going to be an intermediate, I think, yeah, rhyolite, I think that's an intermediate lava. And er, probably the granodiorite, I think that could be an intermediate rock as well.

Right, can you give me a key difference between rhyolite and dacite?

Between rhyolite and dacite. Well, if they're both, both intermediates, then it's going to be a, a grainsize again, `cos the rhyolite is a lava, which like, like the basalt is, extruded out, and so it's going to have a fine grainsize and a, sort of lava characteristics, really fine and lots of glass present usually, due to its rapid cooling.

And lots of glass?

Yeah.

Right, dacite on the other hand is a, is a medium grain

I'd say it was a medium grain, sort of.

Yeah, so, can you give me, in that case, a list of, erm, rocks with lots of glass content.

Yeah, trachyte, the rhyolite, the andesite and the basalt. I think they're all lavas <chuckles> Due to the rapid cooling.

Basalt, so, you'd say that if a rock's a rapid lava erm

A rapidly-cooling

Rapidly-cooling, it erm, it tends to contain glass.

It tends to contain glass, `cos

That's what creates the glass is it?

Yeah, `cos the crystals don't have time to crystalise out and grow, it's just rapidly cooled, quenched, sort of chilled, and er, this forms a, a glass.

Can you give me the difference between andesite and basalt?

Between andesite and basalt.

The key diff-the key difference.

Erm, the basalt isn't going to have any quartz in it, whereas the andesite er will. I think the andesite's a, a acidic lava, so it should have er, should be oversaturated with respect to quartz.

Erm

Whereas the basalt isn't, is undersaturated.

Trachyte has no quartz.

Yeah, I think that's a basic.

Rhyolite is intermediate quartz

I think the rhyolite's an intermediate lava.

Right, right what I'll ask you now is, from the looks of that universe of rocks there, are there any key dimensions upon which you would discriminate between them that we haven't so far covered?

Er, yeah, er, as far as I can tell from this, I mean there's a couple of rocks on here which I haven't heard of, but er, they all appear to be igneous or metamorphic, I would say looking at this list there is no sedimentary rock on there.

Right, right, which are the igneous er

Well, the andesite, the basalt, the dacite, dolerite, the dunite-peridotite, gabbro, granodiorite, granite, the microgranite, the peridotite, the rhyolite and the trachyte.

OK, and which are the metamorphic?

The nepheline-syenite, I'd say the Kentallenite, and the er, the ade-ademelite, number 1.

Right, and those are the ones that you're not...

Well, I know the nepheline-syenite's metamorphic and the, the other two are, I'm sure that that's a metamorphic one, the name seems to have cropped up before, and er, that seems to be sort of metamorphically sound.

Apparantly it's from Scotland, that's all I know.

<laughs>

So, are there any more key dimensions then? That you'd use to distinguish between them?

Yeah, there's also the, the sort of the, depth of emplacement, like I was saying the basalt, the andesite, the rhyolite and the trachyte are all lavas and they're all sort of extruded whereas the things like the dolerite and er, I think the dacite, and the er... maybe the microgranite, they're all sort of a, shallowly emplaced, you know, not too far away from the surface, from the crust, they're all fairly small-bodied intrusions, and so they cool at a quicker rate, that's a slower rate than the lavas, but at a er, quicker rate than the larger igneous bodies which would form the granite, the gabbros.

What's they're emplacement?

They're, they tend to be fairly deep down, er, a large-bodied, very large-bodied sort of things.

Erm, how do you tell the difference between these three things visually?

Well, unless they're exposed at the, <something> surface, you, you don't usually see er, the granite, well granite usually forms in a [pluton] which is like a really big thing, like the Cornwell peninsula, you know er Dartmoor, erm Exmoor, Saint [Orsmoor] and that, that is all one big intrusion, all the way down to Lands End and erm, the the isles off the bottom end of it, that is all one big chunk of granite.

Say somebody chipped a bit off and, thing is, you know, you don't actually see where the rocks are, and they walk in the room now and sling a bit on the table, er, say an extruded rock, how do you tell it's an extruded rock?

An extruded rock, well, it should, it should be fine grained, I mean if you look at the grainsize and it's coarse grained and they're saying it's erm, you know, an igneous extrusive and it's coarse grained, then you can be sort of thinking about well, what sort of environment did it cool under, `cos to give a coarse-grained it's meant to be slow-cooling, whereas if it's extruded it's going to be cooled rapidly.

So that the emplacement of the rock should reflect the grainsize?

Well, generally the larger bodies have a deeper emplacement because it's more difficult to punch a big body right the way up, and they er, they cool slowly and they tend to be nearer to the magma chamber, or they tend to be a feeder chamber for these er, for these other lavas which come off, like you'll get a big, big stock, or a chamber of it, and you'll get little pockets going off, and these'll be quicker cooling than the main bit, so although all the compositionally similar, the grainsize is going to be different due to the cooling rate.

Tend to be larger-grained do they then?

Smaller grained the quicker the cooling... yeah, really slow cooling, the grain's got a long time form and crystalise out, so you get fairly big substantial grains.

Are there any other, sort of major, things relating to say the content of the rocks that you would use to discriminate from among that bunch?

Er... well the, the peridotites tend to have, have the olivines and the pyroxenes in it and erm, they're visually different to the granite, they're fairly er obvious is the difference.

And this, this is, this is, this is the thing that tends to make it green isn't it.

Yeah, the olivines and the pyroxenes.

Right, so would you say that, the main sort of content indicators you're looking at are olivine, pyroxene, quartz content, clinopyroxene and plagioclase feldspar.

Yeah.

Are there any others?

Er, there's various accessory minerals and alteration products which you may see `cos it's very rare that you actually get a fresh rock, I mean it's usually been exposed at the surface, where it must have been to have been found, and then it'll it's also been weathered, and this weathering, `cos the rock will try to remain in equilibrium with its surroundings and so er, usually the weathering process will break down the minerals in it, and you'll get alteration products formed along it, and er, it's kind of a micro-retrogressive metamorphosism, whereas the, where the minerals start to break down and decompose and weather away, `cos the er, the olivines and the pyroxenes have got a lot of er iron and manganese in them, and er, you'll get a, iron streaking through them and iron staining in it and so on.

So that they, the iron would be something that would tend to go with olivine and pyroxene

Yeah, yeah, I mean, an olivine is like a, an iron-magnesium silicate and when it starts to weather, the silicate breaks down, and you get the iron, iron ions, you know iron 3 plus, sort of forming white rust, forming an oxide

Oh right

And so on

So are you, are you likely to find that in any of the olivine-bearing rocks rather than the other ones, of the olivine-bearing rocks.

Well, it, it, it depends on how the olivine breaks down or how the, how well put together the rock was, how, interlocking the grains are, if it's easily weathered then it's going to be broken down really quickly.

So, which ones are easily weathered?

Er, it depends what they've been through.

It, it's not a particular type of rock that's going to

Oh no no no, I mean

No, it's just the history of that

Yeah, it's just the history of it, it depends on the environment and the setting and what's happened to it since er, it's been formed.

Right, are there any other major, visual clues on the hand sample or a thin sample that would...

Well, you, you'd look at the colour, the grainsize, the er, proportions of minerals present in it, use er, these sort of visual characteristics to try to get some idea of what sort of rock it is. Surface texture as well, because the basalt and the rhyolite and the trachyte and the lavas, they're going to er be vesicular in nature, where it cools and degasses, and the gasses bubble up through the lava and form a, airbubbles in it, well gas bubbles which form holes and will become filled in, if there is any percolating, percolating <something>

How do you spell that, vesicular?

v-e-s-i-c-u-l-a-r, vesicular, and these are quite often filled in with er, subsequent sort of fluid and so on and get precipitates in them, and form a [methales].

Now, can you give me a list of rocks on there with a vesicular texture?

Well, basalts can have vesicular textures, basically all the lavas, the basalt, the andesite, the rhyolite and the trachyte.

Any others?

Er, no, `cos for the, for the er sort of vesicular texture to develop, I mean, they've got to be extruded for it to degas properly, and er as the gas bubbles up through the magma, the lava, then sometimes it's trapped in pockets and other times it escapes on the surface and it sort of leaves a, a blister as the bubble bursts. And you get like a little crater in it, not crater like a crater, little iddy-biddy indentation.

Right, any other things you'd like to mention that you'd use to descriminate between them, or are you thinking of those are the main.

Yeah, these er metamorphic ones may have a sort of a foliation developed in them, like a flow-patterning or something, where er, you may have a phenocryst, a larger grain, and the sort of matrix, the fabric of the rock, has flowed around this sort of thing as its been stretched.

Right, and, just a second... in that case, you talked about nepheline, kentallenite and ademelite, that there you'd get the slow bending

Well, it depends on the degree of metamophosism again, but these are things that you'd look at, if you saw that you would be stumped, think, well, you know, `it's probably metamorphic'

So these things that we're talking about here tend to be cues rather than er... they tend to be things that push you in a certain way of thinking rather than confirm.

Yeah, I mean, it's fairly sort of er, I mean nothing's like it's said in the textbook, you know, matrix or, there's always the exception to the rule like, and you just look at these things and you sort of say `this would suggest it if this', but I mean it doesn't say because face present it is definitely.

So what you've got is a continuum rather than discrete classes.

Yeah, yeah, you tend to end up going back to the books and so on, looking at other examples and so on, but it's very difficult to sort of get absolute proof, you just sort of tend to say `this is probably this rock formed under this condition'.

Right, so do you think we've covered the sort of key dimensions across those <unintelligible>

Yeah, well, by what I reckon, I can put forward, yeah.