Self Report 1

To start off with it's obviously a fairly coarse-grained rock ... and you've got some nice big orthoclase crystals in here - this is actually SHAP GRANITE - I know it just because everybody's seen SHAP GRANITE - or it's a very strong possibility that it's SHAP GRANITE ... it's a typical teaching specimen - as I say the obvious things are these very big orthoclase crystals pink colouration and you can certainly see some cleavage in some of them - you can certainly make out there are feldspar cleavages in there - it's a coarse-grained rock anyway, you can see the crystals nice and coarsely - these large porphyritic crystals - you can see, in the ground mass, you can see quartz - get some light on it (holds up to window) quartz, which is this fairly clear mineral you can actually look into it and see through it as opposed to calcite or feldspars where it's more cloudy - you can't actually see any good crystal faces on these cut sections - small flakes of biotite, black micacious looking - small plates, you can certainly see some on this specimen even without a hand lens.

How can you tell that that's present?

If you look you can see small plates - there - you can just see it, and Mica's always a platy cleavage - they come off in small plates - it's black - biotite's not only black - the way it shines and the way the small plates make up the main - the way the small flakes flake off - the white feldspar - you can't actually see that much cleavage - in fact you can hardly see any in some of this to be able to identify it properly you'd have to look under the microscope to see if it really is feldspar - almost certainly it's plagiclase but - you've also got a fair bit of, well I presume it's Pyrite, in here - it's the small shiny mineral - I expect it's going to be cubic if we ...

Can you tell me what you're doing?

Well with a hand lens - obviously you get a much greater detail - you can see it (shows it to interviewer) the colouration of this shiny mineral - it looks either like pyrite or charcoal pyrite - they're very common minerals, you find them in all sorts, especially in these kind of rocks - and we should be able to see small cubes with a bit of luck ... yes, you can make out 3 nice faces on some of these small crystals - cubic crystals ... yes, there doesn't look a lot more in there - as I say if we look under the microscope to confirm all those things which we actually saw ...

Is that the order in which you generally do it?

Yes, especially in a case like this where you can virtually make out all the minerals anyway - you've got a very good chance - obviously, on a finer grained thing you can hardly see the crystals - you're going to say it's a fine-grained whatever and look at it under here and hopefully you'll get a bit more of an idea of what's going on ... under the microscope you can see four very distinct minerals under here - nice brown biotite - typical white - how would you describe it ..........

What is it that marks those out?

It's really experience more than anything - it's very obvious biotite - good cleavage, it's got a mottled extinction colour if you look down here - it's got a sort of heavy surface to it - it's the one in the middle, elongated with a cleavage in one direction - a lot of other ones will have cleavage as well, micas in general - it just is a mica. You've also got small inclusions in there of ... probably appetite by the look of that and there's some green Chlorides associated with this biotite as well - which you can tell because it's got blue biofringance in there - you have got a few opaques which either are just black under this - they have got quite good crystals - square and rectangular shapes, probably a bit of magnetite or titano-magnetite - you can't actually tell, it's just black under here - you'd have to get a poly-section to find that out - the alchaline feldspars, these huge great things, are quite altered actually in some of this - it's quite good to often take out the slide and look at the thin section against the window or whatever - you can quite often tell quite a lot more - you tend to lose colours under here that you see just by holding them up to the light - you certainly do see more and you get a good idea of what you can actually see - you can certainly make out 4 different minerals.

Is colour very crucial then?

Colour and biofringance i.e. the colour you get when you cross the polars is quite crucial - I mean quartz which yiu get quite a lot in here only gives you low biofringance colouring - which are greys and whites - one of the main differences in this sort of thing in just plain polarized light - the quartz is very clear - it's got virtually no gleamages, occasionally you just see a trace - whereas your alchali feldspars under here are very cloudy - feldspars alter a lot more readily than quartz do - so if you look under there you've got nice clear patches nice clear grains - not particularly good chrystal shapes - then you've got a cloudy mineral - a lot of which is plagiclase.

So when you look at a slide sample under the microscope do you just go for the biggest magnification you can get?

No - the ideal is to start off with the lowest one like this - and just get the general overview of what you've got - i.e. you've got quartz, you've got biotite - you've got this alchali feldspar - very big alchali feldspars.

Then why would you change up - under what circumstances do you change to a higher magnification?

In this one there are some far smaller grains - you've got a bit of spheen there obviously the basic rock is made of these 4 basic minerals really - plagiclase, blah, blah, blah - right at the end you're going to have accessory minerals just small little thinge where all the titanium's going to go - all the incompatible evidence really which doesn't fit into the nice lattice work of these simple SI308 whatever alchali-feldspars etcetera - you've got a bit here - you've got some spheen here - and this has got high relief, stands out, looks at you - it's got a brown colouration - slightly pleochoric this one.

What does the presence of that tell you?

Spheen's got quite a lot of titanium in there - it's just one of the accessory minerals really that you're going to get if you've got a big mass of magma and virtually everything's gone into your Quartz which is just silica basically and your feldspars which are basically aluminium, bit of sodium, bit of calcium depending on where you are, and the same with your plag[iclase] and your biotite - biotite's got a bit more mixture - hydrated sodiums and what have you but you've got a lot of spheen in here and some nice - another diagnostic feature of spheen is the little diamond shapes you've got here and you've got these very nice little ones here - and the high relief shows through a lot - probably got some epidotes, appetites I mean, but I'm not sure - it looks as though you've got some associated with the biotite - you've certainly got some very small crystals, very clear - and, again they seem to be high relief, and you can't just by putting the analyser in - they're dark - they're isotropic basically which goes against them being appetites, but I think that's what they are - again, a nice 8 sided crystal shape which is typical of appetites - as I say, if you've seen a few it's pretty obvious - I'm certain that's what these are under here - just fine-grained little things - some are associated with the biotite and some are just seemingly in the ground mass with the feldspar - to actually classify this you'd want to go back really to a bit of hand specimen and a bit of low powered microscopy - and see really the relationship of the feldspars and the quartz - there's essential quartz in here - there's a lot of quartz - 20% I don't know ... a fair amount - a lot of ......

What makes you think quartz is there?

There's quartz under here - you can see quartz in there and you can see it just in here - no cleavage - very low relief - or relatively low relief - low biofringance and I can test it just by doing - using a Bertram lens - high power - get a nice uni-axle cross on there ...... the plagiclase feldspar is also very very obvious, although it is altered here, but - it is invariably in long lath shapes and in plain polarized light that's usually all you see and a lot of alteration - when you put the analyzer in - you get this nice zebra [*** unintelligable ***] twinning and it's always plagiclase feldspar - there are very few other things - there are a few other things but I mean in these [*******] sort of rocks that's exactly what it is - plagiclase feldspar - to find the actual composition of that you'd have to do, which again not so much in this one but in basalts and things when you come to classify them, you have to find the composition which you can do by looking at - doing a thing called the mickle-levy test - and you try to find a crystal in the right orientation align it along with your cross hairs - down the microscope - and twist it round - and say, if these are the lines on your feldspar - you get one that's all the same colour - you twist it round that way and those two will go black and that one won't - and you write that down - then you go back the other way and that one will go black and those two won't and the idea is to do at least ten from each thin section which gets a bit monotonous and quite often you haven't got ten in the right orientation - but then you can look on a chart in the book there - say if you've got an angle of 28 degrees or something - you take the highest one - you get anything up to 28 degrees - and look along them here - 28 degrees you get a content of [******************************************************************************* andesine and a rhiodite

******************************************************************************** borderline - for your basic rocks you're going to have just basic calcium and sodium - your basic basalts getting on for about well ... that's a bit of a mixture you can get all sorts it starts of basically calcic rich and as you get more towards the silica end of the rock scale you're going to get more sodic things coming out and you can classify basalts and things just by that - granite and gabros will have ********* agradorite ********* ......

This levy test - is it a thing you do to confirm your other inspections?

No not really - all you can do down there is to prove that it's plagiclase feldspar - then - if you want - a lot of the classification is done on whether you've got andesine or agradorite or whatever in it - and you've got to do that on 12 or 15 specimens

So a lot of your classification work is chemical?

Yes well in a way there's a purely chemical classification of rocks - you just get your rock and crush it, look at the silica content and your basalts will be from 45-52% silica, your ultrabasics sort of 35-42 or whatever, your middle of the road intermediate type things 52-63, above 63-68% silica rich things, granites will be above 68% something like that - you can do it purely on that - you can also do it purely on your alkalis and things like that sub-alkaline and things like that - it's a bit of a mixture of both really - you can also classify them by taking that chemical analysis and working out a hypothetical rock - if you had that chemical set up and you just allowed it to cool under equilibrium normally if you like - you would get these rock minerals coming out - which you just calculate *** if I produce plagiclase right, and there's more silica left or olivine rather - if there's more silica left you start turning that olivine you've already created back into pyroxine and if there's more and more you carry on and carry on - and then you can, again, relate that to what you see under here because this won't be under equilibrium things won't have changed as well as they should have done so you might have quartz under here were you shouldn't have any true quartz left so it's a bit of a mixture of both.

As I say, it's called SHAP GRANITE - this isn't a true granite - now let me get this right - it's got essential quartz in which makes a a granodiorite type rock anyway - lithero-granodiorite, granite type thing - but this has got - I'm getting a bit confused - I'm not quite sure whether this is the right way around or not - as a matter of fact I'm sure - yes, adamellite should have roughly equal plag[iclase] and orthoclase - hold on ...... granodiorites got essential quartz .... so if I remember correctly, adamellite - just by looking at this very quickly - you've got to find the percentage of your alkali feldspar to your plagiclase feldspar - and mainly, partly because I know this and it is roughly half - is roughly 50% alkali and is roughly 50% plagiclase you'd say this was ADAMELLITE - but to actually do that properly you'd have to sit down and sort of roughly work out how many crystals of each you've got and give a percentage to each - and you can do that very very quickly, or else you can do it point counting sort of - you do sort of 500 - well more than that - and you can get a little sort of point counter which just moves the slide along little bits at a time for you and you just write down all your - or you press a button if it's quartz under the cross-hairs and then alkali feldspar and that gives you a true percentage if you like - then you just stick that into your oh yes it's got 8% plagiclase or whatever 4%, 8% of your total feldspar is plagiclase - and you can then say yes that's an adamellite or no that's a granodiorite as opposed to an adamellite

What do you think this one is then?

Well mainly because I know - and it's classified as an ADAMELLITE which means it should have roughly half and half - looking under this - partly the fact that it's so coarse-grained doesn't help - you sort of get half a grain under each - under the microscope - I mean there's a lot of plagiclase in here.

And that's a very coarse-grained rock is it?

Yes, and these very big orthoclase crystals sort of centimetre and a half or so probably a bit bigger than that.

What is it that makes a rock coarse grained?

Well on this one it'S nice and obvious - you've got these nice big orange orthoclase crystals and you can see - that's all one crystal - you can see in there - that's a mineral constituent of that rock - you've also got little black bits - all little biotite flakes - if you get the true light properly you can look in there and see two different things - you can see a glassy mineral - you can see a white mineral - as I say - just looking at it you could say it was quartz quite easily - the other thing - you can't really see any cleavages or anything like that in there - just from experience you'd have a good guess that that was going to be felds- a feldspar, and seeing these big orthoclases you'd think that it probably would be plagiclase - but you could be wrong you can get white orthoclase and things - white alkali feldspars but again then you look under there and say ah-yes - I can see the big orthoclase crystals - I can see the biotite - yes I can see the quartz - that other thing under there that white thing under there must be a plagiclase and as I say, on the surface you've got this pyrite - charcoal pyrite stuff.

So you are saying that it's adamellite?

Yes - mainly because I've seen it so many times before and I know it is - as I say, to actually be able to - I'd have to check if the feldspar is 50% because I'm not quite sure about that - you've just got to sit there and give it a percentage of things - it'd take a while longer than you want.

How would you describe the colour?

Well it's getting on towards leucocratic - but really leucocratic is something like that approaching whit......

What if a rock has two colours?

I think really it's not a true leucocratic granite - leucocratic granites are white - really the best bet is just to describe it as you see it - you know - just say the colours that are there - you can have things called pink granites, lot of orthocalse - especially in the field you call it what you like so long as you can identify that one there at location a is the same as at location b and you're calling them two things the same - it doesn't matter what you call them in a way .

So these colour classifications are fairly plastic are they?

Oh certainly - you would never - it gives you a good indication - I guess that one tends to be melanocratic [points to a sample] but that's not one of the things I'd call it - I'd just call it a fine-grained basalt - an olivine one - that's the sort of thing you look at - I'd never dream of calling it a melanocratic basalt. Colour is a good indicator of quartz content - leucocratic things have a lot of quartz - melanocratic has olivines, pyroxines. In the middle you get leucocratic with melanocratic crystals.

The rock is adamellite (shap fell) That's the place it comes from. It's a typical teaching rock.

(Tape appears to jump for a second or two)

Look at the feldspar concentrations , ratios rather, and see if it's got essential quartz in or not, the dividing line, although it's , according to some people it's five to twenty percent quartz or something, you go to America and it's slightly different, it goes up to eight percent or something, so there's variation. One of the problems about classific_ ation is the fact that a lot of them are old names, and a lot of rocks that are classified as andesites should be classified as dacites, or the other way round, for example.

So you're very dependent on localities, really.

It shouldn't be, but it is.

It's a lot less clear cut than you imagine.

Oh, yes it is. You can do it purely on the minerals, or purely on chemicals.You could have a name that describes the rock, but you can't apply it because..

Because it's been in the text books so long. What are you looking for at the moment?

It looks like a fairly fine grained...just by the colour it should be fairly silicious, because it's fairly leucocratic, as opposed to the basalts. You've got different coloured crystals in here, some light coloured ones, fine grained ground mass with 1 to 2 millimetre sized crystals, some dark greenish, black greenish as opposed to black, under the hand lense it's a bit hard to see whether they're ... the greenish colour tends more towards a hornblende rather than a pyroxene, but that's purely on the colour and you can't see if ... if you can see cleavage, pyroxene cleavage is at 90 degrees, but in hornblendes it's 120 degrees. You should be able to see it under a microscope fairly easily, but in a hand specimen, with large crystals like that you've got a fair chance of seeing it, but in some basalts you can see it as well, you can see a slightly greener thing, whereas pyroxenes would be black .

Looking at the specimen under the hand lense, you're getting a better impression of colour than you would under the ..?

Not for these things. The cleavage should be a give away whether it's hornblende or pyroxene. If it's pyroxene it'll be a darker brown colouration as opposed to a green.

Does the fact that pyroxene is present rule out a number of rocks?

It points you towards things. It points you more towards a basic rather than a silicious rock. You've got to be careful because you can have a (unclear) or rhyolite or something, which can have pyroxene in, which is basically pure quartz, pure silica, it's got pyroxene crystals. It's rare, but you do get them. You've also got what are probably feldspar crystals in here. They're a bit small to truly identify. There's certainly a red colouration to some of them. There's a small xenolith or something in here as well. From the colouration, and if that is hornblende, the rock is going to be ... I'm not sure. If it's plagioclase and hornblende, then in a fine, silicious feldspar-thick ..

What makes you say it's silicious?

It's just the colour, and fine grainedness of it. It's light in colour, it's a sort of light greenish colour. You've got these almost certainly feldspars in there, and .... there possibly is quartz in the bottom... it's hard to say. It's possibly an andesitic, dacitic type of rock... is there a slide?

(Apparently starts looking at thin section)

Well, it's definitely feldspar.

How can you tell it's feldspar?

Again, you've got these typical twins (? not clear on tape). It's set in a fine grained ground mass of ... mainly feldspar by the look of it.

As you're looking through there, are you doing a ..

Yes. Trying to see what the mineralogy is.

Estimating percentages?

Well, start off with trying to see what the minerals are. The obvious thing is the plagioclase, nice big zone (?) plagioclase crystals. There's another mineral in here, which looks more like a pyroxene than a... it's got - some of these have got a good single straight cleavage, which doesn't tell you either way if it's a pyroxene or (inaudible). It's got a rough colouration, which a lot of pyroxenes have, which is a slight brownish tinge, slight greenish tinge. Slightly (inaudible) when you swing it around. A lot of small opaque minerals associated with these clumps of...

What are you doing at the moment?

I'm trying to work out whether this is pyroxene or hornblende. It's got a porphyritic texture anyway because there's very large crystals, well, relatively large, which are the feldspar crystals, the white ones, and these blacky_green things, set in a very fine, well, relatively fine grained ground mass.

That's what goes to make up a porphyritic texture?

Yes. Porphyritic basically means large crystals set in a finer grained ground mass. The ground mass is made up of fine laths of plagioclase. A number of opaque small crystals of ...

And opaque just has its standard meaning, does it?

Yes, sometimes you see a mineral (which is opaque) but until you do a polished section and stick it under a reflected light microscope or probe it or something there's no way you can tell... What I'm trying to look for under here is to see if I can find any good cleavage, either a 120 or 90 degree cleavage which would tell whether it's a pyroxene or an amphibole. The colouration and some of the crystal shapes seems to point towards an amphibole, but then again...looking under here, at the thin section, it looks very much like a porphyritic basalt, but that goes totally against what you see in the hand specimen.... You've got clumps of feldspar crsytals and some of these .. I'll call them amphiboles for the sake of it .. and around some of them there seems to be some quartz. ... The fact that we don't seem to have any 120 degree cleavage in the whole slide - there's a couple of partings that look possible - no, I'm sure that's amphibole. The reason why I think it's amphibole is partly the colour, partly the shape of the crystals, and the fact that there's a couple down here which don't really have true cleavage, but they have a parting at 120. They've got sort of fractures as opposed to true cleavage.

And what would it tell you if you'd got that?

A lot of plagioclase and a lot of amphibole.. set in a fine plag.. ground mass .. slightly porphyritic.. all this plag has got to be dioritic .. If this was pyroxene, which I don't think it is, certainly looking at this thin section, you'd think it was a porphyritic basalt. I'm sure this is hornblende. Since it is hornblende, it is dioritic, that sort of thing.. whatever you call it, it's going to be a porphyritic micro- something. It's fine grained apart from these porphyrites, or whatever you want to call them. It's got a lot of plagioclase.. I'd go for a porphyritic microdiorite or something like that.

It's an andesite.

An andesite. Bloody hell.

Does that often happen ? Do you get conflicting evidence from a hand sample and thin section?

You do sometimes. I wouldn't have thought I'd have got it that wrong.

The first thing you said was actualy andesite..

....Must be pyroxene.

(Next specimen is handed over)

This is obviously, just in hand specimen, an olivine basalt. It's a dark coloured rock, basically fine grained rock, I very much doubt if you can make out any of the .... you can't see anything in the ground mass as such, it's just very fine grained, you can make out very small crystals but you can't actually identify them as much, although they're bound to be plagioclase and pyroxene and things. You have got these porphyritic green coloured minerals, sort of olive green, it's the colouration more than anything, it's almost bound to be olivine, though you might have some oyroxenes as well.

So what are you looking for now?

I'm looking to see if there is actually two different minerals set in this ground mass. There''s definitely these large olivines, there's possibly a blacker one, which almost certainly will be pyroxene.

You can see that in a hand lense?

specially in that kind of rock, you see a dark crystal, it's almost bound to be pyroxene, that's looking at a rock and saying, right, that's a basalt,it's going to have pyroxene, you're going to have plagioclase in there, without thinking about it really. You can certainly see olivines in there, therte's possibly pyroxenes, it does look a bit possibly micacious,

How do you know if it's micacious?

These little black platelets that you get,

Is mica always black bits?

No, not by a long way. You do have white micas, which you get in these sort of rocks, certainly in the granites, in basalts and gabbros and things you usually have black micas - well, it's black in hand specimen although it's brown under there (the microscope?)

It's the lustre of it, is it?

It's the platiness and the shiny lustre. If they're big they flake off easily, you can put your thumbnail underneath and flake them off.

So the colour's just dependent on where it is?

Well, it's dependent on the mineralogy of it, well the chemistry as opposed to the mineralogy, they're all basically micas, but some have more sodiums and potassiums in, and some have (something else). Anyway, let's have a quick look under here and see what we've got. (Using microscope?)

Now we've got nice big crystals of olivine, moderately high relief well, moderate to high relief, basically colourless. They have no real cleavage pattern but they do have fracture patterns, and along the fracture patterns there's green alterations on this one...that'll be, as a general name it's called bowlingite (???) - bowlingsite ? I'm not quite sure. There's actually two or three fairly fine grained clays and things, it's typical, olivines often break down to this green stuff or they have a red stuff called ivingsite (???) which goes through them, and it certainly helps to identify them, it's one of the things that they do break down to. It"s got high ???inaudible??? colours ... that whole rock has got a fair bit of alteration, some of which is chloride, by the look of it, well, it's got some alteration...You've also got ... a lighter brown mineral which seems to have very good cleavage, and I'm sure it's a pyroxene, a kind of pyroxene,

It's the colouration and the cleavage ?

The colouration sets it apart. The olivine, that's basically clear, it's obvious that there's at least two different minerals, porphyritic minerals, so it's almost certain we did see something different in there. It's got very nice single cleavage, single straight running cleavage, and the rock, the mineral goes into extinction when these are inclined, as opposed to when the cross hairs follow that, so it's an inclined, a clino- pyroxene, I'm sure it's a pyroxene. It's set in a very fine, well, a fine grained ground mass, which has a lot of fine opaques, a lot of long little laths of plagioclase, which again are obvious with the zebra striping and the long thin crystals, and you've also got finer grained crystals of olivine at least in there. Fairly rounded, they're small ones... and again this brown coloured mineral which I'm sure is pyroxene... you've got a fair bit of flow alignment, the thin laths in the ground mass are all aligned in one direction, well, they've got a strong alignment in one direction; around these big crystals they go round and round where the thing has flowed across, so it's possibly a lava but not necessarily, it could be a sill or something.

And the slide sample, that's confirming what you?

Yes, it's got nice olivines, it's got pyroxenes which we thought we might see the ... what I was thinking were micas were almost certainly pyroxenes because there's no micas in here.

And what nakes you think it's pyroxene, how do you know there's no mica?

There's no mica, certainly no large micas, the colour, micas are ??pleocrate?? when you spin them round, they've got that pebbledy textured surface to them, and they've got good straight extinction where these have got inclined extinction, oh, possibly up to about 8 degrees but they're fairly straight normally, these are definitely inclined. the micas again would have a strong body colour, they'd be that browny colour, they would be pleocrate whereas these, although they've got a definite tinge of brownness compared to the virtually colourless olivines, they've still got that strong brown colouration.

Well, you could call this, certainly an olivine basalt anyway. I see no objection to that. There's only one pyroxene. Can we go for an olivine basalt?

It is a basalt. You can cross the olivine out.

In hand specimen you'd call it an olivine basalt, but under there you've got, the pyroxenes are as common as the olivines are, by the look of it.

Have you got time to do another?

Yes, carry on. Right. Well, again this is a coarse grained rock, you can see the crystals very nicely in there, you've got nice biotite flakes, platey, that quite often shows, that sort of silvery lustre on the surface of it, as opposed to being a true black shininess. But in actual fact you've actually got two in here, because that looks, you have got the black shiny mica and you've also got a shiny, what looks like a platey mica, which is possibly muscovite...it does look like there's two. You've got a very black, biotite mica, I'm not sure, it does look like you've got two micas in there, you've got a lot of feldspar, the white crystals set in there, and again like the Shap thing you've got nice quartz as well, nice clear colourless quartz in there. Fairly coarse, and you could call that fairly leococraticy if you like, leucocratic with black spots if you like. And certainly by the fact that it's got a lot of essential quartz in there, a lot of feldspar, looks like you've just got mica as opposed to amphibole,

So when you use the description leucocratic, you're going on the colour base?

It's purely just on looking. It's not a term that you'd use that much. It gives you an idea. They're so loose terms. Unless it's a leucocratic granite, which is white virtually, or a melanocratic gabbro or something like that, certainly in betweenies, mesocratic can absolutely - to my mind anyway.

When we're looking at these classifications, grain size and colour, you've got a set of descriptions you can have, but they're not universally applicable tests to each sample.

No. Grain size you could do, but something like colour is so subjective to your own....

The subjectivity of sample descriptions is something we're a bit concerned about.

To my mind it certainly is. Yes, but it (darkness) isn't one of the things you would take much notice of when describing it. It gives you an idea that you're talking about an intermediate type rock, or a quartz rich rock

But the real use of colour is to say "That's green"

Yes, to me the use of colour is to describe the rock. You've got to be able to write down on a piece of paper what it looks like so that later someone can come along and read the descriptionand pick up the lump of rock and say yes, he's describing that. If mesocratic helps you, or leuco- cratic helps you, then that's fine. It's a helpful aid, but there's a lot more detail that you've got to put in.

Well, looking at this, just looking at it, a typical name for it would be a granodiorite sort of thing. Granodiorite to granite, actually. It depends. If this is all micas, well a granite certainly, if there's a bit of hornblende in there I might revert that back to granodiorite.

What does hornblende look like?

It's this green coloured mineral, hundred and twenty degree cleavage in it. It's certainly got essential quartz in it, it's either a granite or a granodiorite.

You're going straight from the bare eye to the...

yes, it's coarse grained, that would be tedious. It does look like there could be two micas in there, but I think it's large enough to see ...there's certainly a black mica in there which is almost certainly biotite. It does look like there's another one. As I say, we should be able to see that under there anyway, it's coarse enough grained to be able to say that that's a granite, that's a granodiorite type rock. There's a lot of quartz in here, we have the brown biotite and we do also have a white mica in here as well, which is probably muscovite, little bits anyway, associated with that piece of brown biotite. Yes, there's definitely two micas in there...You've got muscovite, and biotite, withsome nice little zircon haloes in it, which is another give away certainly of biotite, you get these little specks of zircons, really, and they're slightly radioactive and you get a brown, well, a dark ring around them. And so you've got this black mica, and these little brown spots. Very often you can't see the zircons, they're too small, but you just see this black ring around them, and as you spin it, you see, they come and go, it's quite obvious...and you can see slight crystals.

Does that point you towards a particular?

Well, it's just another thing that biotite often shows, and very few other minerals show rings like that. Zircons and biotite are sort of associated together, quite often, and this one's just spotty biotite really, but it's just another of the things..

And would biotite confirm your initial ?

Yes. You've got the black biotite in there, which you can see, you've also got things like that, which are obviously micacious, they've flakey, got this platey structure to them, but then you've got the black, and you've got this other coloured one, which is sort of silvery,

would you say then that the biotite and the micacious structure would generally point you towards granite, then?

Well, the white mica would certainly point you towards that direction. And you're not going to get muscovite until you get fairly silicious style rocks. And the fact that you've got all this quartz and all this feldspar, you're certainly talking in that 65% silica range at least. You've got lots of free quartz in here, lot of .. fair bit of white mica, fair bit of alkali feldspar by the look of it, you've got these large crystals of plagioclase again with the zebra type twinning which anybody should get, it's so obvious, there's no problem about it. Both the feldspars are altered. Under plane polarised light you've got quartz which is basically colourless, there's very little alteration, they're just clear patches, you can't actually see any real grain outlines.. then you've got a cloudy nature, some of which follows possible cleavage patterns, it's basically just cloudy, which is your alkali feldspars, and it looks like the plagioclase in fact are more altered than the alkalis. You've got the clear stuff, you've got patchy alteration of the alkali feldspars, and you've got a lot more alteration, just stippled, on the plagioclases, certainly on some. The alkali feldspar, certainly this little bit, is microclined and you can tell that because under plane polarised light it has a cross hatching effect, as opposed to the plagioclases which have these nice, solid, zebra type crosses. This one has a cross hatched effect which is nowhere near as well defined, but it's a sort of a wavy, bi like a tartan. It's fairly distinctive when you see it, and it's always microclined, which is what you've got down here. That's not to say you haven't got any other kind of alkali feldspar as well, but you certainly have got microcline. Some of these plagioclase feldspars are full of fine grained clays where they've slowly been broken down. Again, lots of this white mica, which is virtually the same as biotite, it's got this pebbled surface under cross polarization, it's got high ??biofringements?? it's got virtually straight extinction, which biotite should have, it's got good cleavage as well, just along one direction, but it's vrtually colourless.

And you're looking at that under the lowest level of magnification.

Yes, the crystals are too big, so if you went any higher power you'd have to be looking at just the one crystal. To get the relative percentage of plagioclase to alkali feldspar is going to be quite hard on this, just because you've got one feldspar crystal that fills the whole of the screen, the eyepiece, so you're guesstimating, but to do it properly you'd probably have to point count... I was expecting to see some small accessories, ??spheno-apatite, or something like that, but we don't seem to have ... there's possibly a little bit of apatite in here, which is this colourless, fairly high relief thing, fairly small, low ??biorefringement , it looks as though in the actual apatite crystals you've got lots of little smaller inclusions, included crystals, which I'm not too sure about, possibly apatite, they're very small, they're very small, you wouldn't use them to classify or anything, just out of interest, they're probably, possibly ??apidates, slight colour, yellowish colour to them, fairly high relief, they're a bit small to do a lot on, don't worry about those too much...I think we'll have to plump for a granite for this one, just because it's got pure quartz, feldspar, a little bit of white mica, bit of muscovite, and not a lot else. In hand specimen it looks a little more like a granodiorite, but I'd have thought a granite.

Absolutely right. From Dartmoor, in fact.

Yes, as I say, if it had a little more , if it had hornblende in there you'd probably err towards the slightly middle of the road as opposed to the extreme, the granite type thing, certainly granites can be a lot more whiter than that, there's a fair bit of mica in there, black mica, but since it is only biotite, which you're going to get anyway.

So from that hand sample you were already down to granite or granodiorite.

Certainly, you're certainly in that sort of scale, because you can see the free quartz in it, and there's a lot of feldspar as well, 65% silica sort of thing, probably more than that,

Have you got time for another?

Yeah. Oh, my goodness me. What the hell's this? Well, it's a fairly coarse, certainly large crystals of what appear to be orthoclase, pink feldspar anyway, ....

How do you describe the colour then?

You've got a nice pink colouration, pink coloured mineral in there which is probably a feldspar, colouration would tend towards orthoclase, but until you get it under there it could possibly be plagioclase, orthoclase is normally pink, but you can get white orthoclase so, you can't say pink is orthoclase and white is plagioclase, it's a good bet, but you can't definitely say. There are elongate white crystals in here...

so all the time really we're dealing with probabilities

Yes, I think you are. Certainly in strange, in unknown type things. Some things are obvious, your granites and your biotites, you know exactly and your basalts and things, you've only got four minerals that it could possibly be usually, basalt is going to be made up of plagioclase, olivine, pyroxene and not a lot else, bit of magnatite, and sometimes.. granites are going to be quartz, feldspar, bit of mica

So would you say, when getting the rules out for this, I'm going to end up with a chain of ifs and thens, ifs and ands, which are all based on probability.

Depends how detailed you want it. You could do a fairly simple straightforward one that would point you to granite, to basalt, to intermediates, that would be quite easy and straightforward, I think if it gets beginning to get more detailed you might get, just because the whole thing is a whole spectrum of rocks, and you haven't got nice little compartments, and whatever dividing line you've got isn't a dividing line at all, it's purely man made, you've got a whole spectrum of rock going across there, and you'll get something right in the middle with characteristics of one and you'll also have something sitting right in the middle with the characteristics of the other, and what do you call that sort of thing, do you...

Probability's the most problematic thing in the rule base, because you end up with chains of probabilities..

I think, as I say, it partly depends on how detailed you want to go, you should be able to do a fairly good one without too many ifs and buts I think. It depends what your basic rocks are.

This set.

This set. Oh well, that's not quite so bad then. You've got quite a wide spectrum and there's not that many which are that similar, are there? Anyway, this thing, going back to this again, you've got these pink feldspar crystals, you've got white long laths of crystals which look like plagioclase but being long and thin, not this long and thin, and quite platey by the look of it...it's unusual. I suppose it is plagioclase ..it looks like another feldspar anyway. And you've got a green mineral in here..it's blacky green, and actually the true surface, the weathered surface is very distinctively a dark green colouration.eing this green it could be hornblende, it could be an amphibole, it could be an alkali pyroxene although the greens don't really look too pyroxene..in fact the greens are all sorts of sizes..yes, looking there, looking on the weathered surface where it's a dark, it's got a slight greenish tinge to it, you can actually see the crystals better, slightly elongate, rectangular, that sort of thing, a bit lost in the cut surface, you can't really see the true shape of them..possibly an amphibole but, that colour green is a bit off putting, you see that, you see some funny alkalis, could be ??adrinorgite or something like that, a funny pyroxene which you don't often get, but this isn't obviously a typical.. typical run of the mill rock by the look of it. We'll say we've got orthoclase, no, we'll say we've got two feldspars, and probably hornblende in there.. certainly got feldspars.

have you got any initial ideas what it might be?

Well, if it's hornblende, I mean, it's got a lot of feldspar in there, a lot of pink feldspar, and almost certainly a lot of white feldspar, platey white feldspar, which is a bit offputting, if that's hornblende.. no, let's see what it is. There's obviously this green mineral.. this dark mineral.. the green is amphibole..

Why do you say that?

Shape of some of the crystals, the strong ??biofringement colour, or rather, polarization colour,

What is the shape then?

You've got some quite nice six sided where the 120 degree cleavage comes in. That gives you a fair bet..there's a strong body colour, which, some ??pleocroism suggests it as well..actually not that much good cleavage in here ... you've got a lot, these are long thin laths..are basically plagioclase, but they've got a lot of almost certainly alkali feldspar exsolved out of them..the basically long thin laths do have a lot of zebra crossing in them, which shows it's a plagioclase.ut there's an awful lot of just ex solution little blebs in there, which will almost certainly be an alkali feldspar..and we've got similar..a lot of them, these long thin things which are obviously these white things running through the..we've got some crystals which are just virtually totally altered..or rather, a lot of them are altered..which may be another feldspar. ..it's certainly a problem to think of any sort of name for it. It's also a little bit of a problem to find out...I mean, obviously in the hand specimen you've got these long thin plagioclase laths which you can see down there, these plagioclase, there's a lot of alkali feldspar down there, the green is the amphibole which is obviously green under there..you've also got..under thin section, you see, you've lost that pink colouration..you can make out the differences which I presume are going to be these white things but they don't, they seem to be similar to the plagioclase, they don't seem to be truly alkali feldspar.. I think that's what they are..they seem to have quite a mixture... there's also another mineral, fairly low ??biofringence

What makes you say that?

It's got a low ?biofringe colour of sort of low yellows and it's fairly clear although it has pockets of alteration in it.. originally I thought it was a thick section ??abous quartz, and this was a thick section making it yellow as opposed to grey, but I don't think that is, in fact I'm sure it's not that..there does appear to be a little bit of quartz, possibly in the corners..there's also a possibly another amphibole, brown strongly cleaved mineral, browny greeny strongly cleaved mineral which is highly altered, well, which is altered, with a lot of small magnatites or plates anyway running along the cleavage planes..seem to be straight extinction.. it's got a darker browny green body colour to it.. it's strongly ??pleochroic.. goes virtually black, this browny green colour.. you've got lots of intergrowths between these feldspars, you've got alkali feldspar, you seem to have the majority is plagioclase which has been exsoluted out..you've got lots of little intergrowths and myrmekites and things..if that's myrmekite that's got to be quartz..ight, so you've got a lot of amphibole, a lot of plagioclase and a lot of alkali feldspar ..a lot of feldspar anyway, and one other mineral which I'm not really certain about..which is closely associated with the amphibole which is possibly a second amphibole..such a strong body colour seems to suggest that more than a pyroxene, it's a very strong, strongly ??pleocroic and has been altered, it's virtually all associated with this, or in connection with this other green amphibole..great tracts of alkali feldspar...there's another mineral down here as well as this amphibole, possibly pyroxene type thing, it looks very much like another feldspar but it's got..this low ??biofringence, which is higher than you'd normally expect from an alkali feldspar, alkali feldspars can be low greys, whites,and things, this one's got a definite orange colour, orange biofringence to it, first of all the colour, it's infilling sections around other areas of alteration more than anything, unless it's been a large crystal which has been altered itself, it looks like a feldspar but the colour's too high for it...to give it a name. Bloody hell. Large platey plagioclases, alkalis, lot of hornblende, something funny, ..these pink things seem to be, it's almost..green hornblendes, white plag, some of these yellow orange things seem to be .. god knows.. oh my goodness..lot of feldspar, no quartz or anything like that so it it's going to be a diorite type rock...I really haven't got a bloody clue what to call it...

If you can't say you can name a number of possibilities

Possibly the fact that the hornblende's got so much kind of plagioclase ...a funny sort of..it's not really a syenite...something like a micro gabbro micro diorite that sort of thing, but.. It's..

So it is a syenite! I said a syenite. Oh, it's an ethylene, that's what that mineral was. Ah, yes, if I didn't recognise that as an ethylene then it would have been undersaturated. It looks like quartz but it actually isn't quartz.

That's quite a difficult one, then?

It shouldn't have been quite as difficult as that. It's because I didn't recognise that mineral.