Self Report 3

I'd like you to go through a normal examination of these rocks in the way that you would if you were examining them in a normal analysis and tell me at each step what you're doing, ... it's essential that you say as much as possible about what you're actually doing and why you're doing it. Take your own time.

In the hand specimen this one is noticeable, a pink , a coarse grained, pink grains I think are orthoclase, and there's a lot of white, some looks like quartz, a dirtier white is probably a plagioclase.

Could you say something about why you think they're quartz?

Quartz, well typically they're white, if I had a knife I'd give it a quick test.

Could you specify what the test is?

The test is, a knife won't cut quartz, but quartz will in fact cut glass. We've also got here the gold looking stuff which is almost certainly a pyrite I should think, you've got black small, about 2 millimetres, lathe like probably a hornblende.

Just take your time and ..

There's also a green, weathered sort of stuff. I'm not sure how much of that can be put down to weathering. Certainly sort of quartz, orthoclase, hornblende, I certainly think it's a granite.

Right, so you're now putting it under the microscope. Can you tell me as you do that what you're looking for under the microscope and what you can see.

Mostly I want to check out the observations.. if it is a granite I'd expect to find some muscovite and biotite, check that it was hornblende I think I could see, the black stuff, look for the quartz, yes, there's actually quartz there, I'll put it into cross polars and it's characteristic extinction, colourless under ..

Could you spell out in more detail just how you know that it's quartz?

Quartz is colourless under single polar, I've put the analyser in, its typical colours are grey, it's a grey, and goes extinct to black. So it's quite, there's no twinning there...quite easy to pick out.

Is the no twinning characteristic of quartz?

No, you do have twinning, certainly sometimes - plagioclase is also colourless under single polar and grey under cross polars but that has characteristic twinning, very characteristic twinning.

So if it's plagioclase then it's more likely to be twinned?

Well, it has this parallel twinning, it's Carlsbad.

So if it's parallel twinning then it's plagioclase?

That's it, yeah.... There is in fact a little plagioclase in there, there is plagioclase, but not much, I think you could have a what could be a perthite structure,

What's one of those?

It's a sort of pink grain..obviously believing there's orthoclase in here from the hand specimen, a largeish grain pink under normal polarised light, a mottled appearance, when you have a sort of growth of plagioclase within it, which causes it to be fairly mottled, you can't actually make that much out when you put the cross polars in..so I'd think there could be some sort of intergrowth there.

Are you just noting what minerals are present or are you also noting what proportion they're in?

Well, there doesn't seem to be a lot of plagioclase, there's a fair bit of this sort of perthitic stuff, probably the orthoclase. There's brown to green, brown green birefringent mineral, yes that's birefringence, it changes colour under a single plane polarised light, it seems to have a cleavage, parallel extinction, more or less, but mottled, which suggests it's a mica, bright second order colours with the analyser in, mica, possibly a biotite or type of biotite, I think that's what it is, biotite, big grains of sort of oblong shape greyish color what I was calling what could be the orthoclase with perthite in it, quite abundant, difficult to be sure because this thing's fairly mucked about so it doesn't give a lot of information.

What sort of mucking about has occurred?

What you've got is a solid outline but within that there's no consistent definition, it's mottled colour, changing from greys and pinks in the plane polarised light (inaudible) it's fairly sort of mottled, there is evidence of a plagioclase within this, so I , well, in with the plagioclase so I think it's definitely a feldspar, and as I say, from looking at the rather large pink grains in the hand specimen I'd hazard a guess it's orthoclase. Quite a lot of quartz present, very characteristic of a granite

Do you get lots of quartz in other rocks beside granite?

I certainly wouldn't really expect it in a basic in the mafic range, not in this proportion, I mean a granite is defined, I think it's more than 65% silica so obviously you'd expect there to be some. There is some glass, which is a colourless normally, put the analyser in and it's totally extinct, there's a bit of an accessory mineral sphene, a kind of sphene, which is two little triangles on top of each other

Is that shape diagnostic of sphene?

Well, it's a diamond shape. It's fairly highly coloured, it's grey, characteristic brown rim, with cross polars it's very high order.

Could you explain a bit about the high order business?

High order colours. That's third order, I'll put the cross polars in, it's the birefringence, the colours which are produced by the crossing over of the light are quite indicative of the mineral, and these are fairly high order, they're bright reds and blues, such a high order in some cases that it actually masks the colour, you can't actually make out the colour because it's such a high order.

So is the order the brightness of the colour that you're getting?

Generally, yeah. More sphene here, not a lot, but small grains of sphene knocking about...one or two opaques..some other accessory minerals. One is sort of lozenge shape, fairly sort of grey colour, fairly high order with cross polars, can't quite remember what it is, possibly a zircon, and it's an accessory mineral, and there seem to be one or two others knocking about..there's a grain here, I don't know what it is, it's fairly long and thin outline, no cleavage but it seems flakey, and very bright colours, probably a mica, yeah, mottled extinction, almost colourless so it's probably a white mica.

Why were you looking back at the hand specimen?

Having seen the small, the black grains which I thought were hornblende in the hand specimen I'll see now if I can find some definite hornblende

So what does hornblende look like under the petrological microscope?

I wish I knew! I'm expecting a long six sided green thing, a length cleavage along the diameter thing.

Could you give me a similar description of the other minerals that you're expecting to find? What would you expect quartz to look like under that, for instance?

Quartz, colourless under plane polarised light, low order under cross polars, so it's going to be grey or grey to black, it's usually not a good shape, a bit of a blob really, and you often have them in a blob, a sort of matrix all together, and there's extinction, it's fairly random, you can have four quartz grains together and they won't go extinct together, they tend to vary.

And what about feldspar, which you mentioned?

(inaudible) orthoclase, well I believe it's orthoclase, what we've got here, I think it's orthoclase, but it's a pinkish to greyish colour, fairly large crystals, fairly good shape, sort of rectangular, it seems to be interbedded with a plagioclase, the plagioclase being colourless but with the cross polars you get the tartan twinning, sorry, not the tartan twinning, the Carlsbad twinning, which is a lines of a black, black grey black white, the different bits are very distinct at different stages parallel to each other, and that seems to be within the larger grey, so I think it's an intergrowth, dominantly orthoclase, with some plagioclase, I think that sort of structure's generally called a perthite. Which is related to its melting conditions.

What about mica? What should that normally look like under a microscope?

Biotite is brown to green pleochroic, under plane polarised light. Under cross polars it's second order colours and mottled extinction, often the grains appear flaky, white mica on the other hand the grains can often be very flaky, generally smaller, colourless, very high second order, and again, mottled extinction. There is white mica, not a lot, but there is white mica in here. There is biotite.. also in the biotite there's a, I'm trying to pick up a cleavage, I think I've found what I think is hornblende, hornblende can be confused with the mica, but I think this is some sort of hornblende, it's green, brown, light brown sort of pleachroic, it's got, this one is four sided, fairly long rather than the wispy flaky biotite, it shows a cleavage, parallel to the long diameter, extinction in this is not quite parallel extinction, which suggests it could be a hornblende, I'd call it the hornblende.

You mentioned pleochroic things a minute ago. Could you explain for the record what pleochroic is?

In a plane polarised light some minerals will show a colour, for example what I'm looking at now, it's a yellowish colour, but while I rotate this stage it goes from yellow, green, a deep brown, and back through green, and back to the yellow. And that is one of the characterisics that you would put down to the mineral. And what I've got now is a hornblende mineral of sorts, it's an amphibole, it's got a cleavage, the cleavage doesn't quite seem to be straight, it's not parallel, it's just off parallel extinction. To be really happy with what I've got, I'd need a book and have to use a ?Bertran? lens to identify it.

First of all, with pleochroic, is it the ones that change colour as you rotate them that are pleochroic?

Yeah.

And what's a Bertan lens?

Bertran. A Bertran lens is this at the top, and what it enables you to do is, you want to find a section of the mineral parallel to what we call the three directions a, b and c, which are related to the Miller indices. You have three right angles, and you find a section parallel to the optic axes, which will be totally extinct with cross polars. You find that, take it to full magnification, put the Bertran lens in, and then you just, for example (inaudible), you can find the actual birefringence of the mineral and using a chart you put a number to this, and then you look at the other properties, you work out from one or two other things, and then you look in the book, look at the minerals of a certain birefringence, look at the colour, and then you should be able to name the mineral by doing a series of tests on it. All I can say without being able to do that here is that it's probably an amphibole, plus the fact that there does seem to be an amphibole in the hand specimen.

Could you describe what a typical amphibole looks like so we've got it all together?

Well, hornblende is a typical amphibole and that's the green, six sided straight cleavage, and I think extinction is usually 15 to 20 degrees, it's not parallel extinction.

It makes life a lot simpler for us if we have all the information about something in one go.

Certainly so far you've got an amphibole, you've got biotite, little white mica, a lot of quartz, a lot of orthoclase with a plagioclase within it, you've got zircon and sphene accessory minerals, you've got what I did think earlier was apatite, another accessory mineral which has a hexagonal shape, almost colourless in plane polarised light, and low order colours with the analyser in, I think that's apatite, so it all seems, I'd definitely name this as a granite.

So that's a good typical granite is it?

I think so, yeah. At a guess it's probably the Shap granite.

I don't know myself.

I hope I haven't got this wrong. I'd be very embarrassed if I got this one wrong. I would say it's a granite and it's probably the Shap granite.

Is there anyything else you want to say about it before we turn over the bit of paper and find out if it is or not?

.... There's a few little bits of opaques, probably those little bits of pyrite which are within it. If I had a book I'd probably attempt to identify what seems to be the amphibole. At this stage I'd be happy to be fairly confident and call it a granite.

Right. Number 13: Shap fell : do you call that the same animal?

I'm not actually sure what adamellite is. I would have called that a granite.

It started off as a granite, I don't know whether it was someone being more precise.

There's a typical granite - shap granite which seems fairly similar so I hazarded a guess at the same granite.

(NEXT ROCK)

This is totally different in hand specimen it's very dark - bit of green - finer grained - with the hand specimen the actual grain size ... I Think I ought to call it a fine grained rock - fine dark - there appear to be some greenish grains - at this stage I could hazard a guess and call them Olivines - some darker - dark grains - some paler stuff - not a lot of structure in it

What structure could you find then?

You might pick up alteration weathering, changes in colours - the olivine could weather to a red colour - which can be fairly indicative.

Is that indicative of it being Olivine?

It's just one of the things you can often use - a flow structure could suggest that there's a Basalt lava - from the hand specimen it looks like a Basalt - it's got some white stuff - probably a plagioclase there's some shiny black long things following the cleavage planes glittering in the light of the pyroxenes probably, fairly fine grained, in the hand specimen fairly equisized grains compared to the granite which was a very coarse and porphyritic obviously with the large grains of orthoclase standing out. There don't seem to be any large grains standing out in this. Move on to the thingy.. when I hold it up to the light, it's not particularly good light..

Is that a fairly normal technique, to hold it up to the light

(I.E. THE SLIDE OF THE THIN SECTION)

Well it's often useful to take the thin section to the window and look at it under the hand lens.

What sort of things do you look for when you do that?

You can pick up...there are some larger grains, it's porphyritic, some large colourless grains, larger cloudier grains, a couple of big opaques, so there's certainly two minerals in there so far. The matrix is seems to be made up of lots of fairly colourless, more lathe like things, looking like a possible plagioclase. Seems to show some sort of flow pattern around the larger minerals

How can you tell it's a flow pattern?

It's like, for example, if you aligned loads of logs in a river, where they flow round and you've got long elongate minerals here, they actually seem to be flowed round the larger crystals.

And so what in turn would that imply for you then?

Well, it's just an alignment. If they flowed round, the large grains obviously crystalised out before these other things which flowed round them, so you're starting to get down to crystalisation order, you seem to have two larger types of grain here.

What sort of rocks are likely to have that sort of flow texture in them, then?

I wouldn't really put a flow texture down as a characteristic of any one type of rock. I'm quite happy to say it helps you identify the melting out order here. Actual identification of any of these minerals.. I think the small elongate things are plagioclase, the bigger ones .. the see through ones may be olivine but I'm not altogether sure. I'll put it under the microscope and have a look.... right. Porphyritic, you've got some large grains, small grains of plagioclase, and some of the large grains do seem to be olivine, fairly sort of lozenge shape, it's got cracks characteristic of olivine which are a green colour due to a characteristic alteration of olivine to, it's chlorite here, or serpentine, sorry, it's been serpentinised, which explains some of this, alteration colour, you've got the reds, and the blacks as well, it's fairly well cracked, it's a blue in the cross polars, so I'm fairly sure it's olivine we have here, one of the larger grained minerals here is an olivine...and it's plagioclases around this, so I'd say the olivine crystalised out before the plagioclase.. moving on, there's another large mineral, if I adjust the diaphragm.

So, what does the diaphragm do?

Well, I'm trying to brighten this because I'm fairly sure the other large mineral is a pyroxene, and characteristically if I change the lightness I change the shade of colour, and I can see more clearly now that the pyroxene has more body colour than the olivine, the olivine is almost colourless while the pyroxene here seems a faint pink, and the olivine shows no cleavage, though it's highly cracked, the pyroxene does tend to how a cleavage..

Is that typical for both of them?

Yes.

And is the same true for the body colour?

Yes. Again, that's one way of distinguishing between olivine and pyroxene is, pyroxenes tend to show more body colour while olivines tend to be colourless. They're both second order colours, pyroxene I'm looking at now is a pink, blue and yellow, ?regularly? shaped, which isn't particularly helpful

Which colours go in which orders then?

First order is going to be greys, certain yellows, and you go up to a sort of dull blue, usually you sit there identifying rocks and you have a chart in front of you, but generally your first order are going to be your greys, whites, and things like that, you go up into the second order which are blues, other yellows, bright reds, and so on. You continue up into the third order, then you get into the fourth order, where you start to get very bright. This doesn't seem to be extinct parallel to cleavage, it seems to be extinct at about 15 degrees from the cleavage, that is, it goes, with the cross polars in it goes black, if I align the cleavage up with the cross hairs and then move the cleavage 15 degrees further it goes extinct, so what I'll say is the extinction is 15 degrees away from the cleavage, and it's (inaudible) straight extinction.

And is it normal for extinction to be parallel with cleavage?

It varies, again it's one of the list of properties which you build up for the final identification. Certainly at this sort of stage without a book I'd be happy just to say it was a ortho, clinopyroxene. Again, it's a large grain pyroxene, with small grains of plagioclase around it, so it's crystallised out before the plagioclase.. there's quite a bit of olivine, quite a lot, some smaller olivines as well, some of a sort of roundish shape, cracked. Of the porphyric grains I'd say olivine is in a greater proportion to the pyroxene.. I haven't seen any quartz here, doesn't seem to be much silica, which suggests it's more likely to be an orthopyroxene than a clinopyroxene... I can't to be honest remember, to distinguish between an orthopyroxene and a clinopyroxene, one has straight extinction and one doesn't. To be honest I don't remember which is which but it's inclined etinction - If I had the book I'd just look it up - and find out which pyroxine - that would be an important thing - distinguishing which pyoroxine you're actually looking at - since there is a more - more olivine than pyroxine - if I go to a higher magnification to look at the finer stuff - amongst the plagioclase in the matrix - [************] - small olivines again.

Is it normal to start with the least magnification and work up?

Yes. One of the first things you do is to distinguish the textures - you've got big grains of mainly olivines - some pyroxines - within a matrix of smaller plagioclases - that is a porphyritic - that is one of the first things you do, have a look at the texture - there does seem to be some alignment within these plagioclases they do seem to have crystalized out after the larger grains - so olivine and pyroxine seem to have crystalized out first followed by the plagioclases.

Is the order of crystalization useful in establishing which rock it is?

Yes. You can trace it to an actual temperature - certain Basalts they come from certain environments - so the mid-oceanic range environment is different to the hot spot environment - I believe the hot-spot is much deeper based - so you could expect different melting histories and things like that - seems to be some glass in the matrix, which is basically black in plain polarized light - black under the analyzer and there seems to be a lot of glass in this matrix - the glass seems to be around the plagioclase I think - you've got some glass - the glass represents silica which has cooled very quickly - with glass here, well you're more likely to find silica with a klinopyroxine - klinopyroxines are pinker than orthopyroxines which fits in with more or less what I've said so far but not being able to remember the extinction rule between the pyroxines I'm not that keen to say which is which but I think it's probably klinopyroxine - I would say the glass was the last to crystalize, olivine came first, probably the plagioclase was crystalized out by the glass - they're the 4 major components glass, klinopyroxine, olivine and plagioclase. The porphorytic grains are mainly olivine - plagioclase and glass constitute most of the matrix I'm less sure of the proportions - I'd say maybe as much as 40% plagioclase, 30% olivine, 10% pyroxine , 20% glass - bit subjective when you estimate you tend to give the big grains too much - seems to be - with that mineral composition - a basalt or a basalt type rock - without really knowing the geochemical properties it's a bit difficult to say which kind of a basalt it is.

Are the basalts all very similar to each other then?

Say in hand specimen - the main difference is in the pyroxines present - well, there is one difference, you can tell by the pyroxenes present, as I say I'm not sure, I think it's a clinopyroxene you've got there, don't seem to have any orthopyroxene, but usually the basalts can be distinguished by looking at the geochemical properties, the isotopes, you then there's a thing called tha basalt series, you'd go into it.

Are you game for another one?

We've got a lecture