Self Report 2

Take them in any order.

Right, do you want me to go through switching it off?

No, it's all right... ... there's a hand lens as well.

Start with <something> for starters.

Right, erm, that's, the rocks, the three rocks are on that list.

Oh right, er. I could tell you now if you like <something>

Take them in any order you like.

Right, <coughs>, well I look at that and the colour, for starters means that it's an acidic rock, which means it's like Granite, Granodiorite etc. Err, it's got a high silica content, 'cos you can see the quartz and that white stuff's the feldspar and the quartz.

Yeah, the light crystal, it's the light colour that tells you it's...

That's it, yeah, yeah. Then technically the courseness of the grain means that it's plutonic rock, so that rules out things like Rhyolite and Dacite, Obsidian erm, so therefore it's something of the Granite, Ademalite, Granodiorite series.

Right.

Err, and then you have to subdivide those three by the percentage of err, the plagioclase mineral. These feldspars are, err, the feldspars are the white, now, you can usually tell from hand specimen that a plagioclase is white and if there's much orthoclase present, it'll be pink. So, that's more than likely to be Granodiorite, right, but, what, what you can then do is revert to the, to the hand spe-, the cross section to check that up, so you check the feldspars...

So what is it that points to Granodiorite, it's the fact that it's mainly white, is it? That it's mainly the white type of the...

The feldspar, yeah.

Which one's that?

That's plagioclase. It could, it could well be that there's, that some of it is white orthoclase, so therefore it could either be white orthoclase, and therefore it's a Granite, or it could be half and half of it, and therefore it'll be an Ademalite. This is the same list as yesterday, isn't it?

Yes, that's right.

Right, it could be one of those, it's definitely one of those three, it's just... ... it's got biotite in it, but that's not much good to...

How do you tell it's got biotite in it?

Bio-, it's got biotite in it, in that it's got <cough> under plain polarised light, which is without the analyser in, erm, there's a brown mineral in there, with erm, which is pleochroic, which means when you turn the table around it changes shade, and often changes sort of colour, goes from brown to light brown. But there's also halos around holes, pleochroic halos, that are black, black rings around holes, and they also fade, so it's definitely biotite.

If you put the, if you put the analyser in it changes the, the colours, erm, very often like, things like erm feldspars and quartz, you can see straight through them in plain polarised light, but as you put the analyser in, it changes the colours completely, so therefore you can use those colours to differentiate between the two. There's quite a lot of quartz in here, so that, that makes it definitely an acidic rock, it, it can't be an intermediate rock or a basic rock, it's definitely, definitely an acidic, so I was right in saying it's one of those three.

Anaman, Ademalite, Granite and Granodiorite?

Yeah, it's definitely one of those three.

And the look of the grainsize also points to the, points to those three.

Yeah, that's right... ... ... ... <coughs>

Have you just spotted something else?

Well, no, I'm just basically looking around at the feldspars just to see if I can see one that's err, definitely one or the other. But you can, what you can, how you can tell is that err, the plagioclase feldspars are, are often twinned, which means like they, they're, you see them as black white black white black white, whereas orthoclase ones are just black and white, just with a line down the centre. Err, I can't really get a good <something>. There's microcline, which is like cross-hatched, err, that often causes orthoclase. It looks like orthoclase in here, but the colour suggests plagioclase, yeah, there's a good plagioclase. Yeah, there's another one. Plagioclase, yeah with the, you can see the black white black white black white. Yeah, I think I'd go for Granodiorite.

Right, let's have a quick peek.

With a possible indication towards Ademalite... Granite! <laughs>

Does that mean it was probably the other kind of feldspar?

Yeah, that's right, yeah.

So those three rocks are in fact very similar, you can't really tell unless you can make that distinction.

That's right, yeah. The first, the first one when you look at it, and you think that's white therefore it's Granodiorite is erm, very very arbitrary, you know, it's sometimes right, a lot of times right, a lot of times it's wrong as well, so yeah, you know, I should have really believed in the hand, the cross section rather than the hand specimen, but, that's the way it goes, what's next?

Err, well, from the colour of this... it's erm... basic to ultrabasic.

That's 'cos it's dark, yeah?

Yeah... Oh dear... erm... ... not much more I can say on that. There looks to be some significant proportion of feldspar in there, which is the light mineral.

What about the grainsize? You...

Oh sorry, yeah.

... talked about the grainsize quite a bit yesterday.

Grainsize is, yeah, grainsize with it being quite course again, means that it's a plutonic, sort of basic rock, so it's a similar type of mode of formation as the Granite. That makes it something like Gabbro, Peridotite, erm, that type of thing, what are the other choices in there? Not Peridotite, Dunite-peridotite, yeah. One of those, I hope... ... and always, erm with the basic rocks, the feldspars are, are err, virtually always pyroxenes, err, sorry, plagioclase, erm, more than often they'll have <bredarite>, and you can tell that by, err, the, the angle when one's black, if you get them right, there's cross-hairs on the, on the lens, and if you get them facing with the cross-hair at the top, north-west cross-hair, they should be equally illumniated. If you bring them one way, one, one set of the bands goes black, and bring them the other way and the other set goes black and you can judge by the angle between them going black what type of feldspar it is, that's <unintelligible>. That's basically what you can do. This is probably going to be the Peridotite anyway... erm... there's olivine in here.

How can you tell that?

There's olivine 'cos there's, there's, olivine's got higher, when you take the analyser out and look through plain polarised light, it's as if it's standing out amidst everything else, and it's got a very very strong ooh!, very very strong err, border, you can see all the crack, very heavily cracked, but no obvious cleavage, cleavage is a direct line, straight lines everywhere, whereas this is very very cracked and often weathered, and you can see some weathering in it where it's been weathered to, for, and another to a different type of mineral. And then there's erm, pyroxenes, which you can tell by the very high, erm, colour when the analyser's in, err... it's a very high blue and yellow and purples... ... there's a great deal of plagioclase in the feldspars in here... ... erm ... ... let me think... ... high plagioclase, erm, pyroxene and olivine... oh it's not, it's not ultrabasic because of the high percentage of the feldspar, 'cos feldspar's predominantly silica.

So what does that rule out?

So it rules out Peridotite and the Dunite-peridotite. It's not Basalt because of the, the chemistry's correct, but the grainsize is wrong. Dacite, Diorite, Dolerite, Dunite, Gabbro. Gabbro looks like the favourite so far. Microgranite, Nepheline. Yeah, it's Gabbro.

Right, have a...

Ooohh...

Right, have a third one.

Right you are... ... well the colour from this looks quite, very similar. Erm, looks very very dark. Possibly with a little less of the, the white mineral, but not a great deal less. Erm, so therefore it's basic or ultrabasic. Erm... the colour indicates that it's basic or ultrabasic and the colour also indicates that it's not a lava or a small intrusion, it's a plu-, massive plutonic intrusion as the other, the other ones have been, 'cos it's very course. Right, err, the only thing you can tell about the mineralogy is that it's probably fe-mag minerals, things like err, clinopyroxenes and olivines, stuff like that 'cos of its dark colour.

That's ferro-magnesium, is it?

Ferro-magnesium, yes. Right... yes, there's biotite in it. Again, by the pleochroic nature of biotite, you can see brown changing to light brown as you turn the stage around... ... there's olivine again, the cracked mineral with the high relief. The background has got very much less... oh, that looks like something... oh it's still quite high in the silica content, I don't think I can call this an ultrabasic either, this could be, this could be Gabbro-, Gabbroic... see if there's any minerals that push it into the information... ... <coughs> ... there's clinopyroxene, but that can be in both, ultrabasic and basic... ... ... ... ... it's, it's obvious that it's plutonic by the courseness of the grain but erm, it also helps to see the clarity of the crystals, the crystals are quite well formed in this, which means they had time to form, which means they formed slowly, and therefore it's a pluton.

So this is very similar to the last sample, then?

Yes, almost identical. Well, it's not identical, there's no, there's err, the feldspars are different, but I think there's just as big a percentage, large a percentage... I think I'd still go for a, for a Gabbro for that one as well to be honest.

I don't think we've got two samples.

Not got two the same?

Just check.

Let's have a look at the others, right err... ... ... if it's not, I think I'd go for the Dunite-peridotite because of the high percentage of olivine that's in there. Dunite-peridotite, Gabbro, Peridotite, there's just the three isn't there. Hmm... ... there's some garnet in it <coughs> got some garnet in, peridotite, but I can't see any of that.

What do you look for when you look for garnet?

Garnet's erm, it's an isotropic, you can see it when you've got plain polarised light, but then when you put it in the analyser, it's black, whichever way you move it. Usually the minerals are quite coloured, very bright, but garnet's black, whichever way you look. Can't see any of that... ... I don't know whether these are olivine now, maybe these aren't, these might not all be olivine, because some of them have got good shapes, and olivine is a very, is often very well rounded, and some of these have got like pyroxene shapes, which are like eight sides. Yeah, one two three four five six-sided that is... one two three, that's eight-sided... I think I would have gone for erm, Gabbro first, but I think probably Peridotite, that's the best I can do.

That's interesting, because it's that.

Is it?!

Which is, which presumeably contains more...

Yeah Gabbro..

Yeah Kentallenite.

Wow!

That the Gabbro.

Oh heck... ... yeah right, yeah.