Self Report 4

Right, could you remember to talk all the way through about what you're doing.

Right, OK. First, er, it's rock number 2 in hand specimen, coarsly crystalline, er varied coloured grains ranging from black to, through pink, to a greyish, whiteish colour. Vitreous, all three grains vitreous. Under the hand lens... ... under the hand lens there is no visible cleavage in the grey vitreous coarse crystals, there is highly visible cleavage in the black vitreous crystals and there is indeterminate cleavage in the pink vitreous crystals. Right. Thin section <sighs>...

I notice you're rotating that to the lowest magnification, is that standard procedure?

Yes, standard procedure, due, basically due to the fact that if you, if you go straight on at high magnification there's a lot more error involved and there's no much chance of you seeing the smaller detail. Take the polariser out `cos we're only interested in plain polarised light to start with (right put that down) OK, in plain polarised light we have a clear, low relief groundmass which is vaguely crystalline, and is non pleochroic, erm...

Could you explain that a bit briefly about relief and pleochroism?

Relief erm, relief is very much an arbitrary constant in that it's the comparison of the way the mineral stands out in regard to the balsam mount, the balsam mount given a refractive index of 1.54, anything higher than that will stand out more than the balsam, anything lower will not, and this, the main mineral in this rock, is lower or equal to balsam in relief. Right, we have a greenish irregular mineral which is non-pleochroic again, and...

Sorry, could you just explain what pleochroic is as well?

Pleochroic well erm, with a pleochroic mineral you'd expect it to show a variation in colour in plain polarised light. The best example's biotite which goes from yellow to brown. This mineral, don't know actually, it does show a bit, yes, this mineral goes from green parallel to the north cross hair to a palish greeny-brown parallel to the east-west cross hair, so there is some pleochroism present in that. Right. Third mineral visible is streaked with black, regular, no, no outstanding cleavage, but some trace of them visible it's pleochroic from black parallel to the north-south cross hair, to a mottled brown-black parallel to the east-west cross hair. What else have we got? The fourth, probably accessory mineral because there's not a great deal of it, which is palish brown, non pleochroic in plain polarised light. A series of fractures running through the first mineral we mentioned, the colourless one. Right, that probably sorts out plain polarised light. Put the cross polarised light in. Right, in cross polarised light, our original colourless groundmass is now a lightish grey, which is ... highly fractured, probably showing strain extinction actually. Good specimen this.

Could you say a bit about erm what extinction is, and how it works?

Erm, extinction in cross polarised light, erm what you'll see is that at one or two points in the rotation of the stage, the, well, the technical description is that the vibration planes of the mineral are parallel with those of the polariser, so in effect the light coming through the bottom polariser is unaffected and it hits the top polariser and gives you a blackout. That, then, in this specimen, you'd normally expect it to occur uniformly throughout the specimen, but in the one we've got here, it appears in mottles, which is what erm, is meant to be strain extinction, which shows that it's probably an igneous rock that's been subjected to a later metamophism or something of that sort. But probably say the groundmass is quartz actually, although it is of an unfamiliar form to me, erm, basically because of the grey, the grey... er cross polarised colours, and there's no twinning, which would be plagioclase. The mineral that was green in plain polarised light is still green in cross polarised light. Probably a middle second order interference colour.

Could you say a bit about erm orders of colour?

Yeah erm, what we have is that there's a pre-determined scale of interference colours which erm, varies uniformly for all minerals, and basically the brighter, more intense, more irridescent the colour, the higher it is up the scale, and although the interference colour for one set of mineral will vary between a maximum and a minimum, it's usually a very good diagnostic feature, specially in the higher order colours, erm, things like calcite have up to third and fourth order colours and what you'll see under the microscope and cross polarised light then is a very irridescent bluey-purple colour. This one's erm, much duller, which makes it less readily identifiable, so we can't... The pale brown mineral that we thought was an accessory in the, in plain polarised light, in cross polarised light is a, an intergrowth in the quartz crystals, so it is an accessory mineral, and you wouldn't use that in determination of what this rock actually is. There's some twins exhibited in the green mineral, characterised by the fact that I'm not seeing the homogenous crystal when put into cross polarised light. The two separate halves have different interference colours. This is probably a strong hint towards one of the ferro-magnesium minerals, possibly augite, olivine, maybe hornblende. Right, there's nothing, nope, nothing else in the thin section of that which is. Did you want me to identify this?

If you'd like to say what you think it is and why you think it's that.

Hmm, right, well, looking at it in hand specimen I would have said it was a granite, but the fact that we may have some hornblende there is disturbing. That may be accessory, I w-, I would say it's a granite basically because, looking at hand specimen, you've got the characteristic coarse grains with no cleavage, reduced lustre, you've got you're characteristic orthoclase feldspar, which is the pink, which doesn't show up that well in thin section, and we've got the characteristic flecks of possibly black biotite mica, and then there's maybe some accessory minerals. So I'd either say it was a granite, or on closer analysis, maybe an andesite. Erm, andesite basically because of the presence of the, the questionable ferro-magnesium mineral.

Right, there's a bit of paper at the bottom of that which tells you what it is. Sorry, at the bottom <paper rustles> if you'd like to pull out the er <something> and then turn this thing at the bottom upside down.

Syenite, Olivite and Nephiline-syenite, uh-huh, I wouldn't know what looked like, but yeah, that would make sense because it has the ferro-magnesiums present, which would push it off from the granite side over to the more basic-intermediate types, yeah. That's a confusing one to the, to most geologists you'll show that to, they'll get confused, because it looks like a granite hand specimen, but it, it's actually got the characteristics of a more basic-intermediate rock, so I would imagine you'll get a lot of people telling you that's granite because it's deceiving hand specimen.

Yeah, it's interesting for us to get the processes by which people arrive at a decision.

Yeah, see we're very much preconditioned into anything with pink, white, black in it is a granite. In fact, a lot of people will look at that and they're first thought if they tell you it will be granite, because everybody thinks that's a typical granite, whereas this one here, well number 3, a lot of people might say that's gabbro without looking at it carefully, it might also be a granite, so I'll have a look at block number 1.

Right, if you can, can I get you to say what sort of mineral you think something is, so that we can then put together the statements about it, if it's this colour, and that lustre then it's likely to be such and such.

Yeah sure. Right, this one, on sort of first impressions, without looking at it carefully stands out as a dolerite, and again, it's a preconditioned reflex that having done sort of five-six years of geology, you're preconditioned that anything that looks like that is a dolerite, erm. It's fine grained, well, fi-, fine to medium grained I would say. Erm, obviously igneous because of the crystalline nature of it. Vit-, oscillating it in the light, you get the vitreous sparkles, maybe one or two minerals. The hand lens probably won't show a lot because of the nature of the grain size. And doesn't, no. Basically composed of dark minerals, just because of the main colour erm, will show a lot more in thin specimen hopefully... OK, right, in thin specimen, whereas before we had a, a large groundmass, and one homogeonous crystal which was punctuated by medium to small size crystals, here we've got a groundmass of many small-sized uniform crystals. There's no big crystals standing out, which would suggest that it has cooled slower, which would back up it being medium grained igneous rock. Right, plain polarised light we have... first mineral.. First mineral, which is sort of column-shaped erm, looks to be twinned, er, no pleochroism, medium to high birefring-, erm pleochr-, erm relief, it stands out from the rest of them quite well. We have a black mineral which is quite well cleaved, it curves basically as black squares, triangles, irregular outlines erm, there is no pleochroism, it stays black on rotation, which would suggest that it is either, let's just have a look, yeah, no, if this is an isotropic mineral, the fact that in plain polarised light it stays black, in cross polarised light it stays black all the way in rotation. Now that would identify it as either a cubic mineral or some other mineral that doesn't effect polarised light: it's a, it's a characteristic erm determination that, that if it stays black on rotation in plain and cross polarised light, then it's isotropic, which would probably make it an accessory mineral in this rock. Very few essential minerals are isotropic. Right, third mineral present in plain polarised light is a greenish-grey erm, best way to describe it is probably globular, it's, it's more a conglomeration of very small crystals than a defined crystal shape. That shows no pleochroism, it's of medium relief. And the first mineral we mentioned, the columnar one is very much intergrown in the groundmass, suggesting it was probably the first mineral to grow. That's about it for plain, there are some small specs of an orangy-brown mineral which are accessories that are not worth bothering with really, there's no way to do identification.

How are you able to say whether something's essential or accessory.

Er, essential minerals are ones that in textbooks are defined, er they define the rock, and they are the ones that are present in upwards of 5%, 10%, up to 30-50% in your thin section or your hand specimen. An accessory mineral is one that will be present 1%, 2%, 0.5%, they're very much just oddities that are, erm, developed with the rock, just because of some fluke in the environment. Right, in cross polarised light we have, our original, <latt>-shaped columbar crystals are characteristic plagioclase twins. What we see here is that erm either side of the twin has different extinction, so when one half's black, the other half will be the palish grey of a low first-order interference colour. It, it's a characteristic, there's no way you can mistake anything else, a plagioclase obviously the twinning. So, the main mass of this rock is plagioclase. Right, what else? The secondary mineral, which we described as the globular one in plain polarised light is a middle-second-lower-third order interference colour, and the yellow, speckled yellow, which shows twinning, and on rotation, variable extinction, erm, they're not constantly extincting in one angle because of this, this globular-type nature. Our black mineral in plain polarised light stays black in cross polarised light. Now erm, this could make it, er it's too abundant to be an accessory, if, if it was of low percentage you'd probably say it was pyorite, simply because that was one of the best examples of an isotropic accessory mineral. But it's far too abundant to be that, so... The second mineral, the one we described as globular again, the yellow interference colours, we'd probably say that's going to be ferro-magnesium mineral, probably augite. The twinning, twinning would help be either augite or olivine. The black mineral, that's a tricky one erm, have to use a textbook to work that one out. The, the thing with these, er, what are opaque phases or isotropic minerals is that because they're all black, you can't tell one from another, there's no properties visible in plain polarised light it's black, in cross polarised light it's black, there's no extinction, no interference colours, so basically you've got to determine that on what the other minerals are and whether the assemblage would make sense. But basically from what we've got here, the key of the plagioclase and the key of the olivine, pyroxene, ferro-magnesium mineral would suggest this is a dolerite, as we thought from the hand specimen. Find out whether I get this one right now... <paper rustles> Yeah, dolerite, quartz-dolerite crossed out?

Yes, we took it all, we took all of these to several different experts and they all fought among themselves, so we ended up with some pretty amazing compound names for, for some of them.

Yeah, right specimen number 3. Hand specimen erm, stands out coarse-grained igneous rock erm, you can actually see the vitreous lustre very clearly and the, the grains are so coarse, it's possible to hazard a guess at what they might be in hand specimen. Unlike the dolerite where we had to go to the microscope. Probably biotite-mica, erm it's a palish brown colour in hand specimen. You can see the thin leaves of the crystal's cleavage. Highly vitreous. Probably some muscovite-mica as well, which is a paler, silvery, silvery-brown colour, whereas the biotite's brown to black depending on the thickness of the crystal. Err, definitely quartz present, er, you can easily see the non-cleavage, sort of amorphous crystal form, which is vitreous in lustre. The white groundmass erm, would puzzle a lot of people who hadn't done geology before, because if, if this was a characteristic granite, which I probably think it is, you would normally expect the erm pink feldspar orthoclase to be present, whereas in this one, I would say it was the plagioclase feldspar which is present, which would give the white, fairly well-cleaved groundmass. Erm, I think we have, possibly have some accessory hornblende, which you can make out by the little well-defined three-dimensional crystals, which stand out with a diffuse lustre on the surface. Thin specimen... ... OK... Right, in thin specimen, we have characteristic biotite grains which are pleochroic from straw-yellow parallel to the north-south cross hair to brown parallel to the east-west cross hair. These are also characteristic in that they contain inclusions of a radioactive mineral which has caused little black spots in the actual crystal. That's again a characteristic of biotite, which helps to identify it. We have, the groundmass is basically low relief, which would lend itself to being quartz. No cleavage visible in the crystal, no pleochroism. There's another low relief mineral, which shows a hint of twinning, and possible some of the strained extension that we saw previously. Erm, with this, with the groundmass of this being two, low relief minerals, basically you can't tell that much in plain polarised light. It's a lot easier to go to cross polarised, where they'll be a lot more distinction between the minerals. Right, our original biotite grains are characterised by upper-second to middle-third order colours which are present clearly in the bright yellows, turquoises, some purples. Erm, plagioclase is present, we have some characteristic twins with the, the zebra-striped pattern of alternate extinction to the twins. We have quartz present, which is characterised by a very amorphous type of crystal form, and we have a lot of separate extinctions of small, irregular crystals all meshed together, so as you rotate it it flicks in and out, irregularly from black to the greyish middle-first order colour that you expect from quartz. No accessory minerals stand out. There is some, yeah, some definite strain extinction and some inclusions in the quartz, which are helpful in diagnosing the difference between plagioclase and quartz in that you find inclusions in quartz whereas you don't in plagioclase, although there is some alteration of the plagioclase come to speak of that. What happens is that some of the larger plagioclase crystals when subjected to erm metamorphic fluids, erm, the plagioclase is altered to a sort of clay, and this shows up characteristically in the, in the crystals as a, erm, sort of mossy-type structure inside the crystal plains, which again that doesn't happen to quartz, so that, that is a key difference in that you can look in a quartz and there's very little alteration, and you can look in the plagioclase and there's a great deal of alteration by metamorphic fluids. Basically, I would say it's a, it's a granite, despite the lack of the pink orthoclase feldspar. The presence of quartz and plagioclase would.... there's definitely quartz there, which means that although it looks like one, it can't be a gabbro; there's no ferro-magnesium minerals present, so again, although it looks like a gabbro it can't be, erm, it's too coarse for any of the intermediate rocks, and the composition doesn't seem right for any intermediate rocks, so unless it's something I haven't seen before I'd say that it, it's a quartz, erm granite rather.

OK, are you right about this one?

<paper rustles> I'd better get two out of three right. Yes, Dartmoor granite, good.

Right, OK.