Structured Interview 1

...overview explaining generally how you'd set about analysis and identification of rocks in general and then after that a sort of ten minute overview and then we can come back and pick up on each of the points in more detail.

Right, okay.

so if you'd like to start explaining how you'd set about identifying a rock ... go ahead.

Right...given these set of rocks or just generally?

A rock that you find somewhere

Right,well the first thing probably to try and identify is just the basic sort of what does it look like. Sounds very straight forward but in actual fact you're looking for things like grain size, colour, hardness, and whether it's got any sort of internal structure such as banding or peculiar sort of texture that you can identify. And on that sort of basis you could probably work out in a very broad sense in the field whether it fell into the three major categories of rocks which would be igneous, sedimentary and metamorphic. There are crossovers, sort of grey areas in between, but you'd probably be able to define which of the three. The thing you'd be looking for in the igneous rocks would be a ground mass of interlocking grains with crystals growing... with well formed crysals generally with clean boundaries. In sedimentary rocks you'd be looking for evidence of some sort... the grains would be supported by some sort of a cement or a matrix as it's termed and in general they'd be softer, whereas igneous rocks would be hard. And in a metamorphic mineral you may be looking for evidence of the metamorphism which would be maybe peculiar minerals which you know you don't find in the other two kinds of rocks which occur by reaction produced by the heat and pressure of metamorphism and also a number of characteristic metamorphic textures such as banding, or lineations as they're called, or foliations - planes or lines through the rock,... or folding or shearing. And of course the presence of fossils may also help. Fossils are found in sedimentary rocks, occasionally in metamorphic rocks, so there again you have another constraint.

Er.. apart from a brief sort of overview, the next thing you'd be looking for are the minerals that would be... in the same time that you're looking at the grain size, you'd be looking at that. The grain size will obviously affect whether you'll be able to identify the minerals; if they're coarse grained they're very easy to identify usually. If they're fine grained they're often quite difficult. But assuming, assuming you've got a coarse grained rock, the mineralogy would be very easy to identify, and if you've decided that you're looking at one of these three categories then you can go in and identify the minerals. In igneous rocks you'd be looking at whether you have olivines, pyroxenes, feldspars, quartz, and an assortment of more unusual minerals such as alkaline minerals and perhaps micas and other exotic minerals which may occur...and from your identification of the mineralogy you'll be able to arrive at a basic chemistry for the igneous rock which will then allow you to pigeon hole your rock into a particular slot which would reflect its chemistry, its grain size would reflect its mode of emplacement or its cooling history ...

Er.. onto sedimentary rocks, you'd be looking for, again, grain sizes tend to be quite important in regard to the deposition of sedimentary rocks. Sedimentary rocks can be divided up into a range of grain sizes which you'll be looking for from pebbles or boulders right down through a range of gravels, sands, silts, down to clay, which is very fine grained, so you'd be looking for grain size and there's a whole scale of grain sizes which would... you could then again pigeon hole your particular rock. Er.. once you've identified the basic grain size you then may be struck by a range of grain size in which certain parts of the rock are different grain size to the other which is often quite common in sedimentary rocks you may get one particular grain size reflecting fragments and another grain size may reflect the cement which holds the fragments together and in this way you could describe what these grain sizes were and also the contrast between the two grain sizes which gives you some indication again of the deposition ..er.. you may get slow changes in grain size called grading which again reflect the deposition . So there's a whole host of grain size analyses which are very very important in sedimentary rocks and in many ways sedimentary rocks are classed on this grain size basis. You often find them called sand, silt, clay, which reflects this. And then of course you're looking again at the mineralogy, which is somewhat more restricted than for igneous rocks. The major components tend to be carbonate material such as in limestones, quartz material that you find in sandstones and siltstones, and clay material that you find in shales, clays, and silts. And another thing to look for would be any exotic minerals which you might find which may tell you something about the rock. So once you've worked out the composition and the grain size you've got a good idea of what the rock is. And any fossils you find might be useful as well.

Er.. onto metamorphic rocks. Again the mineralogy is quite important. The chemistry of a metamorphic rock perhaps isn't so important in allocating it to a particular group. What you're looking for in metamorphic rocks is the grade of the metamorphic rock, that is, the temperature and pressure at which it was altered. And the way in which you do this is to look again at the grain size and mineralogy. Particular minerals occur at particular pressures and temperatures in..their stability ranges at a particular temperature and pressure. And metamorphic rocks tend to be grouped together under a range of what are called metamorphic facies which are actually representative of this pressure and temperature regime. And on top of that, chemical variations which are reflected by the mineralogy. So the minerals tell you something about both the chemistry and about the metamorphic history. And if you can also look for the grain size - coarse grained metamorphic rocks tend to be metamorphosed at higher grade than fine grained metamorphic rocks, in general. And if you can look at any sort of deformation which has occurred you might notice some shearing, folding, flattening of grains or fragments, or you might notice bands of little, destruction of the original grain size, it's been ground down to very fine grained rock, which is evidence of very very intense deformation. So metamorphic rocks are a bit more sort of vague in the sense that you don't tie things down definitely with regard to one thing, you're looking at both mineralogy and grain size to tell you about the metamorphic history. And that's basically how, basic field identification.

Right, if we could go back over this, then, in moree detail in more of the bits and follow those through ..er..if you have a specimen then which ...is, say, large grained, what would this imply for you?

Is this in which rock type, would you say? You see, the difference between igneous, sedimentary and metamorphic you would draw different conclusions.

Can we actually go on to how you would identify these particular sets? Now, again, once again. How would you know, for example, something was andesite?

Ah, right. Okay.

If you could give us some sort of if and then type rules.

Well, you mean, specifically for andesite?

Well, for the whole lot.

Each one individually?

Yes.

Right, okay, shall we start with dacite? No, we won't start with dacite, we'll start with something easy. Er.. start with dunite. Number seven. Now dunite would be very easy to identify because it's a rock made totally of the mineral olivine. Very very easy to identify. Coarse grained, green in colour, very dense, and contains just an interlocking mosaic or framework of olivine crystals with nothing else at all really.

Er.. now the mineral the rock peridotite is similar to dunite in the sense that it contains olivine. What you'd also be looking for in peridotite, it's effectively a type of dunite, or vice versa, peridotite containes pyroxines as well as olivine.

And how would you know that it contains pyroxenes, are they...

They're distinctive from olivine, in a hand specimen you'd be able to recognise them.

How would you be able to identify them as that, I mean, what is it that gives them away as

Pyroxene?

Yes

They're darker green than olivine, they have a different crystal form, they have a different er.. hardness, cleavages are different, you'd notice fractures running along them which were different from those in olivine. You're just looking for differences in the minerals, they're very very obvious. They're just a set of properties which are different.

Can you tell us how they're different?

Oh, right, I see, specifically?

Yes, so we know why they're different, not just that they're different.

Why they're different? Well, it really boils down to chem.. er.. crystal structure and chemistry.

Yes, sorry, I mean how you can tell when you see them that these are for example pyroxene and not something else.

Right, right. Well, to identify pyroxenes in hand specimens, they're dark green in colour, they usually form tabular crystals, they have cleavages which are approximately at right angles, they have cleavages at right angles which are planes at which the crystal splits, they are softer, slightly softer than olivine, whereas olivine is rather brighter green it's a glassy appearance. It has a fracture rather like glass, it doesn't break into distinctive planes or cleavage planes, it breaks rather glassy without these cleavages and it's also a bit harder than pyroxine. It hasn't got the tabular habit, it tends to form sort of octagonal or hexagonal prisms rather than tabular crystals.

Right, right, so that would be enough to identify peridotite?

From dunite?

Yes

Yeah. Er.. Right. Now the other rock type which would be similar to those two would be lherzolite. Now this is part of a similar sequence in that lherzolite shows the characteristics of peridotite in containing olivine and pyroxene. But it also contains the mineral garnet which is easy to identify because it forms large, prisms, I think they're twelve sided prisms, I couldn't be certain on that, but they're red, it's a red in colour, and it forms a characteristic many sided prism. Very easy to identify.

Now the other coarse grained rocks. Perhaps the natural progression from there would be to go to gabbro.

Gabbro is similar to peridotite in the sense that it contains pyroxenes, but only rarely does it contain olivine. And replacing the olivine somewhat inthe rock you get the appearance of the mineral feldspar or plagioclase feldspar. And this is very simple to identify in hand specimen, it has a hardness less than pyroxene and olivine, it's white in colour, tabular crystals with right angled cleavages, but basically it's the colour which gives it away. It's always white, no problem with identifying that really. And gabbro has a has a different appearance from dunite, the three I've mentioned before, because of this. It looks lighter, looks speckly coloured. Groups in with the mesocratic rocks, it's got that lighter look to it. In addition it's doesn't contain any alkaline minerals, which is a good criterion when you come to some of these later on. They all have alkaline minerals.

And how can you identify the alkaline minerals? Is there some general characteristic that identifies them all?

No there isn't one characteristic which identifies them all. There are certain common ones such as potassium feldspars or potassium or sodium pyroxenes. They're often ... the potassium feldspars are pink colour and potassium pyroxines tend to be a very very bright green whereas ordinary pyroxenes are dark green, almost black, they can appear black in a hand specimen, whereas alkali, what are they called, potassium pyroxens can be very very bright green in colour.

So would you have to learn those groups individually as special cases?

Yes, you cerainly couldn't learn all the alkali minerals as one group. Each of the different mineral types has an alkaline n-member, as it would be, which is different from all the other alkaline n-members of the other groups.

Well, maybe we could come back to those later on if we get time.

Yes, er.. now those are perhaps some of the rocks which are low in silica. Silica is perhaps, well for the dunite, peridotite and lherzolite well it would be about forty-five percent, for the gabbro it would be fifty percent or so and there's, I notice you've got a group of rocks here which you could term intermediate.

And how do you tell about the amount of silica in a rock? Is it visible when you actually look at it?

Well, you use the overall mineralogy. The presence of silica in a rock is reflected by the minerals. Rocks rich in silica have a lot of quartz and feldspars. Those with intermediate silica have not so much quartz, still quite a few feldspars but you're also getting the appearance of pyroxenes. Less silica still, pyroxenes take over, and at very low silica you get the appearanc of olivine. So that's a good, sort of a sliding scale. It's more of a general appraisal of the whole mineralogy rather than something you can put your finger on in a rock.

Right, so it's not a case of saying that there are so many blobs of silica present or anything like that?

No, you're just using the whole mineralogy to sort of come to a conclusion in your mind as to how much silica there might be.

Right, so you'd say if this is present and that is present then it's likely to be high silica.

Yeah, and also there are the percentages in which they're present. K Right, well we can maybe come back on that one later as well, if we have the time.

Shall I carry on with these?

Yes, if you'd like to carry on with those then we can maybe work through the sequence.

Okay, well there's two rocks, two of these, I've taken the coarse grained ones first deliberately, there are three what I might call, or two, let's say two, intermediate composition rocks. These might have less, about ten percent quartz, also some pyroxenes and quite a lot of feldspar. And the two you've picked, syenite and diorite, are quite easy to identify from each other in that diorite is a non alkaline rock and syenite is an alkaline rock so a diorite would contain quartz, up to ten percent maybe, about, a similar amount or slightly more pyroxene, perhaps some of the mineral amphibole, which is a bit enigmatic in its appearance but comes in in rather intermediate rocks sometimes, and a lot of feldspar, plagioclase feldspar. The rock syenite is different in that it contains perhaps up to ten percent quartz, but you've got the appearance of alkaline minerals. You would get bright green pyroxenes, micas, and alkali feldspars, the pink ones, in syenite.

And then there's a group of four rocks which you might term acidic. These are going to contain a lot of quartz, and no pyroxenes. Pyroxene can't exist in a rock which has got a lot of silica or acidic as they're termed and these would contain no pyroxene or negligible pyroxene. The main components are going to be micas, quartz and feldspars of some description. The easiest one to take first is the granodiorite. As the name suggests it's an intermediate between granite and diorite. Er.. you might have the odd pyroxene. The amount of quartz would be less than in the granite. It's basically a case of not being able to put it in one or the other, it's a half way stage. The granite would have no pyroxene, lots of quartz, lots of both types of feldspar, both alkaline and ordinary plagioclase feldspar and lots of mica. That's quite easy. Microgranite would be identical to granite only finer grained, as the name suggests. So we'd have the same mineralogy but finer grained. A mixture of quartz, feldspar and mica.

How do you draw the line between medium grained and fine grained? Do you have any absolute measurements that you use for that?

Er, well I suppose, yeah. well, probably about, the thing is it's different in different rock types, I think. I would personally put the boundary at somewhere about five millimetres. Five to eight millimetres, something like that. Perhaps a centimetre at absolute maximum. That sort of range. Coarse grained rocks have got really quite impressive crystals, you know, granites, whereas microgranites are more like a mosaic of smaller crystals.

Then the final rock type, adamellite, now I couldn't be certain on this, but I've a feeling that adamellites have got a larger percentage of alkaline minerals than granites. I'm afraid this is one I'm not too well up on. I think that adamellites, you would be able to demonstrate a greater percentage of alkaline feldspar for instance than a granite. A granite might have equal amounts. i think an adamellite might have more alkaline feldspar than plagioclase feldspar.

And then we come on to the finer grained rocks which are the extrusive rocks. Those are all intrusive rocks. Now the finer grained rocks, the extrusive ones, often have a, they're often of the same composition to one of these I've just described only finer grained, and this is where it's going to be a lot more difficult because you can't see the minerals so easily.

The two low silica rocks are basalt and picrite basalt. And basalt is basically a fine-grained equivalent of gabbro. The majority of the rock is made up of plagioclase feldspar, and dark green black pyroxene. It's not alkaline, it's calcium rich rather than alkaline. And so you'd be looking for then an interlocking mosaic of small plagioclase and small pyroxene, and sometimes in these rocks you get what's called phenocrysts, which are large grains, just rather unusual large grains and you may get phenocrysts of pyroxene, more commonly plagioclase, or you may get the odd phenocryst of olivine in basalt, which is sometimes occur.

Now the picrite basalt is similar in many ways and is slightly less rich in silica, and I've a feeling that the major manifestation of this would be the apearance of olivine and a lack, or relative lack, of plagioclase. It's what's called a more primitive rock. Less silica. So it would be darker in colour, slightly denser, you'd be able to see more olivine, and there'd be less in the way of plagioclase.

And then this rock dolerite. Now dolerite is again of a similar composition to gabbro and basalt, but it's normally intruded at shallow levels in the earth's crust, so it's not quite as coarse grained as gabbro, which is intruded deep in the earth's crust, but it's not as fine grained as basalt in that that cools quickly on the earth's surface. So dolerite would be the sort of rock you might find in intrusive rocks close to the earth's surface, dykes, sills, things like that. Same mineralogy, mostly pyroxene and plagioclase feldspar, only in the sort of half way stages as far as grain size goes. There's really, I mean you can put a, er.. it's a very arbitrary division as to where the boundary would be between gabbro and dolerite and dolerite and basalt comes. It's more a sort of eye of faith job, that is, you know, just look for a general grain size.

You mentioned cooling and size of crystals. Could you tell us how cooling affects crystal size and also septh below surface and so on?

Yeah. Er.. basically the quicker a lava or, sorry, a magma cools, could be a lava if it's on the surface, the, er, time taken for it to cool will control the size of the grains. If a lava cools slowly then what happens, a magma cools slowly, there are relatively few what's called nucleation sites in the magma, and the crystals start to grow at these nucleation sites and then grow very slowly but to a much larger size than rocks that are cooled quickly. A lava extruded on the earth's surface cools very very quickly, there are lots of nucleation sites appear all of a sudden, and then all of a sudden the crystals grow very quickly but because there are so many nucleation sites they don't grow very large. So basically a slow - cooled rock produces large grains and a quickly cooled rock produces small grains. And this is related to the depth of cooling below the earth's surface. Generally speaking, the deeper something cools, the slower it cools and therefore the coarser it will be.

Right. Now you mentioned magma and lava. If you could just explain in a few words .. where you draw the line..

There's a distinction, I was being a bit sloppy there, the distinction is that magma is beneath the earth's surface, any liquid, molten rock, that is beneath the earth's surface is called magma, and the name changes to lava when it's extruded out.

Right. Then you also mentioned primitive rocks. Does that tie in with the same sort of thing?

Well, er, primitive is a strange word to choose in many ways, but it reflects the degree of evolution that a rock's undergone. Many of these magmas underneath the earth's surface start off in, with quite a similar composition, they're derived from melts from the earth's mantle, or the top of the earth's mantle, which is usually of a fairly similar composition. A primitive magma is one which reflects the composition of its source without undergoing any further modifications. An evolved magma is one which is modified in some way, by contamination, fractional crystallisation, that's where crsytals separate out, fall away, and in that way change their chemical composition. So a primitive magma is one which reflects the composition of its source area rather than becomes modified. And these usually have a similar compositon because most magmas are derived from the top of the earth's mantle, bottom of the crust, which is a similar composition everywhere.

Right. Are there any other points that you'd like to add to what you've said before we come to, back on to identifying alkaline minerals and talking about silica?

Er.., am I to go through these?

Well, things that you'd use in general as techniques for identifying rocks and techniques that you might not have mentioned so far.

Er, I think, er, I think I've covered the main things that you would use in a hand specimen in the field. Obviously, when you get into the area of analytical work, it's a lot more specific.

Yes, I think maybe we should keep off that

Yes, yes I think I've covered the main things you would use in the field. The one thing I might mention is field relations. When you're in the field you may find an exposure of rock, you may look at it and before you go into, assign it, you may care to look around and see how it's, how it's positioned. If you find a gabbro, if it appears in an intrusion, that's fine. Or you may find a basalt and you may, on the criteria I've already distinguished, you may just write down "basalt equals lava" but that's not necessarily the case. You can often get basalts which are intruded as dykes, but because they've cooled rather quicker than normal, they're finer grained and hence could be called basalts rather than dolerites. So the trick is to look how the rock is in relation to its surroundings. If it's a dyke it's obviously an intrusive rock despite the fact that it's termed a basalt. So there are slight overlaps which you can clear up with field evidence.

Could you give us some more examples of that sort of relational evidence that you'll see in the field. You've given us the example of "if it's a dyke, then it's likely to be intrusive"; can you think of any other cases or examples of that same sort of thing?

Yeah, well, intrusions, you can demonstrate things are either intrusive or extrusive in a number of ways. Often when you get and intrusion, you get baking of the surrounding rocks, which you can, it's called a metamorphic aureole, which you can usually pick up on. And chilling of the intrusive rock, now this chilling produces finer grained rock at the margin of the intrusion which gets coarser and coarser inwards. Er, intrusive bodies obviously are surrounded by the rock in which they are host so they're chilled on all sides and they bake on all sides. Extrusive rocks come to rest on the underlying material, they will bake the underlying material and be chilled against the underlying material but any subsequent material which is deposited on top of those won't be affected. so that's one good way for distinguishing say between a lava flow and a horizontal intrusive body called a sill. If the body has chilling on both sides and baking on both sides then it's a sill. If this chilling and baking only occurs at the bottom along with perhaps some weathering on the top you know it's a lava flow.

Could you tell us, just for the record, how you'd know it had been chilled?

Yeah, the chilling, it's the grain size which gives that away. A chilled margin will have finer grain that the core of the intrusion, which will have cooled more slowly.

Right, and are there any other things you can think of in the way of field relations which would be..

Er... let's think, well, as regards sedimentary and metamorphic rocks, field relations are very very important. For sedimentary rocks you tend to look at a whole succession of sedimentary material to work out an evolution of how the rocks have been deposited. You may get a sequence with coarse sandstone and then you might get a fine silt then a shale then you might get a limestone, so you're looking at the whole succession to come up with a picture of how the earth's surface evolved and how these rocks were deposited.

Would that be a big question to go into?

Yeah.

Yeah, right, well if we bypass that for the time being and stick with igneous then. In that case could we come back to how you'd set about identifying alkaline minerals in a bit more detail?

Yeah, em, well the basic thing with alkaline minerals is that each of these mineral groups I've mentioned, that is micas, feldspars, amphiboles, and pyroxenes, they're all distinct types of minerals but they're like a series which have n members and each type, each of these four different types of minerals, has an alkaline n member. So the feldspars have an alkaline equivalent, the pyroxenes have an alkaline equivalent, the amphiboles have an alkaline equivalent and to an extent the micas although they tend to be more alkaline generally.

And, er, and how does the equivalence work?

well, they all have high concentrations of the alkaline elements such as sodium, potassium, things like that, whereas the other members of the same group might have more calcium and magnesium and very little of the alkaline elements. It's basically the presence of sodium and potassium. It changes their appearance, their colour, it's a totally different mineral in effect.

Are there any underlying generalities that they tend to have in common that you could see just by looking at ?

No. No, not really. If there's one thing that perhaps you could wave it slightly, they tend to be rather brighter green and turquoise colours but that of course falls down with alkali feldspars which are pink, in the sense of pyroxenes and amphiboles they're very bright greens, turquoises.

Are there any other minerals which could be confused with them easily? In terms of being bright green and...

Er, no, I don't think so, because they still show the other characteristics of being a pyroxene or an amphibole. They have the cleavages, and the crystal shape and the hardness, it's just that they're a different colour. They may have some slight differences which is a reflection of the change in the lattice of the mineral, but in general it's going to be fairly minor. Most of the pyroxenes for instance are similar in major features, the colour is one of the major differences caused by slight differences in chemistry which the introduction of alkaline elements produces, this bright green colour. They tend to be also a bit more perhaps more needle shaped. That's another thing you might.. they might have slightly different crystal structure in that respect.

And how useful is the crystal structure for diagnosis and identification?

Very very important. If you can look at the crystal form if you get a good crystal which has been well formed the shape of the crystal is a manifestation of the internal structure which is again a manifestation of the chemistry. So if you can identify the crystal shape, the form, how many sides it's got, whether it's a prism, whether it's a tabular shape, what the angles are between the faces, it's very very good for determining which group of minerals they belong to and then you can use other criteria to narrow it down further.

Could we go into that in a bit more detail then about the things that you'd look for in a crystal. You've mentioned the number of sides. What other features are there in a crystal that you'd take into account when diagnosing?

Er.. well there's a whole range of properties. General shape,

Could you give a bit more detail about general shape? What sort of categories shape would break into?

Prismatic, tabular, needle like, platey, or of course when you're dealing with prisms the number of sides is important. A prism could have twenty four sides, it could have five sides, it could ... the minerals generally fall into either one or the other and then if it's a prism, the number of faces.

And one of the other important criteria you'd use is this business of cleavage which I've explained. It's the way a rock, sorry, a mineral fractures naturally, again it's a reflection of the internal lattice. The structure of the crystal. Er.. certain different groups of minerals have usually quite well defined cleavages. Are they at ninety degrees, are they at sixty degrees, is the cleavage very very flaky, there might be a total lack of cleavage in which case you might go to something called fracture. in a mineral which doesn't have a very good cleavage fracture could be like glass, it could be like metal, it's very descriptive but you're just looking for general points. Colour might be helpful although colour's a bit dodgy in that a very small change in chemistry could produce a wild change in colour. So it's perhaps useful generally, but when you're looking at specific crystals there's often crystals which are particularly dodgy on that score. The mineral apatite, which is calcium phosphate, for instance, can be a whole host of colours, depending on.. Things like olivine generally tend to be green, pyroxenes tend to be dark green.

Er, can I just clear up the difference between tabular and platey?

Yeah. Well, tabular's sort of a brick shaped whereas platey would be like sheets.

So platey's thinner than..

Thin sheets, yeah.

Right, er.. Could you tell us roughly which groups tend to have which sorts of cleavages? You said that different groups have different cleavages.

Yeah, yeah, er.. The olivines have a very poor cleavage, is the thing to notice. Er, they fracture rather than cleave, and when they fracture they fracture raher like glass, it's called conchoidal fracture, a series of circular sort of ridges and like sort of swirling pattern, it's called conchoidal fracture. Very very poor cleavage. It has got cleavage but it's poor. Pyroxenes have cleavages at right angles almost, it's actually eighty seven degrees but it's as near as to make no difference in a hand specimen. The amphiboles have cleavage at sixty degrees and a hundred and twenty degrees so they're quite distinct. It's one of the very useful criteria for distinguishing between amphiboles and pyroxenes. The feldspars, they have generally cleavages at right angles, which obviously makes them similar to the pyroxenes but feldspars being white or pink are obviously a lot easier to identify, pyroxenes being green, black in colour. The micas have a very platey lamellar cleavage, they cleave off into thin sheets, platey cleavage. Quartz is a glass effectively, that has no cleavage, and that fractures in the same way as olivine, conchoidal fracture.

Is there anything else about micas that's distinctive?

Mm, they're very soft. Hardness is another good identification criteria. They have a hardness of two or three on the hardness scale which goes up to ten for diamond. It's very very soft.

And where do the others you've mentioned stand in terms of hardness?

Quartz and olivine are both glassy, they stand at about seven. Pyroxenes ,plag- sorry, pyroxenes, feldspars, and amphiboles sort of between five and six.

Right, and would you use hardness on rocks in general ,or are there any problems with just using straight hardness on different types of rocks?

Well, you've got to really test the hardness of one particular mineral. Particular rocks aren't what you could call hard or soft, they only reflect what the individual minerals show. You could have a rock which was a mixture of a very hard mineral and a very soft mineral. If you scratched it you would get, you would come up with the answer of the very hard mineral, so you've got to test individual minerals.

And ..er.. the last thing I said I'd come back to was the question of how you can tell that something's low in silica or high in silica. If you could give us a bit more detail about that.

You mean a rock in general?

A rock in general.

Well, it's basically a bringing together of all that I've said previously, looking at the mineralogy in general.

(Partly inaudible: checked whether the tape was about to run out).

Well, what you would do to decide whether a rock is high, low, or intermediate in silica is identify the minerals as best you can and then identify the percentages in which the minerals occur. Now there's a sort of sliding scale of silica content in the minerals for igneous rocks. Quartz is high in silica, it's pure silica in fact, and then the feldspars are the next ones down, micas, then the amphiboles, then the pyroxenes and then the olivines. And so if you've got a rock which has got lots of olivine in, you're not going to have much silica, if you've got a rock which has a lot of quartz in, yo're going to have a lot of silica. And with this business of alkaline minerals as opposed to calcic minerals you can pigeon hole rocks into high in silica or medium in silica or low in silica and rich in alkalis or poor in alkalis. And so this gives you a double cross check as to the chemistry of the rock. And so that's basically, it's very qualitative but it usually gives a good indication. For instance, I used the distinction between syenite and diorite, is based on the amont of alkalis, whereas the difference between diorite and gabbro, they're similar in alkali content but the difference is in silica.

Right. Are there any points that you think we haven't covered.

Excuse me, did you know that there's still five rocks?

I didn't.

We didn't cover all the rocks.

No, shall I quickly do those? I think it will be very rapid, actually, because lots of these are pretty similar. Er, I'll just basically group them together. Right.

These are still on the fine grained..

These are all fine grained, these are all volcanic rocks, yeah. Now the difference here is going to be in, you can't really identify the minerals very easily cause they're fine grained and the minerals could be only a twentieth of a millimetre across in these. What you're going to be looking for is colour, density, general sort of, the way it looks, whether it's very light or whether it's sort of getting half way. Andesite is a volcanic equivalent of diorite in which case it could have a bit of quartz in, otherwise it's a mixture of feldspars and a few pyroxenes, perhaps the odd amphibole in there. It will be generally, it will be quite dark, because despite the fact that it's got quite a lot of light minerals in, when you mix minerals up, when you get a lot of very fine grained material darker material tends to show more than lighter material so andesite will still look quite dark. But it won't be as desne as some of the basalts for instance. And you might be able to pick out with a hand lens the grains of feldspar. And of course if it's porphyritic, that is if it contains these unusual large crystals it might have some feldspars in it which will give it away.

Then you've got two pairs left, really. Perhaps it's easiest to deal with the rhyolite first. The rhyolite is the volcanic equivalent of a granite. Very very light, low density. Very very light in colour. And the other thing that rhyolite is, because it's got more silica in, because of structural reasons, it forms a more viscous lava. Basalt lava is very runny, rhyolite lava is very sticky and so you're likely to get flow textures preserved in rhyolite, which you don't in basalt. You're likely to get lineations of minerals, almost as though you could see the streams of the crystals going through. So the viscosity of the rhyolite might give you some idea, you might see this flowage, this sort of treacly flow.

Er.. now trachyte, this is where it gets difficult because they're fine grained and they'll have the same density, I mean, rhyodacite, dacite and trachyte, they're all very difficult to tell apart in hand specimen. Trachyte is more alkaline, it'll contain more of these alkaline minerals, particularly potassium rich feldspars, possibly the odd feldspathoid, which is a feldspar like mineral only with less silica than a normal feldspar. Might have a few of those in. The one way you might be able to tell trachyte is that it has this peculiar texture, which is called trachytic flow texture in which you get the feldspars lined up like a series of dominoes across the rock. And what's happened is that the feldspars have crystallised and then been dragged out by the flow of the fluid. It's called trachytic flow texture and it's often quite useful. It's distinctive from rhyolitic flow texture, which is more of a banded appearance rather than this distinct flow of individual grains. But in general trachyte is more alkaline than these two. That's going to be difficult to tell in hand specimen cause you can't identify the minerals so well.

Dacite and rhyodacite,er, dacite, the difference between dacite and rhyolite is it's got less silica, so it'll have less quartz in, might have the odd pyroxene, perhaps a few more amphiboles. Quite a lot of plagioclase feldspar with the odd alkali feldspar. Rhyodacite is obviously half way between the two, very very difficult to tell the difference between those two, very difficult indeed. You'd need chemical analysis, really, I think.

Er..you've mentioned two textures. What other textures are there that you haven't mentioned so far?

Quite a few. I've mentioned one, the porphyritic texture which I mentioned. That is the case where you've got large crystals seemingly alien crystals, in a fine grained ground mass or matrix. What these represent is, they're crystals which have solidified at depth, then the lava has come onto the surface and most of it has crystalised into this very fine grained stuff, but you've still got a few of these phenocrysts left there. That's one, that's porphyritic texture.

Which of those rocks do you tend to find that one in?

Well, the porphyritic texture could apply to any of the volcanics but in general you tend to find them in, I would say that you're more likely to get basalts and andesites as being porphyritic rather than rhyolites and dacites simply because the minerals which produce phenocrysts are more likely to cool at the same, a different temperature to the general magma of basalt and andesite. Porphyritic rhyolites and dacites, trachytes aren't so common. Because of the way in which the minerals cool. Basalts cool at about three hundred degrees higher sorry, crystalise at about three hundred degrees higher than rhyolites. So if the phenocryst cools somewhere in between there you might not get them in one or the other, it depends on the cooling history. In general, the more silica poor rocks have more phenocrysts.

And er, what other textures are there then after those three?

Well, textures really relate to the way in which minerals interlock. You can get some cases where minerals enclose other minerals, you might have a large grain of one mineral totally enclosing a lot of little grains of another mineral. That's called ophitic texture - o p h i t i c - (spelled out by subject). Or in the case where minerals are partially enclosed it's called poikilitic texture.

Could you give us any examples of that, do you think?

Well, ophitic and poikilitic textures you might find in gabbros, diorites; in general these textures are better, you can pick them out better in the coarser grained rocks, although the phenocrysts may show them in the finer grained rocks. Er, graphic texture, whereby minerals have intergrown at the same temperature to produce a very, a very very, er, easy to identify texture, it's sort of straight intergrowths, looks very, er, sort of blocky. They commonly occur in deep seated rocks. You can get peculiar intergrowths such as myrmekites, which are sort of like worm like intercollations of minerals where the minerals haven't, they're so mixed up and they've intergrown together at the same temperature they haven't been able to form their normal crystal shape, they just form these swirling intergrowths.

Could you give us some more examples of graphic texture from among the sample?

Graphic texture. I think you're most - your tape's finished.

Right, could you give us some examples of graphic texture?

Right. Er, you may get graphic texture in the intergrowth of plagioclase and quartz, for instance. They cool at relatively low temperatures, er, I think quartz crystallises at about seven hundred degrees centigrade, plagioclase slightly higher, but in, if the conditions are right, you may end up with them crystallising together. And you get this intergrowth of square shaped patterns of intergrowths. You might get that for instance in a granodiorite. You may get intergrowths of plagioclase and pyroxene in a similar fashion in a gabbro.

Could you give us some examples of poikilitic textures?

Yeah, poikilitic textures you might, you could find poikilitic textures in any of these, really, particularly I think minerals containing both plagioclase and pyroxenes, you're more likely to get them in the gabbros, diorites, basalts. You get one, you get lots of little crystals of perhaps pyroxene crystalising, and then later in the cooling history because pyroxene crystalises at a higher temperature than plagioclase, lots of little pyroxenes, little pyroxene laths, and then later in the cooling history a big plagioclase crystal grows round them, partially enclosing them, poikilitic texture. That's quite a common occurence between feldspar and pyroxene, so you're likely to get that in the ones containing those. Such as basalts, gabbros, things like that.

Er, and are there any textures that we haven't come to yet? We've done porphyritic and trachytic rhyolite, ophitic, poikilitic, and graphic.

Er, I think you've got the major ones there. There are a whole host of, I mean basically every texture you can name's got a you know, has got a name. I mean, I don't know half of them myself. There's a lot. And they all describe particular ways in which crystals lock together. If there's any glass, you can often get glass in these, and that's called a hyalophilic texture h y a l o philit or phiric, one or the other. Er, and that's where you get glass occuring between the crystals, the glass is where the liquid's been quenched suddenly, the liquid doesn't have time to produce crystals, it just quenches into a glass, and this glass often fills in the spaces betweeen the crystals which have formed earlier, it's called intersertial glass. That's very very common in fact, in fact many many basalts show that texture. Of course you don't get that in the, in these intrusive rocks because the liquid never gets quenched, it cools slowly. The extreme case of course occurs when a lava flows into water, you get quenching suddenly of everything, and you get a volcanic glass such as obsidian produced. And this in turn may what's called devitrify after a number of years and at stress points in the glass you get little crystals starting to grow. And these are called Apache tears.

Do you get those in any of the sample rocks?

No, because none of these are actually glassy. The two main types of glass are the acidic glass, which is obsidian, and the, er , the more silica poor glass, which is called pitchstone. These aren't, these aren't glasses so you wouldn't get that sort of texture. But you can get glass in these, in between the crystals, that intersertial glass.

Right, is there any, which ones do you get intersertial glass in?

Er, the volcanics, particularly I think the basalts and er the basalts and the andesites. The rhyolites you may get more glass more glass so that the glass actually forms a significant proportion of the rock. Because they cool and crystalise at a lower temperature, so when the thing flows into water you may get more of the, more of the lava may still be liquid, so you tend might get a bit more glass.

And how homogeneous do the glasses tend to be? Are they all the same sort of colour, and hardness, and so on?

Well, they obviously depend on the chemical composition, so they vary as much as the individual rock types in their composition and they effectively mimic the liquid. But they're often a very similar colour because in a glass the dark coloured portions which are poor in silica are very finely distributed and like I said earlier this gives the impression of darkness. In a very fine grained rock things tend to look darker because of this even distribution of dark coloured grains or particles.

Right....er.... can you think of any remaining points that we haven't gone over, then, which, any methods that you'd use to be able to identify any particular rock, any things that you'd think about when you were looking at a rock?

Er, nothing specific. I think we've covered the major things there as far as igneous rocks goes. I think we've aso dealt with the other two types. I don't think there's anything specific that we've missed, it's just a general, ?bit( inaudible) you've got to identify minerals, textures, and features on say the surface of the, look at the way in which the rocks relate to perhaps others nearby and then before you jump to conclusions you take all the evidence together then ddo the identification.

Right. And you mentioned in the Q sort something about using unique one off diagnostic features. What sort of things tend to be used as that, is there any sort of general trend?

Yes, the trend is ... I should imagine that just about any unique feature could be used. The presence of a particular mineral is often quite good, like I said in, there's a lot of rocks which contain say garnet, and that changes the name. Er.. percentages are often quite important in geology. Er, whether a rock's got more than ten percent quartz or not, if it's more than ten percent quartz you might call it a granodiorite, if it's less than ten percent you might call it a diorite. Whether it's got more than ten percent alkalis would decide whether it's a syenite or a diorite. It often works in percentages in minerals because there are total, it's just a whole grey area of gradation between rock types, geologists have just had to put boundaries down, twenty five percent, ten percent, so on. It's often identifying percentages which is obviously very qualitative, you're looking you'll pick the rock up in the field and you'll say it's got about twenty five percent so and so, therefore it must be a ... you could say thirty, and it might be a different rock type, it's just how you estimate.

Can we just feed a rule back, to see how we are for necessary sufficiency for rules, for example, if it's low in silica and it contains olivine and it contains pyroxene and it contains garnet and it is coarse grained then it's likely to be lherzolite.

Yeah.

That's sort of necessary and sufficient?

I think the problems with this group you're going to have are in these fine grained acidic ones, rhyodacite, dacite, trachyte, rhyolite, those are the difficult ones.

They're the ones that you tend to need chemical..

Yeah. they're a bit dodgy, because they all look pretty similar.