Structured Interview 3

Well the first one I'm afraid I don't know [Adamellite] - not much help there.

The second one, Andesite, it's a lava, a volcanic lava, it's intermediate composition between acidic and basic, it's tends to be associated with destructive [pac ** something **]

Basalt - it's a lava as well, it's a dark rock and it's basic, a high percentage of irons and magnesiums not so much of quartz, and feldspars I should think - [***]

Dacite, I think, is a type of granite, would be the opposite to a baslat in fact a high silica quartz content, feldspars, I think it's an intrusive rock.

Suppose we had a hand sample and a thin section, would you be able to tell that it's got a high Silica content?

Yes - it tends to have a light colour - if it was in hand specimen that would probably be the first thing I'd notice - it tends to have light minerals rather than things like iron and magnesium - it tends to have fairly large crystals since it's an intrusive rock.

Would a rule be IF a rock has large crystal THEN it is an intrusive rock.

That would be fair to an extent as a rule, there are exceptions but a lava would have fairly small crystals compared with an intrusive rock. .br Diorite-Tonalite - Diorite, again is an intrusive rock, so will have fairly large crystals, visible crystals, it's of intermediate composition and SO will tend to be darker than in colour than the Dacite BECAUSE the high percentage of magnesium and iron not so much quartz - in composition it should be fairly similar to Andesite - but being intrusive would have large crystals.

Andesite is extrusive then?

Yes.

So apart from the crystal size, they tend to look very similar?

Yes I think so. They have similar chemical properties, yes.

O.K., Dolerite - an intrusive basic and it's a very similar composition to basalt, being an intrusive rock will have large crystals.

Basic, what does that mean?

Basic, higher ferro-magnesiums less silica-quartz. Also it's very similar to Gabbro, it will tend to occur in smaller intrusions such as in dykes and sills rather than in huge great batholitic intrusions.

So Dolerite is more likely to occur in small dykes?

Yes, sorry. Dunite-Peridotite, would even be darker in hand specimen - the ultrabasic rocks, ultramafic rocks, have very little quartz, very little silica, very high level of magnesium and iron. It's an intrusive rock so will be fairly crystalline.

This dark colour, and the quartz level are two parallel continua?

It's true to say that a basic rock, one without very much quartz or silica, one with a high level of ferro-magnesium, is dark with one or two exceptions - I don't think you've got any here - Obsidian is. !!!!!!

O.k., Gabbro. It's a very similar chemical composition to Dolerite and to Basalt. It tends to occur in larger intrusions than the Dolerite, and as a result of that [] the crystal structure should be larger. In hand specimen it's dark, tends to have a green tint to it.

What would the green tint tell you?

That you probably had minerals such as olivine and pyroxine in the rock, and they tend to be greenish.

O.k., Granite is the other end of the scale to Gabbro. It's an acidic rock, meaning it's got a high level of silica in it. An intrusive rock, a range in crystal size quite variable, sometimes you get huge crystals, being large batholitic intrusions, something like Dartmoor. They would tend to be of a light colour from the quartz and the feldspars - they are white or transparent but they might also have pink tints to them.

That's both the quartz and the feldspars?

That's the feldspar.

If the feldspar's pink, what does that mean?

Two types of feldspar, orthoclase and plagioclase. The orhtoclase tends to be pink or white, the plagioclase tends to be white.

And if you get a feldspar in a rock, do you tend to get both types?

Yes, I suppose most of the time you would but there are lots of times where you wouldn't you could get a one feldspar granite or a two feldspar granite.

O.k., Granodiorite is in fact very similar to Granite and very difficult to tell apart. It's an acidic, intrusive very similar to Granite.

How would you go about telling the difference?

The difference is in a chemical sense - where......... [] in hand specimen it would be very difficult to tell them apart, it's not a major difference I don't think.

The next one (Kentallenite) I'm afraid I don't know anything about at all.

Microgranite, as the name suggests is a Granite but much smaller crystals due to the fact that it would occur in small intrusions rather than large batholiths like Dartmoor where it's cooled very slowly and there are large crystals, its been in a small dyke or small sill which could be only a couple of feet thick so would have cooled much, much thicker there wouldn't have been enough time for crystals to grow to a large size - so the chemical properties and everything are similar to Granite, are the same as a normal Granite but due to its small size crystals haven't grown to the same size as they would in a normal Granite.

Nepheline-Syenite, well what can I tell you about this? Syenite is very similar to Diorite, it's intermediate, intrusive, it's not got a particularly high levels of silica, fairly high levels of magnesiums, irons, it's an intrusive as Diorite is.

So it would tend to have smaller grain size?

The Syenite has cooled fairly slowly so it has fairly large grains.

And it's not got high silica so it will be very light?

No it will tend to be fairly dark. Now, Nepheline part of it has a dark green colour to it as well.

You mentioned Olivine earlier, and said that that was a green colour as well - say you're looking at hand specimen and thin section - how would you tell whether the green area was nepheline or olivine?

Well in the hand specimen I'm not sure but under a microscope in thin section it would be fairly easy due to various properties. But in a hand specimen probably the only way that you could tell is that you would realise that it was a Syenite is that it would have a fairly large, well higher percentage of Silica than Gabbro would. And so you would class it as intermediate and from that you might know that olivines would never exist in that.

But they do in Gabbro?

Yes, because olivines have very low levels of silica and so wouldn't be found in a rock that contains high levels of silica.

You said that it would be fairly easy to do in thin section, how?

I'm not too sure how Nepheline would look under a microscope but olivine is very distinctive, has very high order colours - greens, tends to have a very distinct outline to the actual crystal, high relief, a distinct shape - a bit like 50p pieces.

So a rule for that might be 'IF this greenish area has very high order colours (greens and blues) in thin section AND very distinct (high relief) outline AND a distinct 50p like shape, THEN it will be olivine.

I think that's true.

O.k., Peridotite, it's ultrabasic, meaning very high level of magnesium and iron, very little silica content.

So it would be dark in colour?

Dark colours yes. It is intrusive, in other words will tend to be crystalline again it might well have a green tint to it - that being olivine.

O.k., Rhyolite, porphyritic. Well Rhyolite is the lava equivalent of a Granite. In other words, an acidic rock, has light colours to it, has the same chemical composition as a granite would, because it's a lava, because it's extruded it cools very quickly and so it's not at all very crystalline. Porphyritic means that it has included parts within it - it means that the lava will have cooled fairly quickly but further down nearer the ground it cools slower and some crystals will grow and others not by the time the lavas cooled so you have a basically fine background - ground mass of the rock but there would be larger crystals within it - and when you see this in hand specimen it's fairly easy to make out, its a very fine crystalline with fairly large crystals within it.

Trachyte...is very similar to Andesite, in the intermediate stage between two hand specimens, in igneous rocks there are Trachyte and Andesite which are two types of lava and there are Diorite and Syenite and to tell them apart I don't really know, I know it's a lava it's intrusive and so it's fairly fine grained and it tends to be a dark colour but how you tell it apart from Andesite I don't know.

Can I just back-track a little? You mentioned pyroxine in relation to Gabbro are there other rocks on the list which contain pyroxine?

Olivine and pyroxine tend to be fairly basic minerals, low in silica content, so olivine will definately be found in Peridotite, possibly pyroxine as well, and I suppose Gabbro, Dolerite and Basalt.

Right. Are you saying you tend to find pyroxine with olivine?

Well the minerals can be quite easily graded as to their silica content. Olivine being at the bottom, pyroxine being the next one up and so on. So yes, it would be quite fair to say that they often occur together.

Those are both low in Silica, so if olivine and pyroxine are present?

The rock is what tends to be called a basic rock.

Can I also ask you about feldspar? You mentioned this in Granite. Particularly you were talking about orthoclase feldspar.

There's orthoclase and plagioclase.

Can you tell me what other rocks are likely to have either of those two?

The feldspars come at the other end of the scale I was talking about so they would tend to be associated with more acidic rocks, the lighter coloured rocks, so Granite, Microgranite. Granodiorite will as well, but I think that the difference between the Granite and the Granodiorite is that the Granodiorite might only have one sort of feldspar.

Which one?

The plagioclase.

But in general you're likely to get both types because they tend to be indicators of acidic rock?

Yes.

Is there anything else that we haven't mentioned that you think is important in discriminating, or do you think that we've covered the main dimensions?

When you're looking at an igneous rock colour is a quite important first impression even though it might be deceptive, crystal size will tell you whether it's an intrusive or an extrusive, if it's got large crystals then you know it's intrusive, if it's crystals then the chances are that it's extrusive. The actual chemical composition if you can actually work it out in hand specimen will be the final deciding factor. So if you have a Granite it's very easy to pick out the feldspars and the quartzes and the micas. You can pick them out, in a lava that's not easy to do. But the colour is quite important then. Lavas as well I suppose when they are acidic they tend to be like treacle and not flow very far very viscous, and then they would have certain features that would be very easy to pick out.

What would they be?

Well we call it ..... if an acidic lava is flowing from a volcana, it would not be able to flow very far and a skin would develop over the lava, and would produce what we call flow banding. And so it would tend to pile up on each other it would not tend to disperse itself, whereas Basalt which would flow for miles and miles and miles would have very different properties, physical properties, it would tend to be very vesicular with lots of holes in it, whereas the Rhyolite wouldn't so that would be a fairly safe way of differentiating between the lavas I suppose.

That's a acidic and a basic lava is it?

Yes.

Where Rhyolite is acidic and Basalt is basic?

Yes.