Laddered Grid 2
OK, and so, I'll start by asking you, how, how could you tell that a rock in either hand specimen or thin section was Dacite?
I don't know, 'cos I can't remember what Dacite looks like.
Right, erm, I'll just pick another one then, say Granite?
Erm, I'd look for something course grained, large feldspar crystals, large proportion of pink crystals which would be the feldspar, free quartz visible, course grained.
Free quartz, right, erm, are there any other rocks on that list which are, which are also course grained?
Yeah.
Which ones are they?
... ... Gabbro... well, it might be Syenite or Trachyte, and it's not Andesite, I can't remember which is which.
Right, maybe Trachyte...
No it's not, I know, it's not.
How would you, for example, how would you tell the difference between Granite and Gabbro?
Colouring and minerals, I think.
What is the difference in colour?
Granite's basically light and Gabbro is basically black.
If we could pursue the colour a bit, what other example of light rocks are there on there?
Quartz, feldspar, well ortho-, erm, alkali feldspars, erm, muscovite mica as opposed to biotite mica, whereas in Gabbro it's darker minerals for instance olivine, hornblende, clinopyroxene, plagioclase feldspar and biotite.
So, you'd find those in Gabbro, and you'd find the light ones in...
In Granite, well, there's some overlap, but in general.
Right, so what other rocks have a similar mineral suite to Gabbro?
Erm, Peridotite, Basalt and Dolerite. And Dunite-Peridotite.
Right, so they, they've all got these dark minerals: olivine, hornblende, clinopyroxene, plagioclase?
Yeah
Erm, what other rocks apart from Granite have got the light mineral suite?
Granodiorite, Microgranite, Rhyolite, then things like Andesite and Trachyte all fall between the two in intermediate categories.
Right, what minerals do the intermediate?
Well, the same as Gabbro and Basal-, erm, Gabbro and Granite, but just the dividing line between the two.
Oh right.
The overlap of minerals.
Right, so which rocks are the intermediate?
Andesite, Trachyte (are there other ones?) and Syenite.
Before we go any further, which are the rocks on the list that you've never come across before?
Do you mean that I can't remember, or that I've never come across?
Well, yes, well either, either.
Well I can't remember anything about them: Ademalite, Dacite.. Kentallenite.
That's also called <Monteblende something Gabbro>, does they not mean anything?
Well, I can make a guess at it, but I don't know what the <something>.
Right, OK. Any others that you?
Don't think so, think those are it.
Right, could you, could I go down the list and ask you for the grainsize of each?
Yeah.
Erm, Gabbro.
Course.
Er, Peridotite.
Course.
Basalt.
Fine.
Dolerite.
Med-medium.
Dunite-Peridotite.
Course.
Er, Granite, you've done, Granodiorite.
Medium.
Microgranite.
Microgranite?
Yes.
Medium.
Medium. Rhyolite.
Fine.
Andesite?
Medium, no course.
Er, Trachyte?
Fine.
And Syenite.
Fine.
OK, could I ask you what do you think is the key difference between Gabbro-Peridotite and Dunite-Peridotite, how would you tell the difference between those?
Mineral suite.
Do you know what the difference is?
Peridotite would be two pyroxene and erm, Gabbro could have all sorts of other stuff. I'm not prepared to say <something>.
Right, Gabbro is other. Erm, Dunite-Peridotite, that, would that have two pyroxene and feldspar?
Yeah.
Is there any difference between Peridotite and Dunite-Peridotite?
Doubtlessly, but I don't know what it is.
Right, what is the key difference between Granodiorite and Microgranite?
Minerals, there's more free quartz in Microgranite than there is in Granodiorite, more larger percentage of orthoclase feldspar.
And that's in Microgranite as well, is it?
In Microgranite, yeah.
Yeah. Erm, what about Trachyte and erm Nepheline-Syenite, what's the key difference there?
Minerals.
Do you know what the difference is?
One's more basic and one's more acidic.
Which is the more basic would you say?
Which ones Ande-?
Sorry, Syenite and Trachyte.
I think the Syenite's the more acidic one.
So that's at the more acidic end of intermediate?
Mmm.
Because there's very little difference presumeably between those, they're both intermediate, they're both fine-grained, it's just that one's...
Well, it, it's the overlap in minerals, one will have more free quartz than the other one, so one would be slightly darker.
Right, so Trachyte will look slightly darker?
Mmm, Trachyte will look more like Basalt.
Right... Right, are there any other erm, sort of dimensions upon which you tell the difference between these rocks?
I'd look for colour, mineral, well, mineral, colour, proportion of minerals and grainsize.
Right, and the sort of proportion of minerals relates to the colour, does it?
Well, I suppose, yes.
You've got this sort of, the light colour rocks containing quartz, things like alkali feldspar, muscovite, yeah, so that's a determinant of colour.
Mmm.
Is there anything else that we haven't mentioned in there, that you think that we ought to have mentioned, that would enable us to tell the difference?