Geoscience Reference
In-Depth Information
coarse-gained rocks, easily with a hand lens for medium-
grained rocks, but with diffi culty even with a hand lens for
fi ne-grained rocks.
7.3.1 Petrologic type
For any crystalline igneous rock, you should attempt to identify
the rock type on the basis of its mineral content. You will fi nd
this easier if you have already had some experience diagnosing
hand specimens with the aid of accompanying thin-sections. In
the fi eld, you are unlikely to be able to go much further than
the simple classifi cation scheme provided in Appendix A7,
Figure A7.1. More advanced classifi cation depends on
petrography done using thin-sections and geochemical analysis.
Here are probably the three most useful tips:
7
•
In medium- or fi ne-grained rocks, pyroxene and amphibole
can be particularly diffi cult to tell apart. Don't worry - it
doesn't matter much. More than 40% of any mafi c mineral
means your rock is mafi c; 20-40% makes it intermediate.
•
In igneous rocks quartz crystals are usually clear (unlike
quartz in veins, which can be milky), but feldspar is often
altered so that it looks white. If the plagioclase is still fresh
it can be clear like quartz. However, on a freshly broken
surface of a hand specimen some of the fractures in
plagioclase will be fl at cleavage planes that will catch the
light as you turn the specimen, whereas all fractures in
quartz will be irregular and curved.
•
Most igneous rocks contain some plagioclase feldspar. If
you can identify this, estimating its percentage will help to
determine the rock type.
7.3.2 Mineral texture and fabric
You should of course check that your rock has an igneous
texture. Except for glassy lava and the matrix in pyroclastic
rocks, this means the rock should have interlocking crystals.
However, it is not true, despite what some texts suggest, that
igneous rocks must necessarily lack foliation. In fact, you
should look carefully for signs of mineral alignment or
arrangement in planes. This may be more subtle than in
metamorphic rocks, but if present it can tell you a lot about the
emplacement process.
'Flow banding' or 'fl ow foliation'
This is a result of shear in pyroclastic deposits (Figure 7.20) or
viscous lavas such as andesite, dacite or rhyolite, so that crystals
(or, in obsidian, the glassy fabric defi ned by microlites, bubbles,
