This rock is a felsic plutonic rock. Plutonic rocks are coarse grained, with easily distinguishable mineral grains. We can see these clearly in Figures A, B and C. A typical felsic plutonic rock is a granite. Not all granites are the same though!

We can distinguish between different types of granite by looking at the amount of the different minerals in the rock, known as the mode or percentage of minerals.

Note that the white and yellow arrows point to two differently shaped dark coloured (or mafic) minerals in image A. The dark minerals are in a rock dominated by light coloured (or felsic) minerals.

Question 1: Define mafic.

Question 2: Identify the two mafic minerals and add them to the first tow rows of Table 1 in your field trip worksheet.

There are also three abundant felsic minerals in this rock. The red arrows in figures B and C point to simple twin boundaries in two different types of feldspar grains. The white arrow in C points to multiple twinning too. There is a third felsic mineral in this rock (best observed in Figure B) which is slightly grey and is the most transparent mineral.

Question 3: Define felsic.

Question 3b: Identify the three felsic minerals and add them to Table 1 in your field trip worksheet.

Mineral mode is the percentage amount of a mineral in a particular rock or sample. We use mode as a way to determine different types of rock. For example, a rock with an abundance of quartz compared to plagioclase would be a granite, whereas if plagioclase is the dominant mineral we would classify the rock as a granodiorite.

Estimating the mode of each mineral can be tricky and is always subjective! We use charts such as this one to help us give an accurate visual estimation of the mode of a rock.

Using this chart as a guide, look at Figure A and have a go at estimating the mode for each mineral in the rock at site 1 and add that to the mode column in Table 1 of the field trip worksheet.

As geologists we take note of the shape of the mineral grains (also referred to as the habit). The shape will give information including an indication of the order in which minerals formed in a rock.

There are three primary shapes which geologists reference for igneous rocks:

Euhedral: Euhedral crystals are well formed, with 'classic' crystal shapes, and straight uniform edges. these will form early in a melt when there is plenty of space for them to form their preferential habit.

Subhedral: Subhedral crystals are not as well formed as a euhedral crustal, but still have a distinguishable shape and some straight edges.

Anhedral: Anhedral crystals form late in the process when there is a shortage of space. They are non-uniform and are often observed to be filling the gaps between other minerals.

This rock has a porphyritic texture. We see a very fined grained matrix of dark minerals (known as the groundmass) surrounding the large well formed plagioclase feldspar crystals (known as the phenocrysts) .

This rock has an 'interlocking' phaneritic texture (coarse crystals which are interlocked with each other). These large crystals, which are easily distinguished by the naked eye formed by cooling slowly in a magma chamber, allowing time for the crystals to grow.