Provide a detailed rock description. Must include: - Modal minerology (e.g. 35%

Question

Provide a detailed rock description.

Must include:

- Modal minerology (e.g. 35% quartz, 25% plagioclast etc.),

- Detailed description (e.g. Sample X1 is a light-coloured rock, with pinkish K-feldspar crystals. The sample is coarse grained (grainsize range: 2-10 mm), phaneritic, and has a weakly porphyritic texture, with K-feldspar phenocrysts being larger than the other minerals on average. Plagioclase, K-feldspar and biotite exhibit subhedral microstructures, quartz is anhedral. The rock exhibits no obvious metamorphic fabrics (foliation is absent))

- other features

- A Rock name

Explanation / Answer

Sample X1 is a dark coloured rock with mafic (dark coloured) minerals olivine (60%), pyroxene(10%). The felsic (light coloured) mineral is Plagioclase (30%).

The rock sample is a coarse grained (grain size of dark coloured grains are hard to imagine from the cental portion of sample but can be done from the left side of the sample), anhedral to subhedral grain shapes of plagioclase grains, phaneritic (as grains can be seen by naked eye; phaneritic texture indicates the rock is a plutonic rock if it is an igneous rock), plutonic igneous rock.

The rock is showing interlocking texture. Alignment of elongated plagioclase grains indicate the cumulate texture. The rock exhibits no obvious metamorphic fabrics (foliation is absent).

The name of the rock is peridotite.

(N.B.-The question arises whether this rock is an igneous rock or metamorphic rock?. From the visual observation, someone can easily interpret that the felsic minerals are aligned in a direction so it can be a metamorphic rock ! But i would suggest to break the rock and see whether the light coloured plagioclase grains are in randomly distribution. According to my observation these grains are showing cumulate texture (in which heavier minerals than magma settle to the bottom of magma chamber under the action of gravity forming layers of large sized cumulate grains and this texture is a cumulate texture)).

Related Questions

Navigate

• Browse (All)
• Browse P
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.