Not far from the Shipquay Gate we will find a very interesting stone composition build from different intrusive igneous or another words plutonic rocks. It is a perfect modern urban piece of art which can be used for a short geology lesson. Let us explore a fundamental classification of intrusive igneous rock minerals which can be observed here.

Source: Owne picture (colors have been modified)

Intrusive igneous rocks have solidified from magma deep under the surface of the Earth. The process of solidification took long time allowing crystals to grow to size which can be observed with a naked eye. As we observe crystals in intrusive rocks, we can also identify their main mineral type depending mineral composition. These two main types of mineral clasification are: felsic and mafic.

Felsic minerals

Felsic minerals are a group of light-colored silicate minerals commonly found in igneous rocks. They consist of lighter elements such as silicon (Si), oxygen (O), aluminium (Al), sodium (Na), and potassium (K). 

Common felsic minerals are:

• Quartz: A major component of felsic rocks. (dark or light grey)
• Feldspar: Another common felsic mineral present in intrusive igeous rocks which can be subdivided in two subtypes:
  • Orthoclase: a potassium feldspar (often pink)
  • Plagioclase: Sodium-rich feldspar (often white, grey)
• Muscovite: A type of mica (golden brown, shiny mineral)

The term “felsic” combines terms “feldspar” and “silica”. Feldspar comes from the German “Feldspat”, which means “field mineral”.

Felsic minerals remain molten at lower temperatures due to their composition.

The most common felsic rock is granite.

Mafic minerals

Mafic minerals are a group of dark/colored, sometime even black silicate minerals or igneous rocks that are rich in magnesium (Mg) and iron (Fe). 

Common mafic minerals are:

• Olivine (greenish)
• Pyroxene (dark brown, dark green, black)
• Amphibole (dark green, black)
• Biotite (shiny, dark brown, black)

The word mafic has been formed by compounding the words: noun magnesium and adjective ferric.

Mafic minerals melt at higher temperatures as felsic minerals. 

The most common mafic rocks are basalt and gabbro.

Tasks/Questions:

At ground zero you will find a sculpture consisting of 4 blocks of rock. The picture (modified colors, so you really need to observe the rocks at ground zero) below shows numbers assigned to the individual blocks in the sculpture.  

1. Block with which number has the highest proportion of mafic minerals?
2. Block with which number has the highest proportion of felsic minerals?
3. Block with which number is a granite?

Sources:
https://opengeology.org/petrology/02-igneous-rocks/