Mugearite

mugearite-sampleMugearite is a fine-grained volcaninc rock. Its composition is similar to basalt, however it is dominated by oligoclase and it might have potassium feldspar which is a mineral that usually is not found in basalt. Mugearite is a rock that is more prone to weathering than basalt, so it is usually not so hard, not so compact and its color is typically paler.

The sample of mugearite shown in the picture was collected at Porto da Cruz.

Where it can be found in Madeira?
It is not a common rock in Madeira. It can only be found at Porto da Cruz. There are 4 distinct areas in Porto da Cruz where mugearite outcrops. These outcrops belong all to the same lava flow. See the mugearite outcrops in the geologic map of Madeira Island:

mugearite-geologic-map

Photos of mugearite outcrops at Porto da Cruz:

mugearite-porto-cruz-1

mugearite-porto-cruz-2

mugearite-porto-cruz-3

When did the lava flow happened?
The mugearitic lava flow happened in the later stages of volcanism in Madeira (this mugearite belongs to the “Unity of Funchal” of the “Upper Volcanic Complex” – volcanism that took place between 1,8 Ma and 7000 years ago). According to Mata (1996) the lava flow occurred about 1,5 Ma ago.

TAS Diagram
The rocks of Madeira Island are plotted in the following diagram of total alkali (Na2O + K2O) versus SiO2 content:
Mugearite-TAS-Diagram
As you can see mugearite is an intermediate rock, more acidic than basalt (which implies lavas with greater viscosity than basalt lava), and it has more Na and K in its composition too. Mugearite is a rock more evolved than basalt (it evolved from a basaltic magma inside a magma chamber through processes of differentiation like crystal fractionation).

Texture
Like basalt, mugearite texture can be typically aphanitic (which means it is not a granular rock, the rock grains are so small that they cannot be observed not even using an hand lens) or porphiritic (this means that large visible crystals, or phenocrysts, float in a fine-grained rock mass).

Mineral composition
Mugearite in Madeira is mainly composed of oligoclase (a type of plagioclase), pyroxene, sanidine (a potassium feldspar) and opaque oxides. The phenocrysts (largers crystals) are oligoclase and pyroxene.

Another sample of a mugearite from Madeira Island can be found here.

A thin section of a mugearite can be observed here at the virtual microscope.

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