Australia’s largest ‘granite’ quarries are at Black Hill near Mannum, where three operators mine an igneous rock of gabbroic composition which, although not technically a granite, is marketed as Austral Black Granite or Imperial Black Granite, depending on the visibility of igneous layering in the final product.
Stone from the Black Hill quarries is a form of igneous rock known as Gabbro. The mineral name Gabbro refers to a large group of dark intrusive igneous rocks chemically equivalent to extrusive basalt. Gabbro is coarse-grained and usually dark-colored, from black to a dark green igneous rock. It is an intrusive rock that means that it has formed as magma, and cooled slowly in the crust.
Biotite is a name used for a large group of black mica minerals that are commonly found in igneous and metamorphic rocks. These micas vary in chemical composition but are all sheet silicate minerals with very similar physical properties. Biotite is a primary mineral found in a wide range of crystalline igneous rocks such as granite, diorite, gabbro, peridotite and pegmatite.
To log this cache you will visit the given coordinates. After that you will have to answer the following earthscience questions. You can log immediately and afterwards send the answers to the cache owner. Please do not include pictures in your log that may answer the questions.
Earthscience Questions - Biotite is black when compared to the other dark minerals in Gabbro.
The Gabbro cube at the listed coordinates has distinctly different surface conditions on the four vertical faces.
A. Examine the four faces. On which face is it easiest to see the biotite?
B. Describe the biotite in the gabbro with its shape and color?
C. What percentage of the rock do you think is made up by biotite?