Igneous Leeds....Mica EarthCache

This EarthCache takes you to a busy street in central Leeds, Park Row. Due to the urban environment, your GPS signal may bounce about a lot, so as an aid you are looking for the street wall behind the bus stop outside 11-12 Park Row. Please be respectful to other members of the public. So lets get down to some geology.

The panels that make up the outside of these buildings are made from granite. 

Granite is a common type of igneous rock. Igneous rock  is one of the three main rock types , the others being sedimentary   and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form with or without crystallisation either below the surface as intrusive (plutonic) rocks or on the surface as  extrusive (volcanic)  rocks. Granite can be white, pink, or gray in colour, depending on their mineralogy.  Granitic  rocks mainly consist of feldspar, quartz, mica and amphibole minerals,  which form an interlocking matrix of crystals.

We are here to look for and learn about Mica. 

Mica is the name of a  group of minerals, which are similar in physcial and chemical manner.   They are all silicate minerals, known as sheet silicates because they form in distinct layers. Silicates are minerals that consist of oxygen and silicon.Mica can be found in all three types of rocks : igneous, metamporphic and sedimentary. In the case of this location, the mica is in an igneous rock.

Mica crystals commonly have basal cleavage, though there are different types of cleavage. Cleavage is the way that a crystal splits along a certain line or plane, so a mica crystal can present as having lines running through it. There are however different types of cleavage. 

• Basal cleavage happens when there is only one cleavage plane. When crystals with this type of cleavage are split, they split into thin sheets. 
• Cubic cleavage happens when three cleavage planes intersect  at 90 degrees. When these type of crystals are split they will form cubes. 
• Octahedral cleavage happens when there are four cleavage planes in a crystal. 
• Rhombohedral cleavage happens when there are three cleavage planes which intersect  at angles that are not 90 degrees. 
• Prismatic cleavage happens when there are two cleavage planes in a crystal. 
• Dodecahedral cleavage happens when there are six cleavage planes in a crystal. 

There are many types of Mica, these are some of them: 

Muscovite :  It can be colourless, white, gray, brown, green, yellow or red, and can be transparent or translucent. It is commonly known as white mica.

Biotite :  it is black in colour. 

Lepidolite : it is a lilac-gray or rose colour.  

Phlogopite : it  is a yellow, greenish, or reddish-brown colour. 

Zinnwaldite : it can be Gray-brown, yellow-brown, pale violet, or dark green. 

This being an EarthCache, in order to log it, I ask that you answer some questions. Please send them to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile.

There is a distinct Mica crystal in the granite panel, using the above information please can you look for it, and then answer the below questions. 

1. What size in cm (diameter and length) is the mica crystal?

2. What shape is it? 

3. Please describe its colour.

4. Using the above information, what type of mica do you think it is?

5. Please count the cleavage planes. It may be useful to take a photograph and then enlarge your image to do this. How many are there?

6.