This EarthCache takes you to Boar Lane in Leeds, to look at the stonework of a public house. In order to locate the correct place for this EarthCache, first find the two furthest east large coloumns, look between them, and then look downwards, where you will see what we are here to look at. The lower part of this building is made from granite, which is an igenous rock. There are plenty of EarthCaches which describe this type of rock, which bascially is formed of interlocking crystals, so rather than repeat the record so to say, we are here to look closely at a feature in the granite, namely a large feldspar crystal.
So what is Feldspar?
Well, its the name of a group of silicate minerals, that make up a large proportion of the earths crust. Feldspars can be found in all three types of rocks : sedimentary, metamorphic and igneous. The types of feldspar can be classified as:
Na-Plagiolase (Albite)
Ca-Plagiolase (Anorthite)
Orthoclase
This classification is due to the mineral content of the different types, and chemically they are all silicates of minerals such as barium, calcium, iron, potassium and sodium.
CLEAVAGE
All feldspar minerals have two directions of perfect cleavage. It is usually easy to distinguish plagioclase feldspars because their two planes of cleavage intersect at 90-degree angles, and their cleavage faces often display striations.
Cleavage is the way that a crystal splits along a certain line or plane. 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.
The Types of Feldspar - Plagioclase
This type of feldspar, depends on the percentage of either Albite or Anorthite in it, which can be further described by the predominant mineral in each:
Albite is also known as Na-Plagiolase, or Sodium Plagioclase. Na is the chemical symbol for sodium.
Anorthiteis also known as Ca-Plagiolase, or Calcium Plagioclase. Ca is the chemical symbol for calcium.
There are different types of feldspar depending on the on the percentage of either Albite or Anorthite in it. The percentage for each is presented as % Ab for Albite, and % An for Anorthite.
| Plagioclase Group Minerals |
| Mineral |
% Albite |
% Anorthite |
| Albite |
100-90% Ab |
0-10% An |
| Oligoclase |
90-70% Ab |
10-30% An |
| Andesine |
70-50% Ab |
30-50% An |
| Labradorite |
50-30% Ab |
50-70% An |
| Bytownite |
30-10% Ab |
70-90% An |
| Anorthite |
10-0% Ab |
90-100% An
|
Plagioclase feldspar tends to be a white / grey colour, though it is possible to have other colours, such as yellowish, pink, reddish brown or nearly black. So what can more diagnostic about the type of feldspar is the prescence of striations and what type of cleavage is present.
The Types of Feldspar - Orthoclase
This type of feldspar is often called potassium feldspar or K-spar. K is the chemical symbol for potassium. It consists of aluminosilicate minerals, which include aluminium, silicon and potassium. It can be pink, or white in colour, and can be easily distinguished from the plagioclase feldspars by its lack of striations, more specifically the lack of fine parallel striations, which are usually present on some cleavage surfaces in the plagioclase minerals.
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. Alternatively, should you meet me at an event (I tend to attend the local Leeds geocaching events), I am happy to discuss answers.
1. Please describe the crystal, in the terms of its appearance, shape, size (diameter and length), and colour.
2. Please describe any cleavage lines or striations that you may see?
3. Using the above information, what type of feldspar is there here, please tell me the rationale for this answer.
4. Please could you take a photograph, with either you or your navigation device in the photograph, with the Boar Lane sign on it