Biotite is a name used for a large group of black mica minerals that are commonly found in igneous and metamorphic rocks. These include annite, phlogopite, siderophyllite, fluorophlogopite, fluorannite, eastonite, and many others. These micas vary in chemical composition but are all sheet silicate minerals with very similar physical properties.
The classic formula for Biotite is:
K(Mg,Fe2+3)(Al,Fe3+)Si3O10(OH,F)2
The group formula including all member minerals:
K(Mg,Fe2+)3(Al,Fe3+)[(Al,Si)3O10](OH,F)2
Individual members are:
Phlogopite:KMg3AlSi3O10)(F,OH)2
Siderophyllite: KFe2+2Al(AlSi2O10)(OH)2
Eastonite: KMg2Al(AlSi2O10)(OH)2
Annite: KFe2+3AlSi3O10(OH)2
Fluorannite: KFe2+3AlSi3O10)F2
Tetraferriannite: K(Fe2+3Mg)(Fe3+,Al)Si3O10)(OH)2
The name "biotite" is used in the field and in entry-level geology courses because these minerals generally cannot be distinguished without optical, chemical, or x-ray analysis.
Biotite is a primary mineral found in a wide range of crystalline igneous rocks such as granite, diorite, gabbro, peridotite and pegmatite. It also forms under metamorphic conditions when argillaceous rocks are exposed to heat and pressure to form schist and gneiss. Although biotite is not very resistant to weathering and transforms into clay minerals, it is sometimes found in sediments and sandstones.
Properties of Biotite
Biotite is very easy to identify, and with a little experience a person will be able to recognize it on sight. It is a black mica with perfect cleavage and a vitreous luster on the cleavage faces. When biotite is separated into thin sheets, the sheets are flexible but will break upon severe bending. When held up to the light, the sheets are transparent to translucent with a brown, gray, or greenish color. Experienced observers can sometimes recognize phlogopite by its brown color.
3D Crystal Atlas
(Click for animated model)
You will normally find the biotite like this in a stone, and not in a big sized mineral as the photos above, so here is what you will be looking for at the stone at gz. (Example photo under, not from gz)
To log this cache.
To get to log this cache you will have to read the cache text and visit the coordinates given. After that you will have to answer the questions which are related to the text and the coordinates given.
When answers are collected, send them to CO for verification. I will accept answers sent via email or through the Message Centre.
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Questions
1. Answer the questions under by visiting the Coordinates.
A. Describe the biotite at GZ with its shape and color?
B. The coordinates lead you to The Royal Plain Fountain stone. Have a look around the monument - what percentage of the rock, do you think is made up by the biotite?
C. What average size are the black 'spots' that you find within the rock?
D. At the back part of the stone, do you think there are more spots to find at this part of the stone than the other side?
2. It’s voluntary to post a photo in your online log. Without revealing any answers!
Royal Plain fountains
The Royal Plain fountains are located on Lowestoft seafront and are a must see for any visitor to the town.
The fountains consist of 74 individual water jets that form a range of displays throughout the day. The attraction is popular with children on a warm summer’s day who can safely play in and out of the water discovering where the next jet of water will come from.
Every 30 minutes from 10am – 10.30pm a musical display takes place with the fountains moving to the sound of the music. These displays really come into their own as night falls and the fountain lights come into effect providing a stunning visual display.
The music is provided by the Royal Philarmonic Orchestra who are the Eastern Region Residents of the towns Marina Theatre.
The fountains are in operation from the 1st April – 31st October.
