Question about the biotite mica group.?

Question

As I understand Biotite has several endmembers due it is varying formula to account for Mg and Fe variations.When biotiite is found in nature is it usually a mixure of the endmembers or is it only found as being a pure form of one type of its endmembers?

Answer 1

While not classified as a solid solution series as such
Biotite is a common rock forming mineral, being present in at least some percentage in most igneous and both regional and contact metamorphic rocks. The typical black to brown color of biotite is characteristic although it is difficult to distinguish brown biotite from dark brown phlogopite. The two are actually end members in a series that is dependent on the percentage of iron. Phlogopite is iron poor and biotite is iron rich. The darker color and density increase with an increase in the iron content. Biotite tends to form in a wider range of conditions than phlogopite which is limited mostly to ultramafic rocks and magnesium rich marbles and pegmatites. Biotite, like other micas, has a layered structure of iron magnesium aluminum silicate sheets weakly bonded together by layers of potassium ions. These potassium ion layers produce the perfect cleavage. Biotite is rarely considered a valuable mineral specimen, but it can accompany other minerals and compliment them. In Bancroft, Ontario Biotite forms large crystals with green apatite and hornblende. Single large plates or "books" of biotite can grow to considerable size and can make impressive mineral specimens. Weathered tiny crystals of biotite can appear golden yellow with a nice sparkle producing a "fool's Gold" that has fooled many.

PHYSICAL CHARACTERISTICS:
Color is black to brown and yellow with weathering.
Luster is vitreous to pearly.
Transparency crystals are transparent to translucent.
Crystal System is monoclinic; 2/m
Crystal Habits include tabular to prismatic crystals with a prominant pinacoid termination. Biotite's four prism faces and two pinacoid faces form pseudo-hexagonal crystal "books". The sides of the crystal often tend to tapper and can have a "hard candy that has been sucked on, look". Also as lamellar or granular rock forming masses providing the luster for most schists and gneiss.
Cleavage is perfect in one direction producing thin sheets or flakes.
Fracture is not readily observed due to cleavage but is uneven.
Hardness is 2.5.
Specific Gravity is approximately 2.9 - 3.4+ (slightly above average)
Streak is white.
Associated Minerals are quartz, feldspars, apatite, calcite, hornblende, garnets and schorl.
Other Characteristics: cleavage sheets are flexible and elastic, meaning they can be bent and will flex back to original shape.
Notable Occurrences include Bancroft and sudbury, Ontario; Sicily; Russia and many other locallities around the world.
Best Field Indicators are crystal habit, color, cleavage, elastic sheets and associations.

A solid solution series is the compositional range between end-member minerals that share the same basic chemical formula but experience substitution of elements in one or more atomic sites. This substitution occurs when an element in a mineral formula can be replaced by another of similar size and charge to make a new mineral. For example, iron and magnesium can readily replace one another in a mineral. In some cases, the substitution can be complete and range from entirely one element to another element, resulting in end-member mineral compositions. One example is olivine, which can vary from Mg2SiO4 (forsterite) to Fe2SiO4 (fayalite). This is known as complete solid solution. Such a mineral can also consist of any intermediate percentage of either end-member. The compositional range between end-member minerals that exhibit complete solid solution is known as a solid solution series.

Another example of a complete solid solution series is between siderite FeCO3 and rhodochrosite, MnCO3. In this and the olivine example, the cation is replaced. Complete anion substitution series are less common, but one example is given by KCl to KBr. Solid solutions can be more chemically complicated as well, with more than one element being replaced. In plagioclase, a complete series exists between albite, NaAlSi3O8 and anorthite, CaAl2Si2O8. In this case, Na+ is similar enough in size to substitute for Ca2+. Because the charges are different, a shift in the number of aluminum and silicon atoms is required to maintain neutrality.

Complete solid solution is also possible with three endmembers. In the pyroxenes for example, compositional variation among of Ca2+, Mg2+, or Fe2+ is often represented in terms of three simplified components: wollastonite (CaSiO3), enstatite (MgSiO3), and ferrosilite (FeSiO3).

The actual compositional variation of a given mineral that forms a solid solution series may be expressed by the abbreviated mineral names with its proportion subscripted. An olivine that has been analyzed and determined to consist of 25% Mg2+ and 75% Fe2+ would be represented by Fo25Fa75. This composition may also be written in terms of the molecular formula: (Mg0.25Fe0.75)SiO4. Graphical forms are also common. In the case of two end-members, a bar diagram is used and the composition is plotted on the bar. When three endmembers are present, a diagram that places each end-member at the point of an equilateral triangle allows compositional variations to be plotted anywhere within the triangle.

Answer 2

Biotite Mica Chemical Formula

Answer 3

This excerpt from Wikipedia should answer your question:
http://en.wikipedia.org/wiki/Biotite

Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K(Mg, Fe)3AlSi3O10(F, OH)2. More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers include siderophyllite.

Marine52 is right too. Here's the link to his reference:
http://mineral.galleries.com/minerals/silicate/biotite/biotite.htm

But basically, phlogopite is pretty common, especially in granites and pegmatites.Annite is less common. You usually find the more iron-rich micas as the solid solution biotite. Siderophyllite isn't extremely common but you do find it.

Answer 4

The two most common minerals in the Mica family are biotite and muscovite. Biotite is black and dark. Muscovite is white and pearly. I am not aware of any other black mica.