Description:
The micas are an important group of minerals. They
represent the classic phyllosilicate mineral and are
usually the first minerals to be thought of from this
subclass of the Silicates Class. Micas are significant
rock forming minerals being found in all three rock
types: igneous, metamorphic and sedimentary. Because
thin flakes of mica are generally flexible and brittle,
it is surprising how resistant and durable mica crystals
can be in withstanding high temperatures and pressures
in metamorphic regimes as well as the punishment of
erosional environments. The term "mica" is
so familiar to the general public that it is often considered
a mineral in itself. Of course it is actually a group
of minerals and most people who are knowledgeable about
minerals know the three most common mica minerals: muscovite,
biotite, and lepidolite and perhaps a few of the less
common micas glauconite, paragonite, phlogopite and
zinnwaldite. The Mica Group is actually a rather large
group of minerals with over 30 members. Being true phyllosilicates,
all micas are thus composed of sheets of silicate tetrahedrons.
The silicate sheets are composed of interconnected six
membered rings. These rings are responsible for the
micas typical six sided pseudohexagonal symmetry, in
actuality they are only monoclinic or triclinic. Each
tetrahedron in the rings shares three of their oxygens
with three other tetrahedrons and all the tetrahedrons
in a given sheet point their unshared oxygen in the
same direction. The structure of micas is stacked like
a building with several different layers. Two tetrahedral
layers (T) with their tetrahedral points pointing toward
each other, sandwich small metal ions such as aluminum
in an octahedral layer (O). This tetrahedral-octahedral-tetrahedral
(TOT) sandwich is stacked with layers of large cations
such as potassium or calcium. This cation layer is known
as the interlayer (i) because it is between the TOT
sandwich layers. This i layer, is needed to balance
the formula due to the substitution of the +3 charged
aluminum for +4 charged silicons in the T layers. The
whole structure can then be illustrated with the following
sequence of layers: ...iTOTiTOTiTOTiTOTi... The Mica
Group minerals are closely associated with the clay
minerals. The clays have a similar structure but include
brucite and gibbsite layers between their silicate layers.
However, often crystals will be intergrown with mica
and clay layers forming a composite crystal; as in a
few layers of pure biotite and then a few layers of
pure vermiculite, then biotite, etc, etc. This type
of arrangement in a single crystal is making identification
and classification of these minerals extremely complicated
and confusing. The mica minerals muscovite, glauconite
and illite are often considered clays due to their clay
like properties. The general formula for the micas is
AB2-3(X, Si)4O10(O, F, OH)2. The tetrahedral layers
by themselves have a formula of (X, Si)2O5. In most
micas the A ion is usually potassium but can also be
sodium, calcium, barium, cesium and/or ammonium. These
ions occupy positions in the interlayer i discussed
above. The B ion can be either aluminum, lithium, iron,
zinc, chromium, vanadium, titanium, manganese and/or
magnesium. These ions occupy positions in the octahedral
layers O. The X ion is usually aluminum but can also
be beryllium, boron and/or iron (+3) and they sit in
the center of the tetrahedrons substituting for silicons
by up to 50%. There are three major divisions within
the Mica Group; The True Micas, The Brittle Micas and
the a new division called The Interlayer-deficient Micas.
The True Micas have a majority of singularly charged
ions in the A position (ions such as potassium and sodium).
The Brittle Micas have a majority of doubly charged
ions in the A position (ions such as calcium or barium).
The Interlayer-deficient Micas, which used to be called
the Hydromicas, have fewer i ions than other micas,
hence the name. The three divisions can further be divided
into dioctahedral and trioctahedral groups. The B ions
occupy octahedrally coordinated sites, bonded to six
oxygens and two of the extra non-tetrahedral anions
(hydroxide, fluorine and/or extra oxygen ions). Dioctahedral
micas have two (or less than 2.5) B ions in their formulas,
whereas trioctahedral micas have three (or at least
2.5 or more) B ions in their formulas.
These are
the Mica Group Members: The True Micas:Dioctahedral:
Aluminoceladonite (Potassium Aluminum Magnesium Iron
Silicate Hydroxide)
Boromuscovite (Potassium Boro-silicate Hydroxide)
Celadonite (Potassium Iron Magnesium Silicate Hydroxide)
Chromphyllite (Potassium Chromium Aluminum Silicate
Hydroxide Fluoride)
Ferro-aluminoceladonite (Potassium Aluminum Iron Magnesium
Silicate Hydroxide)
Ferroceladonite (Potassium Iron Magnesium Silicate Hydroxide)
Muscovite (Potassium Aluminum Silicate Hydroxide)
Variety: Fuchsite
Nanpingite (Cesium Aluminum Silicate Hydroxide)
Paragonite (Sodium Aluminum Silicate Hydroxide)
Roscoelite (Potassium Vanadium Aluminum Silicate Hydroxide)
Tobelite (Ammonium Aluminum Silicate Hydroxide)
Trioctahedral:
Annite (Potassium Iron Aluminum Silicate Hydroxide)
Aspidolite (Sodium Magnesium Aluminum Silicate Hydroxide)
Biotite (Potassium Magnesium Iron Aluminum Silicate
Hydroxide Fluoride)
Eastonite (Potassium Magnesium Aluminum Silicate Hydroxide)
Ephesite (Sodium Lithium Aluminum Silicate Hydroxide)
Hendricksite (Potassium Zinc Aluminum Silicate Hydroxide)
Lepidolite (Potassium Lithium Aluminum Silicate Fluoride
Hydroxide)
Masutomilite (Potassium Lithium Aluminum Manganese Silicate
Fluoride)
Montdorite (Potassium Iron Manganese Magnesium Aluminum
Silicate Fluoride)
Norrishite (Potassium Lithium Manganese Silicate)
Polylithionite (Potassium Lithium Aluminum Silicate
Fluoride)
Phlogopite (Potassium Magnesium Aluminum Silicate Hydroxide)
Preiswerkite (Sodium Magnesium Aluminum Silicate Hydroxide)
Siderophyllite (Potassium Iron Aluminum Silicate Hydroxide)
Tainiolite (Potassium Lithium Magnesium Silicate Fluoride)
Tetra-ferri-annite (Potassium Iron Silicate Hydroxide)
Tetra-ferriphlogopite (Potassium Magnesium Iron Silicate
Hydroxide)
Trilithionite (Potassium Lithium Aluminum Silicate Fluoride)
Zinnwaldite (Potassium Lithium Iron Aluminum Silicate
Fluoride Hydroxide)
The
Brittle Micas:Dioctahedral:
Chernykhite (Barium Vanadium Aluminum Silicate Hydroxide)
Margarite (Calcium Aluminum Silicate Hydroxide) Trioctahedral:
Anadite (Barium Potassium Iron Magnesium Aluminum Silicate
Hydroxide)
Bityite (Calcium Lithium Aluminum Beryllium Silicate
Hydroxide)
Clintonite (Calcium Magnesium Aluminum Silicate Hydroxide)
Kinoshitalite (Barium Magnesium Aluminum Silicate Hydroxide)
The Interlayer-deficient Micas:Dioctahedral:
Brammallite (Sodium Aluminum Silicate Hydroxide)
Glauconite (Potassium Sodium Iron Aluminum Magnesium
Silicate Hydroxide)
illite (Potassium Aluminum Silicate Hydroxide) Trioctahedral:
Wonesite (Sodium Magnesium Aluminum Silicate Hydroxide) |