Mica Group - MonoclinicSheet structures – PhyllosillicatesPlaty morphology, perfect basal cleavage – layer atomic structures

Minerals in this group show considerable variation in chemical and physical properties but all are characterized by a platy morphology and perfect basal cleavage, which is a consequence of their layered atomic structure. Phlogopite, muscovite and lepidolite are of consideral economic importance

Phyllosilicates (sheet silicate or layered silicate) A group of silicate minerals characterized by

silicate (SiO4) tetrahedra which are linked together and arranged in the form of a flat sheet (the Greek phyllon means “leaf”). The group includes the mica and clay mineral groups. Phyllosilicates are generally flaky, soft and of low density. They form at relatively low temperature and may replace earlier minerals as a result of alteration They are major constituent of fine grained argillaceos rocks and of some metamorphic rocks.

Mica Group

• Muscovite• Paragonite• Glauconite• Lepidolite• Phlogopite• Biotite• Zinnwaldite• Margarite• Clinotonite• Xanthophyllite• Vermiculite

Most common micas- muscovite, paragonite, glauconite, lepidolite, biotite and zinnwaldite

• General formula X2Y4-6Z8O20(OH,F)4

X = K,Na,Ca, Ba, Rb, CsY = Al, Mg, Fe, Mn, Cr, Ti, LiZ = Si, Al

Structure

• Layer of octahedrally co-ordinated cations is sandwiched between two identical layers of linked (Si,Al)O4 tetrahedra

X Y ZDI-OCTAHEDRAL MICAS

Common Mica Muscovite K2 Al4 Si6 Al2

Paragonite Na2 Al4 Si6 Al2

Glauconite (K, Na)1.2-2.0 (Fe, Mg, Al)4 Si7-7.6 Al1.0-0.4

Brittle Mica Margarite Ca2 Al4 Si4 Al4

TRI-OCTAHEDRAL MICASCommon Mica Phlogopite K2 (Mg, Fe+2)6 Si6 Al2

Biotite K2 (Mg, Fe, Al)6 Si6-5 Al2-3

Zinnwaldite K2 (Fe, Li, Al)6 Si6-7 Al2-1

Lepidolite K2 (Li, Al)5-6 Si6-5 Al2-3

Brittle Mica Clintonite, Xanthophyllite

Ca2 (Mg, Al)6 Si2.5 Al5.5

Paragenesis - Muscovite - Igneous rocks - Granites, Granitic Pegmatites and aplites. Phlogopite, Peridotites Biotite - Gabbros, norites, diorites, granites, pegmatites Lepidolite, Zinnwaldite - Pegmatites and high- temperature veins

Metamorphic Rocks - Muscovite, Paragonite and biotite: Phyllites, Schists and gneisses. Phlogopite: metamorphosed limestones and dolomite Sedimentary Rocks Muscovite and Paragonite - Detrital and authigenic sediments. Glauconite : greensands

Chemistry : MUSCOVITE (Monoclinic) One of the most common micas it can vary considerably in composition, as a result of atomic substitutions. Na always present. Higher the replacement of K, higher temperature of formation. Na content of muscovite may thus be a useful geological thermometers.

Al replaced by Mg and Fe. Less commonly by Cr (giving a bright green colour, var. Fuchsite or mariposite) Diagnositic features: the perfect cleavage .

Sericite is a variety of muscovite which formed from the alteration of feldspar.

Uses of Muscovite

The combination of perfect cleavage, flexibility , elasticity, low thermal conductivity, and high dielectric strength makes muscovite a unique mineral and one which is essential in industry. Used in electrical industry for condensers, as insulating material between commutator segments and in heating elements.

Paragonite• A mineral, related to muscovite. • Wide solvus separates muscovite from paragonite, such

that there is little solid solution along the vector Na+ K+ and apparent micas of intermediate composition is most commonly a microscopic (or even sub-microscopic) intergrowth of two distinct micas, one rich in K, and the other in Na.

• Paragonite is a common mineral in rocks metamorphosed under blueschist facies conditions along with other sodic minerals such as albite, jadeite and glaucophane. During the transition from blueschist to greenschist facies, paragonite and glaucophane are transformed into chlorite and albite.

Glauconite• It is an iron-potassium phyllosilicate of

characteristic green color with very low weathering resistance and very friable.

• Its name is derived from the Greek glaucos (γλαυκος) meaning 'gleaming' or 'silvery', to describe the appearance of the blue-green color, presumably relating to the sheen and blue-green color of the sea's surface. Its color ranges from olive green, black green to bluish green.. It can be confused with chlorite (also of green color) or with some other clay minerals.

• Glauconite is considered a diagnostic mineral indicative of continental shelf marine depositional environments with slow rates of accumulation. For instance, it appears in Jurassic/lower Cretaceous deposits of greensand, so-called after the coloration caused by glauconite.

• It develops as a consequence of diagenetic alteration of sedimentary deposits, bio-chemical reduction and subsequent mineralogical changes affecting iron-bearing micas such as biotite,

• It is also influenced by the decaying process of organic matter degraded by bacteria in marine animal shells. Glauconite forms under reducing conditions in sediments and such deposits are commonly found in near shore sands,

• Glauconite has long been used in Europe as a pigmentation agent for artistic oil paint, especially in Russian "icon paintings".

• It is also found as mineral pigment in wall paintings from the ancient Roman.

• Glauconite, a major component of greensand, is also a common source of potassium in plant fertilizers.

Margarite• Calcium rich member of the mica group • It forms white to pinkish or yellowish gray masses or

thin laminae• It occurs commonly as an alteration product of

corundum, andalusite and other aluminous minerals. • It has been reported as forming alteration

pseudomorphs of chiastolite (variety of andalusite) along with muscovite and paragonite.

• The margarite in this occurrence forms preferentially along the dark graphite rich inclusions with the chiastolite crystals.

Phlogopite

• It is a yellow, greenish, or reddish-brown member.

• It is also known as magnesium mica.• Phlogopite is the magnesium end member of the

biotite solid solution series, • Iron substitutes for magnesium in variable

amounts leading to the more common biotite with higher iron content.

• Similar characteristic properties of biotite.

Paragenesis• Phlogopite is an important and relatively common end-member

composition of biotite. Phlogopite micas are found primarily in igneous rocks, although it is also common in contact metamorphic aureoles of intrusive igneous rocks with Mg-rich country rocks.

• Basaltic association• The basaltic occurrence of phlogopite is in association with picrite basalts

and high-alumina basalts. • Phlogopite is stable in basaltic compositions at high pressures and is often

present as partially resorbed phenocrysts or an accessory phase in basalts generated at depth.

• Ultrapotassic association• Phlogopite mica is a commonly known phenocryst and groundmass phase

within ultrapotassic igneous rocks such as lamprophyre, kimberlite, lamproite, and other deeply sourced ultramafic or high-magnesian melts.

• Phlogopite in this association is a primary igneous mineral present because of the depth of melting and high vapor pressures.

Biotite• Common phyllosilicate mineral within the mica

group, More generally, it refers to the dark mica series,Primarily a solid-solution series between the iron-end member annite and the magnesium-end member phlogopite More aluminous end members include siderophyllite.

• It is sometimes called "iron mica" because it is more iron-rich than phlogopite. It is also sometimes called "black mica”

• Biotite is used extensively to constrain ages of rocks, by either potassium-argon dating or argon-argon dating.

• Because argon escapes readily from the biotite crystal structure at high temperatures,

• These methods may provide only minimum ages for many rocks.

• Biotite is also useful in assessing temperature histories of metamorphic rocks, because the partitioning of iron and magnesium between biotite and garnet is sensitive to temperature.

Biotite occurs in greater variety of gelogical environments than any of the other micas. Formed under a wide range of temperature and pressure conditions, and it occurs abundantly in many regionally and contact metamorphosed sediments. Found in many igneous rocks but most commonly in the intermediate and acidic families. Biotite of volcanic rocks are in general poorer in Fe 2+ and richer in Fe 3+ and Ti than those in their intrusive equivalents.

Lepidolite

- Commonly pale lilac colourCan be distinguished from other micas by its lithium content (flame test) and by its lilac colour and common occurrence in complex granite pegmatites.

Ore of lithium and also used as a raw material in glass and ceramics.

Zinnwaldite

• potassium lithium iron aluminium silicate hydroxide fluoride is a silicate mineral in the mica group.

• Occurrence• It occurs in greisens, pegmatite, and quartz veins

often associated with tin ore deposits. • It is commonly associated with topaz, cassiterite,

wolframite, lepidolite, spodumene, beryl, tourmaline, and fluorite.

Clintonite• is a calcium magnesium aluminium phyllosilicate mineral. • It is a member of the margarite group of micas, "brittle" micas. • It occurs as variably colored, colorless, green, yellow, red, to

reddish-brown masses and radial clusters.• The brittle micas differ chemically from the micas in containing

less silica and no alkalis, and from the chlorites in containing much less water;

• In many respects, they are intermediate between the micas and chlorites.

• Clintonite and its iron-rich variety xanthophyllite are sometimes considered the calcium analogues of the phlogopites.

• Typical formation environment is in serpentinized dolomitic limestones and contact metamorphosed skarns.

• It occurs with talc, spinel, grossular, vesuvianite, clinopyroxene, monticellite, phlogopite, chlorite, quartz, calcite and dolomite.