Posted by mining | Posted in mica | Posted on 19-01-2012
The mica group of sheet silicate (phyllosilicate) minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition. The highly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms.
The word “mica” is derived from the Latin word mica, meaning “a crumb” and probably influenced by micare, “to glitter”.
Mica classification
Chemically, micas can be given the general formula
X2Y4–6Z8O20(OH,F)4
in which X is K, Na, or Ca or less commonly Ba, Rb, or Cs;
Y is Al, Mg, or Fe or less commonly Mn, Cr, Ti, Li, etc.;
Z is chiefly Si or Al, but also may include Fe3+ or Ti.
Structurally, micas can be classed as dioctahedral (Y = 4) and trioctahedral (Y = 6). If the X ion is K or Na, the mica is a “common” mica, whereas if the X ion is Ca, the mica is classed as a “brittle” mica.
Trioctahedral micas
Common micas:
Biotite
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.
The physical properties of Biotite
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.
Lepidolite
Lepidolite is an uncommon mica and has only in the past decade become available on the mineral market in large quantities. Lepidolite is an ore of lithium and forms in granitic masses that contain a substantial amount of lithium. The lithium content in lepidolite does vary greatly however and low lithium lepidolite is nearly useless as an ore of lithium. The typical violet to pink color of lepidolite is characteristic and is the only field test available to identify lepidolite from other micas. Pink muscovite or very pale lepidolite may confuse an identification.
Lepidolite, like other micas, has a layered structure of lithium aluminum silicate sheets weakly bonded together by layers of potassium ions. These potassium ion layers produce the perfect cleavage. lepidolite crystals accompany such other lithium bearing minerals such as tourmaline, amblygonite and spodumene and can add greatly to the value of these specimens. A rock made of granular pink lepidolite and red to pink tourmaline is used as an ornamental stone for carving. Single large plates or “books” of lepidolite can have appealling violet color and make attractive mineral specimens.
The physical properties of Lepidolite
Color is violet to pale pink or white and rarely gray or yellow.
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. Lepidolite’s four prism faces and two pinacoid faces form pseudo-hexagonal crystal “books”. The sides of the crystal often tend to tapper. Also as micaseous, lamellar or granular rock forming masses.
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 2.8+ (average)
Streak is white.
Associated Minerals are quartz, feldspars, spodumene, ambygonite and tourmaline especially elbaite.
Other Characteristics: cleavage sheets are flexible and elastic, meaning they can be bent and will flex back to original shape. Also some specimens may show triboluminescence.
Notable Occurrences include Brazil; Ural Mountains, Russia; several African localities and California, USA.
Best Field Indicators are crystal habit, color, cleavage, elastic sheets and associations.