Ontario Mineral - Biotite

Rocks minerals Ontario Biotite

K (FE, Mg)3 AlSi3 O10 (F, OH)2, Potassium iron magnesium aluminum silicate hydroxide fluoride.
heat insulator for industrial purposes.
vitreous to pearly
black to brown and yellow with weathering
crystals are transparent to translucent
Crystal System:
monoclinic; 2/m
Crystal Habits:
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.
not readily observed due to cleavage but is uneven.
2.5 - 3
Specific Gravity:
approximately 2.9 - 3.4+ (slightly above average)
Other Characteristics:
cleavage sheets are flexible and elastic, meaning they can be bent and will flex back to original shape.
Associated Minerals:
quartz , feldspars , apatite , calcite , hornblende , garnets and schorl
Local Occurance:
Bear Lake Occurrences, Burgess Mine, Davis Hill, Davis Quarry, Egan Chute, Goulding-Keene Quarry, Grace Lake Roadcut, North Baptiste Lake Occurence
Best Field Indicators:
crystal habit, color, cleavage, elastic sheets and associations


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.