Mica Group of Sheet Silicates
|Biotite is K(Mg,Fe)3AlSi3O10(OH)2 Potassium iron magnesium aluminum silicate hydroxide. Phlogopite is KMg3AlSi3O10(OH)2 Potassium magnesium aluminum silicate hydroxide
|Biotite is typically brown to black while phlogopite is pale yellow to golden brown. Both can be separated into thin translucent sheets, although phlogopite sheets will transmit light more readily than biotite, and less readily than muscovite.
|Single perfect cleavage to produce thin flexible sheets or flakes.
|2.5 to 3 (soft)
|Phlogopite is 2.7 (feels ‘light’), the more iron-rich biotite is only slightly heavier (2.8-3.4)
|Pearly to submetallic, crystals are translucent and usually occur as thin flakes or as thicker ‘books’ of sheets.
|White to gray
Did you know...
Biotite and phlogopite are among the best naturally-occurring insulating materials. Both are widely used throughout industry but small differences in their composition lead to phologite’s widespread use in electrical systems while biotite is only used to insulate heating systems.
Dark colored minerals with a layered crystal structure, biotite and phlogopite both display a single perfect cleavage direction. This cleavage allows biotite and phlogopite samples to be easily separated into thin flexible light-transmitting translucent sheets. Students can effortlessly reduce samples of these minerals to a mound of broken sheets, much to the dismay of earth science teachers struggling to maintain their mineral sample collections.
Description and Identifying Characteristics
Biotite and phlogopite are the darkest members of the mica mineral group. Typically brown to black in color, biotite occasionally occurs in a dark green variety. Phlogopite is lighter in color, typically being pale yellow to golden brown. Both exhibit the perfect single cleavage that characterizes the mica group and allows them to be easily separated into thin flexible sheets that transmit light. Their cleavage is a reflection of the minerals’ crystal structure. This structure consists of aluminum silicate sheets that are weakly bound together by layers of positive ions. In phlogopite, these positive ions are primarily potassium and magnesium, while in biotite they are dominantly potassium and iron with only minor magnesium. Because the chemical bonds between the aluminum silicate sheets are much weaker than those within the sheets, both of these mica minerals can be easily separated into thin sheets that are both flexible and elastic. If bent, they tend to return to their original shape once the applied stress is removed.
Related minerals, biotite and phlogopite are end members of a ‘mineral series’. A mineral series consists of minerals that share the same crystal structure and only differ from one another in their relative proportions of a few key elements. As an example, magnesium and iron ions have the same charge and are roughly the same size, so they can substitute for one another in the crystal structure of the biotite-phlogopite mineral series. Biotite, the more abundant of the two minerals, is the iron-rich end member of the series while phlogopite is the magnesium-rich end member.
In Our Earth: The Geologic Importance of Biotite
A common, widespread, rock-forming mineral, biotite is a significant mineral in granites, diorites, and igneous pegmatites. In pegmatites, biotite crystals can be very large. It also occurs in schists, gneisses, and hornfels that resulted from either regional or contact metamorphism. The mineral is named after Jean Baptiste Biot (1774 - 1862), a prominent French scientist who studied the mica mineral group.
Less common than biotite, phlogopite primarily occurs in marbles that resulted from the metamorphism of dolostone. Phlogopite may also be found in iron- and magnesium-rich ultramafic igneous rocks, metamorphosed ultramafic igneous rocks, and magnesium-rich igneous pegmatites. The mineral name comes from the Greek word ‘phlogopos’, which means "fiery looking".
In Our Society: The Economic Importance of Biotite
Of the two darker micas, phlogopite has more industrial uses than biotite. Its perfect cleavage, flexible nature, and high heat and electrical insulating properties are similar to those of muscovite. Phlogopite is often preferred for use in electrical applications, however, as it wears out at about the same rate as copper elements and has higher heat resistance than muscovite. Unlike phlogopite, biotite conducts electricity because of its iron component. As a result, it cannot be used as electrical insulation. Less valuable varieties of phlogopite and biotite are ground up to be used as filler, lubricant, or as dusting material to keep rubber and asphalt goods from sticking to one another during storage and transport. Altered biotite, produced through hydrothermal or weathering processes, produces vermiculite, a porous material that is used as an insulating material in gypsum wall board and in other heat insulating applications. When heated, vermiculite will expand to many times its initial volume, in an accordion-like expansion of its layered structure. This makes it a very effective insulating agent. Vermiculite is also commonly added to potting soil to improve soil quality and drainage as it readily absorbs liquids fertilizers, herbicides, and insecticides.
Commonly confused with...
Biotite and phlogopite are easily recognized as mica minerals, because of their perfect single cleavage and the ease with which they are separated into thin flexible sheets. Color is the only easy way to distinguish them from one another and other micas like muscovite.
Muscovite is another common mica mineral that has a single perfect cleavage similar to biotite and phlogopite. It also occurs in the same igneous and metamorphic rocks that biotite is found in. However, muscovite is much lighter in color than biotite, typically being colorless to white. Darker shades of muscovite are more often confused with lighter-colored varieties of phlogopite. The only easy way to distinguish the two is by their occurrence. Typically found in granites and granite pegmatites, muscovite also occurs in detrital sedimentary and metamorphosed detrital sedimentary rocks. In contrast, phlogopite usually forms in metamorphosed dolostones or dolomitic rocks, although it can also occur in mafic igneous or metamorphosed mafic igneous rocks such as serpentine.
Chlorite is another mica mineral that might be confused with phlogopite, but chlorite usually has a very distinctive green color. It also cleaves into inelastic brittle flakes rather than the thin flexible sheets of phlogopite and biotite.
Graphite is not a mica but a metallic mineral. In low-grade metamorphic rocks, like phyllite and schist, the two may be confused with one another simply because both reflect light. However, unlike biotite, graphite has a distinctive greasy feel and will easily leave marks if rubbed on paper.