Mica Group of Sheet Silicates
|Chemical Composition||KAl3Si3O10(OH,F)2 Potassium aluminum silicate hydroxide fluoride|
|Color||Usually colorless, white or silver, occasionally yellow or brown.|
|Cleavage||single perfect cleavage to produce thin sheets or flakes.|
|Hardness||2 to 2.5 (very soft)|
|Specific Gravity||2.8-3 (feels slightly lighter than expected for sample size)|
|Luster||vitreous (glass-like) to pearly, crystals are transparent to translucent and usually occur as thin flakes or thicker ‘books’ of many flakes thickness.|
Its perfect single cleavage and easy division into thin flexible sheets usually sets muscovite apart from other minerals, except for other micas. Muscovite can be usually be distinguished from the latter on the basis of its lighter color and very transparent nature.
Biotite is another common mica mineral with a single perfect cleavage similar to that of muscovite. The two often occur together in the igneous and metamorphic rocks, but they can be distinguished by their color. Muscovite is very light colored to colorless, while biotite is much darker (typically brown to black).
Chlorite is another mica mineral that might be confused with muscovite, but chlorite usually has a very distinctive green color. It also cleaves into inelastic flakes, rather than the thin flexible sheets of muscovite.
The mica mineral most similar to muscovite in form, habit and color is phlogopite. Muscovite is usually lighter colored than phlogopite, which tends to be light brown to golden brown in color and is not as transparent as muscovite. The two also differ in their associations. Phlogopite usually forms in metamorphosed dolostones or dolomitic rocks, although it can occur in mafic igneous or metamorphosed mafic igneous rocks such as serpentine. In contrast, muscovite is typically found in granites and granite pegmatites, or in detrital and metamorphosed detrital sedimentary rocks. Trying to definitely distinguish whether a light brown mica sample is muscovite or phlogopite may require chemical tests.
Clear crystals of selenite gypsum may initially be mistaken for muscovite because of their transparency, light color, low hardness, and good single cleavage. Selenite gypsum, however, cannot be separated into thin elastic sheets like those of muscovite.
Graphite can occur in low-grade metamorphic rocks, such as phyllite and schist, where it may have a shimmering gray or silver appearance that can be mistaken for muscovite flecks. Graphite, however, cannot be separated into thin transparent sheets like muscovite. Graphite is also so soft that it will leave marks on paper, while muscovite does not.
Did you know...
Before humans learned to make glass from quartz, the finest window panes were formed from large sheets of muscovite. Although not perfectly transparent, these window panes were far superior to an uncovered opening or window coverings made from cloth and animal bladders. A common rock-forming mineral found in felsic igneous rocks, pegmatites and metamorphic rocks, muscovite is the lightest-colored member of the mica mineral group. Micas are a group of sheet silicate minerals distinguished by their perfect single cleavage. This cleavage allows mica minerals to be easily split into thin parallel sheets. In India and Eastern Europe, muscovite can be found in crystals large-enough to be split into sheets to be used as windowpanes. While our modern society no longer uses muscovite windows, this mineral is widely used throughout the electric and construction industries and even provides the sparkle of many cosmetics.
Description and Identifying Characteristics
Often called ‘white mica’, muscovite is the lightest colored mica mineral. Micas are characterized by a crystal structure consisting of aluminum silicate sheets weakly bound together by layers of positive ions (usually potassium, but sometimes sodium). Because the chemical bonds between sheets are so much weaker than those within sheets, mica minerals can easily be separated into very thin, flexible pieces. This single, perfect cleavage serves to distinguish the mica group from other minerals and so fascinates students that mica samples often have to be continuously replaced as older samples are reduced to a pile of thin flakes. Within the mica group, individual members can be identified by their color. Muscovite is so light in color that light will easily pass through its thin transparent to translucent sheets.
In Our Earth: The Geologic Importance of Muscovite
A common, widely distributed, rock-forming mineral, muscovite is a significant component of most granites, granite pegmatites, and many hydrothermal deposits. It does not occur in other igneous rocks, but is very common in metamorphic phyllites and schists where it is largely responsible for the shiny luster of those rocks. It also occurs in metamorphic gneiss where it contributes to the rock’s light colored bands. Because muscovite is chemically relatively stable, muscovite in granitic igneous rocks may survive the rock’s weathering to become fine sedimentary grains. Despite their fragile appearance, these small pieces of muscovite can survive to become a significant component of some sandstone, siltstone and shale.
Muscovite in igneous rocks is usually associated with quartz, potassium feldspars, biotite and amphibole minerals. In pegmatites, beryl and tourmaline may be added to that list. Within metamorphic rocks, muscovite is typically found with other members of the mica group, such as biotite and chlorite. When present in fine-grained sedimentary rocks, muscovite is intimately intermixed with other mica and clay minerals and it is often impossible to identify muscovite as a discrete mineral. As a result, muscovite in sedimentary environments is usually just part of the undifferentiated ‘clay’ component.
In Our Society: The Economic Importance of Muscovite
Muscovite’s name originated from ‘Muscovy’, the older Russian name for the province of Moscow, where the mineral occurs in large sheets that were used and known as ‘Muscovy glass’. For centuries, in Eastern Europe and India large thin transparent sheets of muscovite taken from coarse pegmatite veins were used for windows in homes and churches until replaced by newer glass production methods. However, because muscovite has such a high thermal resistance, it continued to be used for oven and furnace door “isinglass” windows until very recently, when new synthetic materials finally led to its replacement.
Muscovite’s perfect cleavage, flexible nature, and excellent insulating properties against both heat and electricity make it an essential resource for many industries. In the past, sheets of muscovite were particularly useful as insulating material in radio tubes. Presently they are still widely used in electrical condensers and heating elements.
Like most micas, however, the majority of muscovite is ground up to be used as filler, roofing material, lubricant, or a dusting material to keep rubber and asphalt goods from sticking to one another during transportation and storage. Ground mica also ends up in paint, wallpaper, and some unusual consumer products. Muscovite flakes are used to create the sparkle seen in artificial snow for Christmas decorations and the glitter of many cosmetic products such as nail polish, glitter, glossy lipstick and shimmering eye shadow.
Muscovite in the Upper Midwest:
Through central and northern Minnesota and Wisconsin, muscovite is fairly common as an important component of those states’ granite and metamorphic rock. No museum-quality or economic deposits of muscovite occur in the region however.