The Mineral CUMMINGTONITE
- Chemistry: (Mg, Fe)7Si8O22(OH)2
, Magnesium Iron Silicate Hydroxide.
- Class: Silicates
- Subclass: Inosilicates
- Group: Amphibole
- Uses: Are limited to some asbestos uses and as mineral specimens.
Cummingtonite is the name of a series as well as the name of a mineral.
The mineral cummingtonite, is basically the middle member of the Cummingtonite
Series, which include the minerals
All three minerals' formulae are essentially the same: (Mg,
However the percentage of magnesium to iron varies in the series and determines
the mineral whereas, little else does. In grunerite, the iron rich member,
the ratio of Mg/(Mg +Fe) equals 0.00
to 0.29; but in magnesiocummingtonite, as the name implies it is
magnesium rich, the ratio equals 0.70 to 1.00. Cummingtonite
by default has a ratio of 0.30 to 0.69, the majority of the
middle and as can be expected is the most common member of the series.
Cummingtonite shares its formula with another mineral called anthophyllite.
Cummingtonite and anthophyllite are polymorphs, a situation where two minerals
share the same chemistry but have different structures (poly=many,
are the most famous examples of polymorphism. In the case of anthophyllite
and cummingtonite, anthophyllite is orthorhombic and cummingtonite is monoclinic.
The two minerals can have similar distinctive brown colors and are hard
to differentiate from each other, although cummingtonite is typically darker
and slightly denser. Despite their distinctive brown color, the two are
often indistinguishable from other amphiboles without optical or X-ray
Cummingtonite is a common component of some regional
If more intense metamorphism were to occur, the minerals hypersthene,
produced at the expense of cummingtonite. Retrograde metamorphism can conversely
restore cummingtonite as well as other iron-magnesium amphiboles.
One form of cummingtonite (a variety called "amosite")
is asbestos-form and can be used as asbestos. Asbestos has many industrial
uses despite some health risks and is made from different minerals, all
with a fibrous habit. Serpentine
asbestos are considered the better varieties due to their greater flexibility
and tensile strength, but cummingtonite asbestos has its uses and is being
mined for this reason in South Africa.
- Color is a dark grayish or greenish-brown and dark green.
- Luster is silky to vitreous.
- Transparency: Crystals are translucent to opaque.
- Crystal System is monoclinic; 2/m.
- Crystal Habits include fibrous, lamellar and radiating masses.
Twinning is common and is either
simple or lamellar.
- Cleavage: is good in two directions at 56 and 124 degree angles.
- Fracture is splintery.
- Hardness is 5 - 6.
- Specific Gravity is approximately 3.1 - 3.6 (average to slightly
- Streak is white.
- Other Characteristics: Non-pleochroic.
- Associated Minerals are
- Notable Occurrences include Cummington (hence the name), Hampshire
County, Massachusetts; Homestake gold mine, Lawrence County, South Dakota
and La Paz County, Arizona, USA; Scotland; South Africa and Sweden.
- Best Field Indicators are crystal habit, fracture, cleavage,
color, density and hardness.