Tremolite is a relatively common mineral in some metamorphic
rocks. It occurs from the conversion of dolomite, silica
and water into tremolite, calcite and carbon dioxide
by way of the following formula:
5CaMg(CO3)2 + 8SiO2 + H2O ------> Ca2Mg5Si8O22(OH)2
+ 3CaCO3 + 7CO2
It belongs to a series with the minerals actinolite
and ferro-actinolite. A series occurs when two or more
ions can freely substitute between each other. In this
case, when iron is predominant the mineral is ferro-actinolite
and when magnesium is predominant the mineral is tremolite.
Actinolite is the intermediate member and the most common
followed by tremolite. Tremolite's formula is often
written as the same as actinolite (with iron and magnesium),
but specimens of tremolite can be found quite pure (that
is, free of iron) and so here the formula reflects the
pure end of the series. The entire series can be represented
with the actinolite formula: Ca2(Mg, Fe)5Si8O22(OH)2.
Without the iron in the formula tremolite will have
the typical creamy white color. With just a small amount
of iron tremolite will be green. Increasing iron content
will raise the specific gravity, index of refraction
and darken the color. A variety of tremolite is composed
of microscopically fibrous crystals called asbestos.
Other minerals also form asbestos such as serpentine
and the other minerals of the series mentioned above.
Serpentine asbestos is more widely used and of a better
grade in general. Although asbestos has been shown to
cause cancer in humans when inhaled in high enough concentrations,
it still has many valuable applications. Asbestos is
used for fire retardant materials and brake shoes and
pads. Its prior use as insulation has been all but eliminated
due to health concerns. A variety of actinolite, nephrite,
is one of the two minerals called jade. The other jade
mineral is jadeite. Although nephrite is considered
to be actinolite, the lighter shades of nephrite probably
contain significant amounts of tremolite. Another variety
of tremolite is called "mountain leather"
or "mountain cork" and is an oddity in the
mineral world. The tremolite fibers form a felted mass
that has all the appearances and feel of a piece of
cloth. The mountain leather will even have attached
calcite crystals that seem to be knitted in to the fabric.
A violet variety of tremolite is called "hexagonite"
and makes for a nice collection specimen with its attractive
color and crystalline habit. Tremolite is an important
temperature indicator for petrologists. Because at high
temperatures tremolite is unstable and will convert
to diopside, CaMgSi2O6. The presence of only tremolite
and no diopside indicates that the rock has not endured
terribly high temperatures. Tremolite can be a fun and
very interesting mineral to collect.