THE MINERAL TREMOLITE

  • Chemistry: Ca2Mg5Si8O22(OH)2 , Calcium Magnesium Silicate Hydroxide.
  • Class: Silicates
  • Subclass: Inosilicates
  • Group: Amphibole
  • Uses: Asbestos and as a mineral specimen.
  • Specimens
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.

PHYSICAL CHARACTERISTICS:

  • Color is usually white or gray but can be greenish, colorless, yellow and violet.
  • Luster is vitreous or silky to dull.
  • Transparency: Specimens are translucent to transparent.
  • Crystal System is monoclinic; 2/m
  • Crystal Habits include flattened prismatic and elongated crystals with a dome-like termination that is actually two of the four faces of a prism. Fibrous crystals form radial aggregates, masses and hair like clusters. Also as a felted mass (asbestos and "mountain leather").
  • Cleavage: is perfect in two directions at close to 60- and 120-degree angles (diamond-shaped).
  • Fracture is uneven.
  • Hardness is 5 - 6.
  • Specific Gravity is approximately 2.9 - 3.1 (very slightly above average for translucent minerals).
  • Streak is white.
  • Other Characteristics: Index of refraction is 1.60 - 1.63.
  • Associated Minerals are calcite, grossular, talc and serpentine.
  • Notable Occurrences include the area around Wilberforce, Ontario, Canada; De Kalb, St Lawrence Co., New York, California, Arizona and Canaan, Connecticut, USA; Tyrol and Piemonte, Italy; Tremola Valley, Switzerland (hence the name); Tanzania and Finland.
  • Best Field Indicators are fibrosity (asbestos), color, cleavage, crystal habit and hardness.
TREMOLITE specimens:
(hover for more info)
TREMOLITE specimen tre-1
$ 25.00
Dims: 0.7" x 0.3" x 0.2"
Wt: 9.6 g w/ specimen box
Magadi District, Kenya
This very small thumbnail specimen is basically a single broken piece of a green crystal of iron-rich Tremolite. It has a seemingly hexagonal cross-section, and its two cleavage planes are clearly visible when one looks at the cross-section. They create four complementary angles: two opposing 60-degree angles, and two opposing 120-degree angles. The material within the areas of both 60-degree angles is a paler green color than the rest of the material, due to extreme internal fracturing. The crystal shows a pearly luster and translucence. It is incomplete and heavily internally fractured. It does, however, come from Kenya, which is not a well-known locality for large crystals of this mineral.
no photo
tre-1 ($ 25.00)
Magadi District, Kenya
TREMOLITE specimen tre-2
$ 60.00
Dims: 2.0" x 1.4" x 1.3" (5.1 x 3.6 x 3.3 cm)
Wt: 1.67 oz. (47.5 g)
Harcourt County, Otario, Canada
At least 10 slender, prismatic Tremolite crystals rest on what I think is a feldspar base. Almost all of the crystals appear to be damaged and incomplete- there may be one or two that are intact. Though none appear to have an intact termination, their prism faces represent a good triclinic prismatic form, with well-defined edges and clean faces that possess a vitreous luster. All have a pale-green coloration which seems to be the standard for individual crystals. Along with the Tremolites are 3 or 4 black epidote crystals that are in fair condition, as most are incomplete. They have the standard black coloration and vitreous luster of this material. The host rock shows some definite crystalline form, but most of its surface area is uneven and shapeless.
no photo
tre-2 ($ 60.00)
Harcourt County, Otario, Canada
TREMOLITE specimen tre-3
$ 80.00
Dims: 3.0 x 2.1 x 1.7" (7.6 x 5.3 x 4.3 cm)
Wt: 6.40 oz. (181.5 g)
Colebrook Hill, Rosebury, Tasmania, Australia
Countless tiny Tremolite needles line the crevices and hollows of this hand specimen. These needles to not generally exceed 0.2" (5 mm) in length but are in very good condition due to the protective nature of their surroundings. They are too small for me to effectively study their form, but likely have good monoclinic form. All have a dull, gray-green coloration and a pearly luster- when clustered together, their luster reminds me a little of velour or crushed velvet. The Tremolite is visibly compact in a few areas, too, and is accompanied on the piece by several small ferroaxinite crystals, many of which are exposed and damaged. Their triclinic form is good, though, and all show a dark brown color and a bright, vitreous luster. A small amount of crystalline quartz is also present. The base rock appears to be made up mostly of massive Tremolite, but there are bits of broken Axinites within, along with a small amount of a metallic sulfide.
no photo
tre-3 ($ 80.00)
Colebrook Hill, Rosebury, Tasmania, Australia
TREMOLITE specimen tre-5
$ 30.00
Dims: 5.0 x 4.4 x 2.1" (12.7 x 11.2 x 5.2 cm)
Wt: 1 lb., 2.3 oz. (519 g)
Gumeracha, Southern Australia
This large cabinet specimen consists of a chunk of almost bladed, compact Tremolite. The material shows definite crystalline characteristics, but no actual crystals can be isolated. It has a pale, creamy color over most of its area, but some portions appear to be rust-stained. These stained areas are likely weathered and show a dull luster, whereas those which are freshly exposed possess a bright, pearly luster. There is no base or host material present.
no photo
tre-5 ($ 30.00)
Gumeracha, Southern Australia
TREMOLITE specimen tre-4
$ 40.00
Dims: 2.6 x 2.6 x 1.0" (6.6 x 6.5 x 2.5 cm)
Wt: 6.1 oz. (174 g)
Canaan, Connecticut, U.S.A.
This hand specimen consists of a partly-exposed, fan-like Tremolite aggregate that extends through a granular calcite matrix. This agggregate has dimensions of 2.5 x 2.3 x 0.1" (6.4 x 5.9 x 0.3 cm) and is in fair condition, as it shows considerable breakage along its exposed edges. Though tightly aggregated, it is obviously crystalline and has a white color and an almost silky luster. If exposed, the aggregate would likely be translucent. Canaan, Connecticut is considered one of the better-known U.S. localities for this mineral.
no photo
tre-4 ($ 40.00)
Canaan, Connecticut, U.S.A.
TREMOLITE specimen tre-6
$ 45.00
Dims: 2.4 x 0.9 x 0.7" (6.1 x 2.3 x 1.8 cm)
Wt: 22 g
Brumado Mine, Bahia, Brazil
Several intersecting Tremolite sprays make up this hand specimen. These sprays contain Tremolite needles that are generally in good condition and reach lengths of 1.9" (4.9 cm). They are too compact for effective study, but likely have reasonably good monoclinic form. Their color is a dull gray-white and their luster is silky. There is no host rock present.
no photo
tre-6 ($ 45.00)
Brumado Mine, Bahia, Brazil
 

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