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Geology is the Way

Tremolite

Monoclinic

Ca2(Mg5-4.5Fe0-0.5)Si8O22(OH,F)2

Tremolite is a calcic amphibole and the magnesian end-member of the tremolite – ferro-actinolite series, a continuous solid solution dictated by the substitution of Mg by Fe2+. Tremolite was named in 1789 in reference to Tremola Valley (Switzerland) by Johann Georg Albrecht Höpfner, a Swiss journalist and pharmacist who investigated some samples that supposedly came from this valley. However, modern investigations concluded that the type material of tremolite actually derived from Campolungo, 13 km to the south.

Structure and chemistry
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Calcic amphiboles are characterized by an empty A-site (the large cation site) that is smaller than in other amphiboles. Mg and Fe2+ occupy the M1, M2, and M3 octahedral sites, whereas Ca occupies the larger M4 sites. The solid solution between tremolite and ferro-actinolite is driven by the substitution of Mg by Fe2+. Tremolite is the magnesium-rich end member, containing no more than 0.5 Fe per formula unit, whereas ferro-actinolite is the ferrous end-member, characterized by more than 4.5 Fe per formula unit. Intermediate compositions, containing between 0.5 and 4.5 Mg (or 4.5 – 0.5 Fe) per formula unit, are known as actinolite. Several other substitutions exist in calcic amphiboles and cause their composition to deviate significantly from the ideal tremolite – ferro-actinolite solid solution. In first order, Al can be incorporated in the structure of calcic amphiboles, via the  substitution of Si by Ca which is compensated either by the substitution of (Mg, Fe) by Al or the incorporation of Na in the A site. This chemical variability can be considered as a solid solution with hornblendes , whose extent is greater at higher temperatures. Other substitutions that occur in calcic amphiboles are Ca by Na, compensated by the coeval substitution of Mg, Fe by Al, and a very limited substitution of Fe, Mg, by Ca. In addition, Mn and Zn may substitute Fe and Mg in the octahedral sites.

Tremolite - talc

Prismatic tremolite crystals surrounded by very fine-grained talc in a tremolite-talc hornfels. CPL. Width: 3 mm. Norsi, Island of Elba, Italy.

Properties
Habit: prismatic, acicular, fibrous
Hardness: 5 – 6
Cleavage: {110} perfect (intersecting at 56° / 124°); {100} parting
Twinning: {100} simple, lamellar: common; {001} lamellar: rare
Color: colorless or grey
Luster: vitreous
Streak: white
Alteration: clay minerals, chlorite, talc, serpentine
In thin section…
α(α^a): ~1.599
β(//b): ~1.610
γ(γ^c = ~28°): ~1.620
2Vα: ~82°
Color: colorless
Pleochroism: non-pleochroic
Birefringence (δ): ~0.027 (second-order colors)
Relief: high
Optic sign:
[Mindat]

Field features

tremolite crystal morphology

Sketch of a tremolite crystal showing the position of crystal axes, refractive indices, and trace of cleavage planes. Modified after Optical Mineralogy: Principles and Practice.

Tremolite in the field occurs as acicular, fibrous (more rarely prismatic) white to grey crystals that easily split in tiny needles thanks to their prismatic cleavage (warning: tremolite is an asbestiform mineral). Larger crystal may show a vitreous luster. Tremolite occurs mostly in impure marbles, calcsilicate rocks, skarns, and metamorphosed mafic and ultramafic rocks.

tremolite schist

Small needles of tremolite in a schist from the Adirondack Lowlands, New York State, USA. Field of view: 2 cm across. Photo © James St. John.

Tremolite in thin section
[Visit this page for more details of the general features of amphiboles]
There are only two transparent, non-pleochroic amphiboles: tremolite and cummingtonite. The two minerals have very similar birefringence and angle of extinction, and both have positive sign of elongation. They differ for the optic sign (tremolite is negative, cummingtonite is positive) and the refraction indices, giving cummingtonite a higher relief. These two amphiboles occur in different metamorphic rocks. Tremolite is found in Ca-rich metamorphic rocks, like marbles, calcsilicate rocks, ultramafites, and metabasites, whereas cummingtonite is restricted to Ca-poor mafic and ultramafic rocks. Another difference is that tremolite is less commonly affected by multiple twinning than cummingtonite.

CPL
CPL
CPL
PPL
PPL

Basal section of tremolite, showing well-developed cleavage planes, surrounded by very fine-grained talc in a hornfels. Note the complete lack of color and pleochroism at PPL and the second order interference colors at CPL. Width: 1 mm. Norsi, Island of Elba, Italy.

CPL
CPL
CPL
PPL
PPL

Prismatic section of tremolite, showing high (second-order) interference colors. Tremolite is here surrounded by talc and brownish mixtures of oxides and clay minerals. Width: 1 mm. Norsi, Island of Elba, Italy.

CPL
CPL
CPL
PPL
PPL

Prismatic and basal sections of tremolite. Width: 1 mm. Norsi, Island of Elba, Italy.

In the video shown below, you can see the oblique extinction and positive sign of elongation (length slow) of tremolite in the same sample. Width: 1 mm.

Occurrence
Tremolite is a common product of contact and regional metamorphism of marbles, where it forms due to decarbonation of dolomite in the presence of quartz or SiO2 supplied by metasomatic fluids. In these rocks, tremolite is stable at amphibolite- and amphibole-hornfels-facies conditions and at higher grade it is replaced by diopside and forsteritic olivine. Tremolite may also form during greenschist-facies metamorphism of mafic and ultramafic rocks. However, in mafic rocks, the higher content of iron generally produces actinolite. In ultramafic rocks, low-grade regional and contact metamorphism produces distinctive parageneses, such as tremolite-talc and tremolite-carbonate-antigorite.

References

        

Petrography websites:
Alexstrekeisen.it
Virtual Microscope

it_IT Italiano
Mineral Properties
Minerals

 

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