Ferro-actinolite is the

ferrous In chemistry, the adjective Ferrous indicates a compound that contains iron(II), meaning iron in its +2 oxidation state, possibly as the divalent cation Fe2+. It is opposed to " ferric" or iron(III), meaning iron in its +3 oxidation state, suc ...

iron-rich

endmember An endmember (also end-member or end member) in mineralogy is a mineral that is at the extreme end of a mineral series in terms of purity of its chemical composition. Minerals often can be described as solid solutions with varying compositions of ...

of the actinolite-tremolite continuous solid solution series of the double chain calcareous

amphibole Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is ...

group of

inosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consid ...

minerals. All the series members belong to the

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...

crystal system. The following formula comparison indicates the position of individual well-known members within the series: :

tremolite Tremolite is a member of the amphibole group of silicate minerals with composition: Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Tremolite forms a series with actinolite and fe ...

: ☐Ca₂(Mg_5.0-4.5Fe²⁺_0.0-0.5)Si₈O₂₂(OH)₂ :

actinolite Actinolite is an amphibole silicate mineral with the chemical formula . Etymology The name ''actinolite'' is derived from the Greek word ''aktis'' (), meaning "beam" or "ray", because of the mineral's fibrous nature. Mineralogy Actinolite is ...

: ☐Ca₂(Mg_4.5-2.5Fe²⁺_0.5-2.5)Si₈O₂₂(OH)₂ :ferro-actinolite: ☐Ca₂(Mg_2.5-0.0Fe²⁺_2.5-5.0)Si₈O₂₂(OH)₂ Some other substitute

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

s that may replace either Ca, Mg, or Fe include potassium (K), aluminium (Al), manganese (Mn), titanium (Ti), and chromium (Cr). A fluorine (F) anion may partially replace the hydroxyl (OH).

Physical properties

Ferroactinolite prisms are much darker in color than actinolite due to their higher iron content affecting opacity, but may be dark green in thin slices or around the edges. Its crystals are brittle, with a hardness of 5–6 on the

Mohs scale The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by ...

, and have a white streak. Ferroactinolite is pleochroic and has a higher

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, ...

and surface relief than actinolite.

References

Mindat.org - Ferro-actinolite

Ferro-actinolite in the Handbook of Mineralogy

IMA Master List
Amphibole group {{mineral-stub