Actinolite and Tremolite

The word Actinolite comes from the ancient Greek aktis which means beam and refers to the fibrous structure. The suffix is derived from lithos and means rock.
Tremolite and actinolite belong to the tremolite- ferro- actinolite series. Within in this series Tremolit (Grammatite) (Ca₂Mg₅[(OH)₂/Si₈O₂₂]) and Actinolite (Ca₂(Mg, Fe)₅[(OH)₂/Si₈O₂₂]) are the end members of this  solid-solution series. The chemical difference is based on the absence or the containing amount of iron. These minerals are inosilicates (chain silicates) and belonging to the clino- amphibole (calcium- amphiboles) and displaying a typical monocline prismatic crystal habit.
The hardness is around 5 to 6 on the Mohs scale and the specific gravity for Tremolite is around 2,95 - 3,12 and 3,05 - 3,25 for Actinolite.
Tremolite displays more white, grey and pale green colors, whereas Actinolite shows colors ranging from colorless, grey to vivid green and black-green shades. With increasing iron content the colors become darker. Both end members exhibit a perfect cleavage and are often found as twins.
As further sub-varieties of this solid-solution-series exist the chromium- actinolite (smaragdite) and the chromium-manganese-tremolite also called hexagonite.
Amiant or byssolite are terms for fine fibrous crystals from this solid-solution series which are colloquial declared as asbestos (collective name for all fine fibrous needle-like crystals of the serpentine- and tremolite-ferro-actinolite series).
Nephrite is a massive micro-crystalline actinolite-aggregate, which was often mistaken as Jade, but in comparison nephrite is lacking the close grained surface as well as the fine brilliancy, besides these characteristics jade e.g. jadeite is chemically a pyroxene mineral with the Formula NaAlSi₂O₆.
The so-termed Mountain Leather (Mountain Cork, Bergholz) are parallel fibrous actinolite- or tremolite- felts, the term is also used for fibrous serpentine minerals (chrysotile).
In the geology tremolite as well as actinolite are often used as temperature markers, rocks consisting one of these amphiboles, have experienced a moderate and limited temperature throughout their formation otherwise this amphiboles would have been transformed by the metamorphosis into diopside. The most occurrences of the actinolite are originated by metamorphosis. Mostly the parent rock has been a basic rock which went through a regional metamorphosis and finally is transformed into greenschist (collective name for close grained, green schist’s, mainly consisting chlorite, epidote, actinolite, talc, glaucophane and albite-feldspar). The chromium variety of actinolite (smaragdite) is typically found in ultra basic rocks associated with chromite. Although these two minerals are used as temperature marker the grade of metamorphosis is depending not only on temperature and pressure additionally the consistency or contamination (Chromium) of present solid and fluid phases as well as the emerging inter-phases play a major role during the regional metamorphosis.
In general the regional metamorphosis takes place in the epizone (zone of relatively low temperature and high pressure; greenschist facies) in iron- and magnesium-rich parent rock.
Furthermore the formation can take place by contact-metamorphosis, typically in hornfels associated with epidote, titanite, albite and chlorite.
Classical other parent rocks are amphibolite, dolomite and schist. More unusual is the genesis by uralitisation (alteration e.g. paramorphosis of pyroxene into an amphibole by metasomatose) in plutonites, this is a process of restructuring of the pyroxene augite into actinolite, caused by fluid phases migrating through the parent rock.

The tremolite genesis takes place in similar way by contact- or regional- metamorphosis but mainly in clacium-rich metamorph parent rock (dolomite), which was contaminated by quartz.
Also it can form by regional metamorphosis in ultra basic rocks, consisting of olivine and pyroxene. Apart from that tremolite can form in albite-epidote-facies or hornblende-facies by regional-metamorphosis. Also, but rarely found in silicate-poor pegmatite’s.

The actinolite crystals you can see on this website are found in Ohn Bin Yee Htwet mine and partly display a strong green color with fine transparency. Tremolite crystals are found in the same mine but show only colorless to grey- green shades.

Further actinolite occurrences are at the Baikal Lake in Russia, China, New Zeeland, and Silesia.
Tremolite occurrences are in Tessin / Switzerland, Paakila / Finland, Italy and Namibia.

Further informations:

Mindat: Actinolite
Mindat: Tremolite

See all crystals in the Actinolite / Tremolite category