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Formula: ☐Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22
(OH)2
Mg/(Mg + Fe2+) is 0.5 to 0.9
Byssolite is a fibrous form of actinolite.
Inosilicate (chain silicate), amphibole
Crystal System: Monoclinic
Specific gravity: 3.03 to 3.24 measured, 3.07 calculated
Hardness: 5 to 6
Streak: White
Colour: Green, green-black, grey-green, or black. Colourless, pale green to deep green
in thin section.
Solubility: Insoluble in water and hydrochloric acid
Common impurities: Mn, Al, Na, K, Ti
Environments:
Metamorphic environments
Hydrothermal environments
Actinolite is an amphibole mineral that is produced by low-grade
regional
or contact metamorphism
of magnesium carbonate. Hydrothermal actinolite occurs in veins and as replacement of mafic minerals. In
active geothermal systems hosted by intermediate to mafic volcanic rocks, actinolite is present at temperatures
above 280oC
(AofA).
It may be found in
schist.
Associated minerals are
calcite, quartz,
epidote,
glaucophane,
pumpellyite and lawsonite.
Actinolite is a mineral of the
albite-epidote-hornfels,
prehnite-pumpellyite,
greenschist,
amphibolite and
glaucophane-bearing
blueschist facies.
Localities
The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia,
are skarn deposits at the contact between
granodiorite and volcanic rocks.
Actinolite is a primary silicate that forms aggregates
up to 6 cm, intergrown with diopside and
grossular
(AJM 22.1.33).
At Kwun Yum Shan, Yuen Long District, New Territories, Hong Kong, China, the deposit is a hydrothermal deposit which lies
along a fault zone withi altered acid volcanic rocks, consisting mainly of
chlorite, biotite,
sericite and actinolite with
scattered quartz
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council).
At the Lin Ma Hang mine, North District, New Territories, Hong Kong, China, the
lead-zinc deposit is a hydrothermal
deposit which lies along a fault zone within altered acid volcanic rocks, consisting mainly of
chlorite, biotite,
sericite and actinolite, with
scattered quartz.
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned
iron mine, with
both underground and open cast workings. The iron ores contain
magnetite as the ore mineral and occur predominantly as masses of all sizes
enclosed in a large skarn body formed by contact metasomatism of
dolomitic limestone at the
margins of a granite intrusion. In parts of the underground workings
magnetite is also found in
marble in contact with the
granite. The skarn rocks
consist mainly of tremolite,
actinolite, diopside and
garnet.
Actinolite is the dominant constituent of the skarn rocks and the
chief gangue mineral of the iron ores. It
is intimately associated with tremolite, and other associated minerals include
diopside, biotite,
garnet, chondrodite,
fluorite, magnetite and
pyrite
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
At Coed-y-Brenin deposit, Ganllwyd, Gwynedd, Wales, UK, actinolite occurs within narrow veins, less than 5 mm in width, cutting the
amphibole porphyry. Specimens consist of 2–5 cm
long, plumose sprays of fibrous actinolite, in which 1–2 mm pyrite crystals are embedded.
The actinolite is surrounded by clinochlore and a little
quartz is also present
(JRS 21.117).
In the Bronx, New York City, USA, actinolite occurs associated with quartz and
calcite.
(R&M 84.3.221.
At the Belvidere Mountain Complex in Vermont, USA, actinolite occurs in coarse-grained
amphibolite, as pods along fault-zone contacts with
serpentinite and as fine-grained masses with
chlorite in fine-grained
amphibolite
(R&M 90.6.525-526).
At Goose Creek, Leesburg, Loudoun county, Virginia, USA, there are several mines. Byssolite was quite
commonly found in the Luck
Fairfax Plant, both inside prehnite pipes and surrounding the exterior of
the pipes; it is also commonly found there as matted fibres and as inclusions in
prehnite, quartz and
hydroxyapophyllite-(K).
At the Mitchells quarry large masses of byssolite have been found containing
actinolite crystals to about 0.6 cm across and 3.3 cm in length and clusters of the same. Also suspended
within the byssolite as “floaters” were titanite crystals to 1 cm,
tiny calcite crystals, and tiny clusters of
epidote crystals.
Actinolite crystals to 1.7 cm in length, in and out of matrix, have been collected at the Luck Bealeton Plant
(R&M 98.2.123).
Alteration
Actinolite may be altered to chlorite
(AofA).
chlorite (clinichlore), actinolite
and albite to glaucophane,
iron-poor epidote, SiO2 and H2O
9Mg5Al(AlSi3O10)(OH)8 +
6☐Ca2Mg5Si8O22(OH)2 +
50Na(AlSi3O8) →
25☐Na2(Mg3Al2)Si8O22(OH)2 +
6Ca2Al3[Si2O7][SiO4]O(OH) + 7SiO2 +
14H2O
This is a metamorphic reaction
(DHZ 3 p156).
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