ACTINOLITE-TREMOLITE

General Formula:
Ca2(Mg,Fe2+)5Si8O22(OH)2
Sample: M-14 System:
Monoclinic
Actinolite
Low to medium grade amphibolite consisting essentailly of actinolite-tremolite. The large grain in the centre of the image displays two cleavages which intersect at 56 - 124°.
Field of view 2.7 mm, plane light
Actinolite
Low to medium grade amphibolite, notet the range of interference colurs displayed by the actinolite in the image.
Field of view 2.7 mm, crossed polars
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Optical Properties

Colour
Pleochroism
colourless to pale green to dark green, darker colours and stronger pleochroism associated with high Fe contents
X = colourless, pale yellow green
Y = pale yellow-green, pale blue-green
Z = pale green, green, blue-green
Form occurs as columnar, bladed or acicular grains, elongated parallel to c axis, may be fibrous, basal sections are diamond shaped, with typical amphibole cleavage
Relief
RI
moderate to high positive
nalpha = 1.599-1.688
nbeta = 1.612-1.697
ngamma = 1.622-1.705
Cleavage two amphibole cleavages on {110}, intersect at 56 and 124°
Birefringence
Interference Colours
0.017-0.027
maximum interference colours are upper 1st to mid 2nd order
Twinning simple and lamellar twins
Interference Figure
Optic Sign
2V
biaxial
negative
2VX = 75-88°
Optic Orientation X ^ a = +5° to -6°
Y = b,
Z ^ c = +10° to +21°
optic plane = (010)
elongate sections are length slow
Composition the primary compositional variation is the relative proportion of Fe2+ and Mg Alteration alters to talc, chlorite and carbonates
Occurrence common occurrence is in contact and regional metamorphosed limestone and dolomite. Also found in metamoprhosed mafic and ultramafic rocks. It is the common fine-grained alteration product of pyroxenes. Distinguishing Features actinolite closely resembles hornblende, however the latter usually has a smaller 2V angle and may have a higher extinction angle.


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