| General Formula: Ca2(Mg,Fe2+)5Si8O22(OH)2 |
Sample: M-14 | System: Monoclinic |
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| 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. |
