Page 58 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS
AMPHIBOLE GROUP
Ca-rich amphiboles * EXTIN CTION The extinction angle of slow' cleavage varies from 21 o to 11 o depending
ANGLE upon the Mg : Fe ratio; the higher the ratio, the higher the extinction
Tremolite-ferroactinolite Ca,(Mg,Fe),Si 8 0,(0H,F), monoclinic
0.55: 1: 0.29, f3 = 105° angle. Thus y'cl = 21 o to 17° in tremolite and 17° to 11 o in ferroactino-
lite. Most amphiboles are nearly length slow. In most thin sections, the
c prismatic section will rarely be correctly oriented to give a maximum
I extinction angle; for example the extinction will vary from straight on a
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I (100) section to a maximum angle on an (010) section.
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I *TWINNING Amphiboles are frequently simply twinned with { 100} as twin plane.
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This is shown under crossed polars by a plane across the long axis of the
basal section, splitting the section into two twin halves. Multiple twin-
ning on { 100} may also occur.
OCCU RREN CE Tremolite (and actinolite) are metamorphic minerals forming during
both thermal and regional metamorphism, especially from impure
dolomitic limestones. At high grades tremolite is unstable, breaking
down in the presence of calcite to form diopside or in the presence of
dolomite to give olivine. Tremolite-actinolite form during the
------ b = 13 metamorphism of ultrabasic rocks at low grades. Actinolite is a charac-
teristic mineral of greenschist facies rocks, occurring with common
hornblende, and may also occur in blueschist rocks in association with
glaucophane, epidote, albite and other minerals. Amphibolisation (or,
uralitisation) of basic igneous rocks is the name given to the alteration of
pyroxene minerals to secondary amphibole by the pneumatolytic action
of hydrous magmatic liquids on the igneous rocks, and the amphibole so
formed may be a tremolite or actinolite.
Nephrite is the asbestiform variety of tremolite-actinolite. Precious
jade is either nephrite or jadeite.
n. 1.599-1.688
np 1.612-1.697
ny 1.622-1.705 llurnblende series Na 0 _ , Ca,(Mg 3 _,Al,_ 0 )(Si 6 _ 7 Al,_,)O, ( 0 H,F),
I) = 0.027-0.017
(' ( ·ommon' hornblende)
2V. = 86°-65° - ve
The hornblende series is the name given to amphiboles which define a
OAP is parallel to (010)
'field' of composition the boundary end-members of which are rep-
D = 3.02- 3.44 H = 5-6
resented by the four phases:
COLOU R Colourless to pale green (tremolite). Ferroactinolite is pleochroic in
shades of green. hastingsite Ca,Mg 4 Al(Si 7 Al)O,(OH,F),
• PLEOCHROISM Related to iron content - the more iron rich, the more pleochroic the tschermakite Ca,Mg 3 Al,(Si 6 Al,)O,,(OH,F) 2
mineral, with a pale yellow, f3 yellowish green, y greenish blue. edenite NaCa,Mg,(Si 7 Al)O, (OH,F) 2
HABIT Elongate prismatic with aggregates of fibrous crystals also present. pargasite NaCa,Mg.Al(Si 6 Al,)O, (OH,F) 2
*CLEAVAGE The usual prismatic cleavages { 110} and intersecting at 56° on the basal
plane. Iron (Fe'+) may replace Mg in hornblendes but this has been omitted
RELI EF Moderate to high. from the formulae for simplicity. The hornblende field can be rep-
resented in a graph by plotting the number of sodium atoms in the
ALTERATION Common (see introduction).
BIREFRINGENCE Moderate: second order green is maximum interference colour seen on formulae against either the number of aluminium atoms replacing sili-
a prismatic section parallel to (010). con or the number of aluminium atoms replacing magnesium (Fig. 2.6).
INTERFERENCE Large 2V seen on (100) prismatic section. It is best to find an isotropic
FIGURE
section, examine one optic axis and get sign and size of 2V.
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