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Amphibole

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Amphibole is an crucial institution of usually darkish-colored, inosilicate minerals, forming prism or needlelike crystals,composed of double chain SiO4 tetrahedra, connected at the vertices and normally containing ions of iron and/or magnesium in their systems. Amphiboles may be inexperienced, black, colorless, white, yellow, blue, or brown. The International Mineralogical association presently classifies amphiboles as a mineral supergroup, inside which might be businesses and several subgroups.

The minerals of the amphibole group crystallize in the orthorhombic, monoclinic, and triclinic systems, but the crystals of the different species are closely similar in many respects. Chemically they form a group parallel to the pyroxene group, being silicates with calcium, magnesium, and ferrous iron as important bases, and also with manganese and the alkalis. The amphiboles, however, contain hydroxyl. Certain molecules that are present in some varieties contain aluminum and ferric iron. The amphiboles and pyroxenes closely resemble one another and are distinguished by cleavage. The prismatic cleavage angle of amphiboles is about 56° and 124°, while the pyroxene cleavage angle is about 87° and 93°.

Amphibole Origin and Occurrence

Exhibiting an extensive range of possible cation substitutions, amphiboles crystallize in both igneous and metamorphic rocks with a broad range of bulk chemical compositions. Because of their relative instability to chemical weathering at the Earth’s surface, amphiboles make up only a minor constituent in most sedimentary rocks.

Types of Amphibole

Amphibole group

  • Anthophyllite – (Mg,Fe)7Si8O22(OH)2
  • Cummingtonite series
  • Cummingtonite – Fe2Mg5Si8O22(OH)2
  • Grunerite – Fe7Si8O22(OH)2

Tremolite series

  • Tremolite – Ca2Mg5Si8O22(OH)2
  • Actinolite – Ca2(Mg,Fe)5Si8O22(OH)2
  • Hornblende – (Ca,Na)2–3(Mg,Fe,Al)5Si6(Al,Si)2O22(OH)2

Sodium amphibole group

  • Glaucophane – Na2Mg3Al2Si8O22(OH)2
  • Riebeckite (asbestos) – Na2FeII3FeIII2Si8O22(OH)2
  • Arfvedsonite – Na3(Fe,Mg)4FeSi8O22(OH)2

Physical Properties for Hornblende

Chemical ClassificationSilicate
ColorUsually black, dark green, dark brown
StreakWhite, colorless – (brittle, often leaves cleavage debris behind instead of a streak)
LusterVitreous
DiaphaneityTranslucent to nearly opaque
CleavageTwo directions intersecting at 124 and 56 degrees
Mohs Hardness5 to 6
Specific Gravity2.9 to 3.5 (varies depending upon composition)
Diagnostic PropertiesCleavage, color, elongate habit
Chemical Composition(Ca,Na)2–3(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2
Crystal SystemMonoclinic
UsesVery little industrial use

Physical Properties of Glaucophane

Color Grey to lavender-blue.
Streak Pale grey to bluish-grey.
Luster Vitreous
Cleavage Good on [110] and on [001]
Diaphaneity Translucent
Mohs Hardness 5 – 6 on Mohs scale
Diagnostic Properties Distinguished from other amphiboles by distinct blue color in hand sample. Blue pleochroism in thin section/grain mount distinguishes from other amphiboles. Glaucophane has length slow, riebeckite length fast. Darkest when c-axis parallel to vibration direction of lower polarizer (blue tourmaline is darkest w/ c-axis perpendicular to vibration direction of polarizer). There is no twinning in glaucophane. Glaucophane also has a parallel extinction when viewed under cross polars.
Crystal System Monoclinic
Fracture Brittle – conchoidal
Density 3 – 3.15

Optical Properties of Hornblende

Photomicrograph in thin section of hornblende
Property
Value
Formula(Ca,Na)2-3(Mg,Fe+2,Fe+3,Al)5Si6(Si,Al)2O22(OH)2
Crystal SystemMonoclinic, inosilicate, 2/m
Crystal HabitMay be columnar or fibrous; coarse to fine grained.
Cleavage{110} perfect – intersect at 56 and 124 degrees. Also partings on {100} and {001}.
Color/PleochroismPleochroic in various shades of green and brown.  In PPL a thin section of Hornblende ranges from yellow -green to dark brown. Green varieties usually have X= light yellow green, Y=green or grey-green and Z=dark green. Brownish varieties have X=greenish-yelow/brown, Y=yellowish to reddish brown and Z=grey to dark brown.
Optic SignBiaxial (-)
2V52-85°
Optic OrientationY=b
Z^c
Refractive Indices
alpha =
beta =
gamma =
delta =
1.614-1.675
1.618-1.691
1.633-1.701
0.019-0.026
Max Birefringence2nd to 4th order with highest interference colors in thin section in upper first or lower second order.
ElongationPrismatic crystal that can be, but is not necessarily, elongated.  Crystals are often hexagonal.
ExtinctionSymmetrical to cleavages
Dispersionn/a
Distinguishing FeatureCleavages at 56 and 124 degrees which form a distinctive diamond shape in cross section.  Hornblende is easly confused with biotite.  Distiguishing factors are the lack of birds eye extinction and the two distinct cleavages.  Simple twinning is relatively common. Crystal habit and cleavage distinguish hornblende from dark-colored pyroxenes.

Optical Properties of Glaucophane

Glaucophane under the microscope
Color / Pleochroism Lavender blue, blue, dark blue, gray or black. Distinct pleochroism: X= colorless, pale blue, yellow; Y= lavender-blue, bluish green; Z= blue, greenish blue, violet
Optical Extinction  
2V: Measured: 10° to 80°, Calculated: 62° to 84°
RI values: nα = 1.606 – 1.637 nβ = 1.615 – 1.650 nγ = 1.627 – 1.655
Optic Sign Biaxial (-)
Birefringence δ = 0.021
Relief Moderate
Dispersion: strong

Amphibole Uses

The mineral hornblende has only a few makes use of. Its primary use might be as a mineral specimen. However, hornblende is the most plentiful mineral in a rock known as amphibolite which has a huge number of uses. It is overwhelmed and used for dual carriageway construction and as railroad ballast. It is reduce for use as size stone. The highest excellent pieces are reduce, polished, and sold under the name “black granite” for use as building going through, ground tiles, counter tops, and other architectural makes use of.

Distribution

Very widespread, but many locality references lack qualifying chemical analyses. A few historic localities for well-crystallized material include:

  • At Monte Somma and Vesuvius, Campania, Italy.
  • From Pargas, Finland. At KragerÄo, Arendal, and around the Langesundsfjord, Norway.
  • In the USA, from Franklin and Sterling Hill, Ogdensburg, Sussex Co., New Jersey; from Edwards, Pierrepont, and Gouverneur, St. Lawrence Co., New York.
  • From Bancroft, Pakenham, and Eganville,
  • Ontario, Canada.
  • From Broken Hill, New South Wales, Australia.

References

  • Dana, J. D. (1864). Manual of Mineralogy… Wiley.
  • Smith.edu. (2019). Geosciences | Smith College. [online] Available at: https://www.smith.edu/academics/geosciences [Accessed 15 Mar. 2019].
Cite this article as: Geology Science. (2019). Amphibole. [online] Available at: http://geologyscience.com/minerals/amphibole/ [9th December 2019 ]
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