Hornblende
Grossular-Amphibole-Group
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°.
Contents
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 Classification | Silicate |
| Color | Usually black, dark green, dark brown |
| Streak | White, colorless – (brittle, often leaves cleavage debris behind instead of a streak) |
| Luster | Vitreous |
| Diaphaneity | Translucent to nearly opaque |
| Cleavage | Two directions intersecting at 124 and 56 degrees |
| Mohs Hardness | 5 to 6 |
| Specific Gravity | 2.9 to 3.5 (varies depending upon composition) |
| Diagnostic Properties | Cleavage, color, elongate habit |
| Chemical Composition | (Ca,Na)2–3(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2 |
| Crystal System | Monoclinic |
| Uses | Very 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
|
Property
|
Value
|
| Formula | (Ca,Na)2-3(Mg,Fe+2,Fe+3,Al)5Si6(Si,Al)2O22(OH)2 |
| Crystal System | Monoclinic, inosilicate, 2/m |
| Crystal Habit | May be columnar or fibrous; coarse to fine grained. |
| Cleavage | {110} perfect – intersect at 56 and 124 degrees. Also partings on {100} and {001}. |
| Color/Pleochroism | Pleochroic 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 Sign | Biaxial (-) |
| 2V | 52-85° |
| Optic Orientation | Y=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 Birefringence | 2nd to 4th order with highest interference colors in thin section in upper first or lower second order. |
| Elongation | Prismatic crystal that can be, but is not necessarily, elongated. Crystals are often hexagonal. |
| Extinction | Symmetrical to cleavages |
| Dispersion | n/a |
| Distinguishing Feature | Cleavages 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
| 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].
