Hornblende

(Ca,Na)2-3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH)

by Robert Haynam



Provenance
Composition
Physical Properties
Crystallography

Optical Properties
XRD
Pictures

References
Links



Provenance:
Hornblende is an important rock forming silicate mineral common in many igneous and metamorphic rocks. It can found in rocks such as granite, syenite, diorite, gabbro, basalt, andesite, gneiss and schist. It is an essential component in amphibolites (medium-grade metamorphic). The name comes from an old German word for any dark prismatic mineral occurring in ores but not containing any recoverable metal. It is derived from the German words horn, a possible reference to the color of horn, and blenden, to “dazzle” or “deceive” because it resembles minerals in some metallic ores.

Similar Species: There exists a large chemical variation in the hornblende series and though the compiled data has been for the more common members of the series the also exist edenite, paragisite, hastingsite, and tschermakite. These have slightly differing chemical composition and chemical/optical properties from the more common magnesio/ferro hornblende series but fall under the general term hornblende. They are all a variation of the general formula given for hornblende.    

 

 

Economic importance: Hornblende has minimal economic importance aside from being a component of many rocks that may have some economic potential such as in construction. Historically it was mistaken for useful minerals occurring in metallic ores but it does not contain any recoverable metals.      

 

Uses: The fact that hornblende is a common rock forming mineral makes it useful to geologists in a variety of ways. Its form and occurrence in a rock give clues as to the conditions under which the rock formed. If it is found in sediments and it can be traced back to a parent rock then something can be said about the depositional dynamics of and area at some time. There are numerous ways in which hornblende can be used to work geologic puzzles but of course this is not a unique utility for a mineral.



Composition:

Molecular Weight = 821.16 gm
   Calcium     9.76 %  Ca   13.66 % CaO
   Magnesium  11.84 %  Mg   19.63 % MgO
   Aluminum    5.75 %  Al   10.86 % Al2O3
   Iron        1.70 %  Fe    2.43 % Fe2O3
   Silicon    23.94 %  Si   51.22 % SiO2
   Hydrogen    0.25 %  H     2.19 % H2O
   Oxygen     46.76 %  O
             ______        ______ 
             100.00 %      100.00 % = TOTAL OXIDE



Physical Properties:

            Cleavage:
{110} perfect    angles: 56º/124º*

            Luster: vitreous-silky

            Hardness: 5-6

            Density: 3-3.4

            Color: brown, green, greenish brown, greenish black, dark green*

            Habit: columnar to fibrous; course to fine grained*

            Streak: white   

            Translucence: will transmit light on thin edges

            Diagnostic Features: *




Crystallography:

            Crystal System: Monoclinic

            Point Group: 2/m

            Space Group: C 2/m

            Axial Ratios: a:b:c =0.5475:1:0.2924

             Cell Dimensions: a = 9.96Å, b = 18.19Å, c = 5.32Å, Z = 2; beta = 104.87° V=931.5

             2D Cell Shape: centered rectangular net

             Bravais Lattice: centered monoclinic                          


Optical Properties:

Type: Biaxial(-)

                RI Values: a = 1.61-1.71, b = 18.01, g = 1.63-1.73;

2V = 30º-90º

Max Birefingence: 0.0140-0.0180 

Dispersion: usually r > v; may be r < v

Pleochroism(x): yellow-green

Pleochroism(y): olive-green

Pleochroism(z): deep green

 



 
XRD:




Due to sample impurities and significant composition range for hornblende definitive XRD identification of the sample was not possible. Quartz and mica were identified but no specific amphibole could be matched conclusively.  



 
 
Pictures:

























References:

Bachmann, O. and Dungan, M.A. (2002) Temperature-induced Al-zoning in hornblendes of the Fish Canyon magma. American Mineralogist, 87, 1062 - 1076.

 

Bhadra, S. and Bhattacharya, A. (2007) The barometer tremolite + tschermakite + 2 albite = 2 pargasite + 8 quartz: Constraints from experimental data at unit silica activity, with application to garnet-free natural assemblages. American Mineralogist, 92, 491 – 502.

 

Kohn, M.J. and Spear, F.S. (1989) Empirical calibration of geobarometers for the assemblage garnet + hornblende + plagioclase + quartz. American Mineralogist, 74, 77-84.

 

Rutherford, M. and Devine, J. D. (2003) Magmatic Conditions and Magma Ascent as Indicated by Hornblende Phase Equilibria and Reactions in 1995-2002 Soufriere Hills Magma. Journal of Petrology, 44, 1433-1453. 

 

Roy, M., van de Flierdt, T., Hemming, S.R. and Goldstein, S. L. (2007) <sup>40</Sup>Ar/<sup>39</sup>Ar ages of hornblende grains and bulk Sm/Nd isotopes of circum-Antartic glacio- marine sediments: Implications for sediment provenance in the southern ocean. Chemical Geology, 244, 507-519.

 

Dutrow, B. and Klein Cornelis, K. (2007) The 23rd Edition of the manual of Mineral Science. Wiley and Sons Inc.

 

http://www.webmineral.com/data/Magnesiohornblende.shtml   11-18-07

 

http://www.mindat.org/min-1519.html   11-18-07  



Links

http://www.webmineral.com/data/Magnesiohornblende.shtml

http://www.mindat.org/min-1519.html