The chemical formula of Hornblende is indicated by Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2 or Calcium Magnesium Iron Aluminum Silicate Hydroxide. Hornblende is actually a Silicate mineral. Hornblende is also a member of the Amphibole group of minerals. The term Hornblende was derived from the German word horn and blenden, which means to blind or dazzle. The word blende is often used to refer to a brilliant non-metallic luster. Mineral Hornblende is known to crystallize in the monoclinic system of crystallization. In optical mineralogy, the monoclinic system of crystal formation comprises crystals having three axes of unequal lengths. Two of which are usually found in a position that is oblique or not perpendicular to one another. However, both of which are commonly found perpendicular to the third axis. Hornblende is most commonly used as mineral specimen and it is most often found exhibiting nice and interesting images when viewed with the aid of polarizing light microscopes for mineralogists.

 

            Even though Hornblende is widespread, it is not often used as a collection mineral because good crystals are somewhat difficult to find. Hornblende crystals are almost always found opaque in appearance, black in color and not very attractive. But then a few Hornblende specimens are extraordinary and often found exhibiting interesting microscope images when viewed under petrographic polarizing light microscope. Thus it can also possibly make for valuable specimens. Some Hornblende crystals can be found in acicular clusters or needle thin crystal aggregates that can be majestically exhibited under polarizing light microscopes. Many times, a specimen of a more valuable mineral is found accented by the black crystals of Hornblende that is opaque in appearance.

 

            Mineral Hornblende is almost always found black to dark green in color and is less attractive even when viewed with the aid of petrographic polarizing light microscopes. Hornblende is most often found exhibiting a vitreous to dull luster when it is closely evaluated in reflected light of polarizing microscope for mineralogists. Hornblende is also usually found showing imperfect cleavage in two directions at about 56 degree and 124 degree when it is closely evaluated under petrographic polarizing light microscope. Hornblende also exhibits uneven fracture when it is evaluated under polarizing microscope used in optical mineralogy. The specific gravity measure for Hornblende usually gives an approximate value ranging from 2.9 grams per cubic centimeters to 3.4 grams per cubic centimeters, which is considered somewhat above average for translucent minerals. The hardness measure of Hornblende when it is evaluated using the Mohs scale method is usually ranging from 5 to 6. Hornblende specimen is most commonly found leaving a brown to gray streak when it is rubbed on a white porcelain streak plate.

 

            Generally, Hornblende crystals are found opaque in appearance. But the thin crystals of Hornblende and some exceptional specimens can appear translucent. Mineral Hornblende that is translucent in appearance is usually found pleochroic when viewed between crossed nicols of polarizing light microscopes. Large Hornblende crystals are commonly found having an almost grainy or striated appearance when viewed under petrographic polarizing light microscopes. The crystal habit of the mineral Hornblende as described in the field of optical mineralogy usually includes short stocky prismatic crystals as well as long thin crystal forms that can be seen clearly visible when valuated with the aid of petrographic polarizing light microscope. Hornblende crystals can have a hexagonal cross-section that can be seen visible when viewed with the aid of polarizing light microscopes for geologist. The typical termination, if seen under petrographic polarizing light microscope, appears as the two faces of a dome but it is actually two of the four faces of a prism. Hornblende can be also found in granular form, massive and occasionally acicular aggregates.

 

            Hornblende is most commonly found associated with several other interesting minerals including quartz, micas, augite, feldspars, magnetite and many medium grade metamorphic minerals. Hornblende is a common constituent of many igneous and metamorphic rocks. The best field indicators of mineral Hornblende usually include cleavage, color and crystal habit, most especially the cross-section. Hornblende notably occurs at numerous localities including Bancroft in Ontario, Canada as well as Norway, New York in USA, Bohemia, and in Mount Vesuvius in Italy.