Hornblende - Encyclopedia

    Class : Silicates
    Subclass : Inosilicates
    Crystal system : Monoclinic
    Chemistry : (Ca,Na)₂(Fe²⁺,Mg²⁺,Fe³⁺,Al³⁺)₅(Si₇Al)O₂₂ (OH,F)₂
    Rarity : Very common

Hornblende was long considered the most common amphibole, present in magmatic rocks (syenites, granodiorites, diorites, gabbros) and metamorphic rocks (amphibolites, metabasalts). But hornblende covered such a wide range of composition that it was divided into several species of well-defined compositions (edenite, ferro-edenite, pargasite, hastingsite, tschermakite, kaersutite, etc...), the name hornblende being retained but restricted to a mineralogical subgroup comprising the ferro-hornblende - magnesio-hornblende series. “Basaltic hornblende”, common in volcanic rocks, refers to brown-black hornblendes rich in titanium (up to 4%): most often it is ferrohornblende or kaersutite. The precise identification of the preceding minerals, however, requires efficient analytical means, combining X-ray diffraction and microprobe analyses. This great complexity comes from the fact that the composition of hornblende derives from the tremolite - ferroactinote series by multiple replacements : Na in A and M4 sites, significant substitution of Si by Al in T sites and presence of Mn, Fe³⁺ and Ti⁴⁺ in Y sites (M1, M2 and M3), as well as the variable content of OH and F. Its name derives from German mining terms : horn (horn) evoking the elongated shape of the crystals or their high resistance and blende (sphalerite) for its resemblance to this mineral. A second etymology is proposed : horn (horn) and blenden (to blind, dazzle) because this dark green, shiny mineral, apparently promising and associated with ores, turned out to be disappointing due to its lack of economic value. Hornblende occurs in prismatic crystals, often elongated or stocky with a hexagonal section, rarely doubly-terminated, with frequently rounded edges. It is also common in masses or aggregates that are more or less fibrous to bacillary, sometimes grainy. The color is green to green-black, brown or black.

Main photo : Magnesio-hornblende from Traversella Mine, Piedmont, Italy © Fulvio Malfatto

No twin known for this mineral species.

No fakes recorded for this mineral species.