The pyroxenes are single chain silicates, while the amphiboles are double chain silicates. ... On fragments of pyroxene, the cleavage faces tend to meet at nearly right angles. In contrast, hornblende and amphibole cleavage fragments have cleavage faces that meet at angles of nearly 60 degrees and 120 degrees.
Amphibole minerals' defining characteristics are their dark color, hardness and well-developed cleavage. These usually serve to distinguish them from most other common rock-forming minerals with the exception of the pyroxene mineral group.
Long prismatic, acicular, or fibrous crystal habit, Mohs hardness between 5 and 6, and two directions of cleavage intersecting at approximately 56° and 124° generally suffice to identify amphiboles in hand specimens. The specific gravity values of amphiboles range from about 2.9 to 3.6.
Identification: Pyroxenes typically occur as stubby prismatic crystals. They are generally dark green to black in colour, though they can range from apple-green and lilac to colourless, depending on their chemical composition. Pyroxenes have a hardness between 5 and 7.
How Does Amphibolite Form? Amphibolite is a rock of convergent plate boundaries where heat and pressure cause regional metamorphism. It can be produced through the metamorphism of mafic igneous rocks such as basalt and gabbro, or from the metamorphism of clay-rich sedimentary rocks such as marl or graywacke.
Hornblende, calcium-rich amphibole mineral that is monoclinic in crystal structure.
It is used as paving stones and as a veneer or facing on buildings (both for interior and exterior use). It is also used as crushed stone for the usual crushed stone applications such as road and railroad bed construction. In this application it is used locally, near the source of the amphibolite.
General considerations. Amphiboles are found principally in metamorphic and igneous rocks. They occur in many metamorphic rocks, especially those derived from mafic igneous rocks (those containing dark-coloured ferromagnesian minerals) and siliceous dolomites.
1 : hornblende. 2 : any of a group of complex silicate minerals with like crystal structures that contain calcium, sodium, magnesium, aluminum, or iron ions or a combination of them.
The pyroxene group includes minerals that form in both the orthorhombic and monoclinic crystal systems. Orthorhombic pyroxenes are referred to as orthopyroxenes, and monoclinic pyroxenes are called clinopyroxenes.
Characteristically, pyroxenes are dark green to black in colour, but they can range from dark green to apple-green and from lilac to colourless, depending on the chemical composition. Diopside ranges from white to light green, darkening in colour as the iron content increases.
The photos above show a large twinned pyroxene (aegirine) crystal with the classic 8-sided ("stop sign") shape and high relief compared to the light white-green surrounding material. The twin divides the pyroxene into two domains with different optical orientations, easily seen in the XP view.