How does hornblende form

Hornblende is a very common mineral found in many geologic environments. It is found in many intrusive igneous rocks ranging in composition from granites to diorite to gabbros to syenites. It occurs as phenocrysts in several varieties of extrusive igneous rocks, such an andesite. It occurs as a detrital heavy mineral in sedimentary rocks. It forms in a number of gneissic rocks formed at moderate grades of metamorphism. Amphibolites are common metamorphic rocks consisting in large part of dark amphibole.

The hornblende is associated with plagioclase, biotite and zircon Wilcox, Some of the amphibolite has been used to make bookends, tie tacks and pendants. Crystal Form : The crystal form of Hornblende is a prism type that looks like a diamond when observed in cross section, or the portion of the mineral that does not have a cleavage surface.

Luster : Vitreous, dull or submetallic. Special Features : The diamond cross section is a unique property of hornblende and the angles of the cleavage. Other unique features of hornblende include the dark to almost black color. Varieties : The independent elements within hornblende are in such small abundance that they often are just blanketed with the term hornblende to describe them 1.

Certain hornblendes contain potassium, titanium, and fluoride. Mineral Group: Hornblende is contained within the silicates group which contains various amount of silicon and oxygen, this is also the largest group of minerals 3.

Amphiboles are a group of minerals that include elements of iron, magnesium, silicon, oxygen, and hydroxyl.

A defining feature of amphiboles are their cleavage. Amphiboles intersect at 56 to degrees differentiating themselves from pyroxenes and other groups 5. Hornblende is also part of the Inosilicates group. Insliciates are a silicon atom surrounded by 4 oxygen atoms and can be chained infinitely 4. Pleochroic in various shades of green and brown. In PPL a thin section of Hornblende ranges from yellow -green to dark brown. Prismatic crystal that can be, but is not necessarily, elongated.

Crystals are often hexagonal. Some crystals are double terminated, or they come to a point at both ends. Example photo below. Cleavages at 56 and degrees which form a distinctive diamond shape in cross section. Black, dark green, dark brown, dark gray. Lighter colors are encountered in the extended Hornblende group, which includes members such as Edenite and Pargasite.

Crystal System. Crystal Forms and Aggregates. As prismatic or tabular crystals with a diamond-shaped cross-section. Rarely in individual crystals; almost always in dense groups of platy or grainy crystals. Also columnar , radiating , acicular , fibrous , in vein s, and massive.

May be slightly translucent on thin cross-sections under strong back-lighting. Specific Gravity. Vitreous , submetallic , dull. Uneven , splintery. In Group. Silicates ; Inosilicates ; Amphibole Group. Striking Features. Color, crystal aggregates, and cleavage angle of amphible s.