Amphibole
Monoclinic, orthorombic
A0-1B2C5T8O22(OH,F)2
Amphiboles are a group of rock-forming chain silicates that occurs in many igneous and metamorphic rocks. Their complex structure allow to incorporate several different elements in solid solution, producing a wide chemical variability. The name itself derives from the Greek ἀμφίβολος – amphibolos (double entendre, ambiguous), in reference to the chemical and morphological complexity of this group of minerals. The term was coined by the French crystallographer René-Just Haüy.
Structure and chemistry
Amphiboles are double chain silicates with general formula A0-1B2C5T8O22(OH,F)2. The double chain consists of (Si, Al)O4 tetrahedrons (T site) linked to form chains with the formula (Si, Al)4O11 that extend infinitely along the long-axis (c-axis) of the mineral. (Si, Al)O4 tetrahedrons alternate with opposite orientation in the double chains, producing rings of six tetrahedrons that constitute the chain. The center of each ring is occupied by a large hydroxyl site, containing OH– or F–: in the structure there are 2 hydroxyl sites every 8 tetrahedral sites. Double chains are separated from each other by strips of octahedral sites (M sites). There are four different types of octahedral sites in the structure with different size: M1, M2, M3 (corresponding to C in the formula), and M4 (corresponding to B). M1, M2, and M3 are smaller and occur sandwiched between two double chains of tetrahedrons, with (Si, Al)O4 vertices of opposite chains pointing towards them. This first order structure produces a series of sandwiches of tetrahedral chains with octahedral sites in between (T-O-T), consisting of 5 octahedral sites every 8 tetrahedrons. These T-O-T ‘sandwiches’, also known as I-beams, are stacked on top of each other through a large cation site, A, centered on the double chains, and two M4 sites, larger than the other octahedral sites and occupying a peripheral position with respect to the chains. This structure is generally monoclinic but some amphiboles lack cations in the A site and contain Fe and Mg in all M-sites, resulting in an orthorhombic structure. Similarly to the pyroxenes, the T-O-T chains have stronger bonds internally with respect to those with the neighboring T-O-T chains. These produces two planes of weakness oriented parallel to the long axis. However, the double chains of amphiboles are larger than the single chains of pyroxenes. As a result, the planes of weakness in the structure intersect at 124-126° and 54-56°, determining the typical intersecting prismatic cleavage planes of amphibole.
Group of prismatic amphibole var. richterite crystals. Wilberforce, Ontario, Canada. Size: 4.4 x 3.1 x 1.2 cm. Photo by Robert M. Lavisnky.
Properties
Habit: prismatic, acicular, fibrous
Hardness: 5 – 6
Cleavage: two prismatic sets of cleavage planes ({110}) intersecting at 54-56° and 124-126°
Twinning: simple, lamellar twins
Color: black and dark to light green, brown, red, yellow, and blue, grey, colorless
Luster: vitreous, metallic
Streak: white, grey, colorless, grayish blue, pale yellow, grey-green
Alteration: clay minerals, chlorite, talc
In thin section…
Color: colorless to dark brown, green, red, yellow, blue, lavender blue
Pleochroism: strong in colored varieties, weak to absent in light-colored varieties
Birefringence (δ): 0.006-0.047 (low to high interference colors)
Relief: high
Optic sign: + or –
*optical properties vary significantly in different amphibole types
[Mindat]
Amphiboles may contain several elements and show highly variable chemistry. In first order, we can distinguish four broad groups of amphiboles based on the cations occupying the B or M4 sites: calcic amphiboles (B = Ca), sodic amphiboles (B = Na), sodic-calcic amphiboles (B = both Na, and Ca in appreciable contents), and iron-magnesium-manganese amphiboles (B = Fe, Mg, Mn). The other structural sites may contain Na, K (A site), Mg, Fe2+, Fe3+, Al and also Mn, Zn, Cr, Ti, and Li (C site or M1 – M2 – M3), while Si occurs in the T site, together with Al. This complexity is steered primarily by five main substitutions: Fe → Mg, Al → Si, Al → [Fe, Mg], Na → Ca, and the introduction of Na, K in the empty A site. Additionally, other substitution occur (e.g. Mn → [Fe, Mg], Fe3+ → Al). Many substitutions involve different sites at the same time to maintain the electric neutrality of the structure, e.g. the Tschermak substitution (Al → Si in the tetrahedral site compensated by Al → [Fe, Mg] in the octahedral sites).
Iron-magnesium-manganese amphiboles
Anthophyllite (Mg, Fe2+)7[Si8O22](OH, F)2 – Gedrite (Mg, Fe2+)5Al2[Si6Al2O22](OH, F)2
*note: these are the only orthorhombic species among rock-forming amphiboles.
Cummingtonite – Grunerite (Mg, Fe, Mn)7[Si8O22](OH, F)2
Calcic amphiboles
Tremolite – Ferro-actinolite Ca2(Mg, Fe2+)5[Si8O22](OH, F)2
Magnesiohornblende – Ferrohornblende Ca2(Mg, Fe)4Al[Si7AlO22](OH)2
Tschermakite – Ferrotschermakite Ca2(Mg, Fe)3Al2[Si6Al2O22](OH)2
Edenite – Ferro-edenite NaCa2(Mg, Fe)5[Si7AlO22](OH)2
Pargasite – Ferropargasite NaCa2(Mg, Fe)4Al[Si6Al2O22](OH)2
Magnesiohastingsite – Hastingsite NaCa2(Mg, Fe)4Fe3+[Si6Al2O22](OH)2
*note, except for the tremolite – ferro-actinolite series, hornblende is a general term for the Ca amphiboles listed above.
Kaersutite (Na, K)Ca2(Mg, Fe2+, Fe3+, Al)4(Ti, Fe3+)[Si6Al2O22](O, OH, F)2
Sodic amphiboles
Glaucophane Na2Mg3Al2[Si8O22](OH, F)2 – Riebeckite Na2Fe2+3Fe3+2[Si8O22](OH, F)2
Eckermannite – Arfvedsonite Na3(Mg, Fe)4(Al, Fe3+)[Si8O22](OH, F)2
Sodic-calcic amphiboles
Richterite – Ferrorichterite (Na)CaNa(Mg, Fe3+, Fe2+, Mn)5[Si8O22](OH, F)2
Magnesiokatophorite – Katophorite (Na)CaNa(Mg, , Fe2+)4Fe3+[Si7AlO22](OH, F)2
Note: listed here are only the most common rock-forming amphiboles. The list is still long.
Field features
Amphiboles show a wide range of mineral properties, like color, hardness, and habit, which vary depending on their composition. However, they all have some features in common: (1) prismatic habit, (2) two prismatic cleavage planes intersecting at nearly 60 – 120 ° on basal faces. This allows to distinguish them from pyroxenes. The cleavage planes are oriented parallel to some prismatic faces (e.g. 110) and inclined with respect to others (e.g. 010). Amphiboles tend to be dark-colored (black to dark green, brown, grey, and blue) and show metallic to vitreous luster, but light-colored varieties exist in nature. The hardness of amphiboles is around 5 – 6 on the Mohs scale. Amphiboles can be confused with pyroxenes, if cleavage traces are not well visible.
Amphiboles in thin section
Amphiboles at the microscale can be easily identified based on the same features that allow their recognition in the field, in particular the presence of perfect prismatic cleavage planes that intersect at nearly 60 – 120°. Another fundamental feature of amphiboles is the strong pleochroism at PPL: different members of the group may show variable hues from brown to yellows to reds, greens, and blues. However, Mg-rich varieties can be weakly pleochroic or not pleochroic at all, appearing in some cases colorless. Optical properties such as relief, color, and birefringence tend to increase in intensity with the iron content. Most amphiboles show oblique extinction with respect to the long axis, as they crystallize in the monoclinic system. Only two rock-forming amphiboles, anthophyllite and gedrite, crystallize in the orthorhombic system and have, hence, straight extinction. Amphiboles have high positive relief.
CPL
PPL
Above: heart-shaped amphibole (left) and lamellar phlogopite (right) in a lamprophyre with primary carbonate groundmass. Amphibole is commonly colored, pleochroic, and shows moderate to high interference colors. Aillikite from Aley, British Columbia, Canada. Field of view: 9 mm. Photo by Alessandro Da Mommio.
Amphiboles and asbestos
Many amphibole minerals have a tendency to form acicular to fibrous crystals, which can produce tiny silicate fibers. There is evidence that these fibers are linked to asbestosis, a serious lung conditions caused by the exposure to asbestos. Asbestos is a commercial and material term that groups together different minerals that from a mineralogical point of view belong to different groups or supergroups. The current definition of asbestos includes chrysotile (serpentine group) and cummingtonite – grunerite (amosite), crocidolite (the fibrous variety of riebeckite), anthophyllite, tremolite, and actinolite, all belonging to the amphibole supergroup. These minerals used to be quarried because they are excellent thermal and electric insulators and their fibrous habit made their extraction and processing easy. However, the tiny silicate fibers they produce when processed can be breathed. These fibers cannot be eliminated by the human immune system and over time they produce a serious inflammation of the lungs (asbestosis), which may lead to shortness of breath, respiratory failure, and death. This condition became extreme over the years in the workers of asbestos production plants, as the method of processing was to crush fibers, resulting in a white dust that was constantly inhaled.
Occurrence
Amphiboles are common minerals in intermediate igneous rocks and in metamorphosed mafic rocks. Igneous amphiboles such as hornblende occur in rocks such as diorite and tonalite, but are present also in some gabbros and alkali basalts. Alkali amphiboles such as eckermannite – arfvedsonite may occur in peralkaline igneous rocks. Around igneous systems, skarns and other metasomatic rocks may also contain a wide range of amphiboles. In metamorphic rocks, amphiboles are commonly associated with mafic rocks, where glaucophane – riebeckite typically marks the blueschist-facies, tremolite – ferro-actinolite the greenschist-facies, and hornblendes occur in the amphibolite-facies. These amphiboles may also occur in metapelites and some metasandstones. Ultramafic rocks may also produce various Ca-amphiboles during metamorphism at medium- to high-grade. Impure carbonatic rocks may also produce a wide range of Ca-amphiboles during metamorphism and metasomatism.
Hawthorne, F. C., Oberti, R., Harlow, G. E., Maresch, W. V., Martin, R. F., Schumacher, J. C., & Welch, M. D. (2012). Nomenclature of the amphibole supergroup. American Mineralogist, 97(11-12), 2031-2048.
Schott, J., Berner, R. A., & Sjöberg, E. L. (1981). Mechanism of pyroxene and amphibole weathering—I. Experimental studies of iron-free minerals. Geochimica et Cosmochimica Acta, 45(11), 2123-2135.
Tindle, A. G., & Webb, P. C. (1994). PROBE-AMPH—a spreadsheet program to classify microprobe-derived amphibole analyses. Computers & Geosciences, 20(7-8), 1201-1228.
Resources
An introduction to the Rock-Forming Minerals. Deer, Howie & Zussmann.
Optical Mineralogy: Principles & Practice. Gribble & Hall.
Transmitted Light Microscopy of Rock-Forming Minerals: An Introduction to Optical Mineralogy (Springer Textbooks in Earth Sciences, Geography and Environment). Schmidt.
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