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Amphibole Group

Complex family of hydrous double-chain silicate minerals. Amphiboles are common in terrestrial metamorphic and igneous rocks, forming in the presence of water. They are rarely found in extraterrestrial materials, having been discovered in R chondrites and graphite nodules in certain iron meteorites. Martian rocks are the main exception with kaersutite amphibole commonly reported in shergottites.

The amphibole structure consists of doubled Si4O11 chains running parallel to c-axis. These chains are bonded to octahedral strips consisting of three regular octahedral sites (M1, M2, M3) and one larger 6- to 8-fold site (M4). In addition there is an even larger 10- to 12-fold A site that is usually empty.

 

 

 

 

OH groups occur in the interiors of the rings in the double chains.

The result is an “I-beam” structure like that of pyroxene. The M123 cations are coordinated by oxygens and OH- groups of the adjacent double chains, forming a TOT strip. Amphibole TOT strips are approximately twice as wide as pyroxene strips, yielding typical near 120° {110} cleavage.

Amphiboles are composed of the same cations as pyroxenes, but also have OH- groups, resulting in lower densities and refractive indices than their pyroxene counterparts. Their general formula is A0-1X2Y5Z8O22(OH,F,Cl)2. The A site holds large cations such as Na+ and K+ and is commonly not completely filled. The X site (M4) holds a large to intermediate-size cations such as Ca2+, Na+, Mn2+, Fe2+, Mg2+, and Li+. The Y sites (M1, M2, M3) hold intermediate-sized to small cations such as Mn2++, Fe2+, Mg2+, Fe3+, Al3+, and Ti4+. Lastly, the tetrahedral Z sites in the chains hold the smallest cations Si4+ and Al3+. As indicated by the formula, substitution of F and Cl can occur for HO.

 

The cation substitutions that produce the wide range of amphibole compositions can be complex. Multiple coupled substitutions are usually required to maintain charge balance. Simple cation substitutions include:

  • Na+ ↔ K+ (A)
  • Ca2+ ↔ Mg2+ ↔ Fe2+ (M4)
  • Mg2+ ↔ Fe2+ and Fe3+ ↔ Al3+ (M123)

Coupled substitutions include:

  • Mg2+,Fe2+)M123 + (Si4+)T ↔ (Fe3+,Al3+)M123 + (Al3+)T
  • ( )A + (Si4+)T ↔ (Na+)A + (Al3+)T
  • (Ca2+)M4 ↔ (Na+)A + (Na+)M4
  • 2(Ca2+)M4 + (Mg2+,Fe2+)M123 ↔ (Na+)A + 2(Na+)M4 + (Al3+)M123

Classification of Ca-rich amphiboles is done in a manner analogous to that for quadrilateral pyroxenes by plotting the Ca, Mg, and Fe end-member components. No amphiboles plot above the above 2/7th Ca line when both M4 sites completely filled with Ca.

Cation substitutions complicate the situation. The most common amphibole, hornblende, has very variable composition owing to significant substitution of Na+ and K+ in A site and Fe3+ and Al3+ in M123 sites. Hornblende is the result of two main substitutions starting from tremolite: Si4+ + (Mg2+,Fe2+) ↔ 2Al3+, and Si4+ +  A-site ↔ Al3+ + (Na+,K+). Hornblende includes following series: magnesiohornblende-ferrohornblende, tschermakite-ferrotschermakite, edenite-ferroedenite, pargasite-ferropargasite, and magnesiohastingsite-hastingsite (mainly listed to show the spectacular mineral names). One way of classifying hornblende is to consider the A-site occupancy and the amount of Si in the T-site. In the diagram below yellow shows the range of observed composition with orange indicating the more abundant ones (Hb = hornblende (sensu stricto), Ts = tschermakite, Ed = edenite, and Pa = pargasite).

There are alkali-rich varieties in addition to the more common calcic amphiboles. Alkali amphibole occurs primarily in sodic-metamorphic rocks and in alkaline igneous rocks. Alkali amphiboles are divided into two groups based upon M4 and A site occupancies. If the M4 site contains <0.5 (Na+K) and there is no Na in the A site, the mineral is considered a low-Na alkali amphibole. If the M4 site contains >0.5 (Na+K) and there is Na in the A site, it is considered a high-Na alkali amphibole. There is no solid solution between calcic and alkali amphiboles. The two types of alkali amphiboles are further subdivided. Classification of low-Na alkali amphiboles is accomplished using the diagram below; the most common types are glaucophane and riebeckite.

Classification of the rarer high-Na alkali amphiboles is done with the diagram below.

A summary of amphibole chemistry is given below (green = calcic amphiboles; blue = alkali amphiboles).

Content above used with permission from J. H. Wittke.