Amphibole Group

Amphiboles are the common constituents in both Igneous and Metamorphic rocks.

• These are hydrous ferromagnesian silicate minerals.
• Amphiboles are belongs to the double chain silicates in the Inosilicates.
• The angle between two set of cleavages is 124^o and is characteristic of amphiboles.
• “Tremolite” (hydrous silicate of Ca and Mg),”Actinolite” (hydrous silicate of Ca, Mg and Fe), “Hornblende” (hydrous silicate of Na, Ca, Fe and Al) are the important members of amphibole group.
• Due to extensive substitution of one atom by the atom of similar size of valency, number of isomorphs series have been generated within this group.
• Most amphiboles are not asbestose but Most asbestose are amphiboles.

General formula of Amphiboles is , W_0-1X₂Y₅(Z₈O₂₂)(OH,F)₂

• where , W = large A site which may be vacant or contain varying amounts of Na/Ca (equivalent to 10-fold coordination)
• X = Ca, Na, Mg or Fe , the content of M4 site
  • 8-fold = X is usually Ca or Na
  • 6-fold = X is usually Mg or Fe
• Y = Fe⁺², Mg, Fe⁺³, Al⁺³ , the M1, M2and M3 sites. They are O in the TOT layers.
• Z = Si and Al in the tetrahedral usually.

Structure

Structurally the amphiboles are double chain silicates which looks like the two single chain united together by corner sharing with a mirror plane along the joint.

The mirror plane is preserved in all amphiboles even when the chains are not straight. These chains are parallel to the C-axis and are stacked in a alternate fashion like pyroxenes.

Some tetrahedra shares 2 while some of them share 3 oxygens, resulting in the Si : O = 4 : 11

• The geometry leads to 5 different structural sites between chains, Octahedral ‘M1’, ‘M2’, ‘M3’ sites are between the point of the chains, larger ‘M4’ and ‘A’ site are between the bases.

• Two chains with intervening cations in octahedral M1, M2, M3 sites form analogous to TOT layer silicates. These are stacked and bonded together by cations in the M4 and A sites to form the amphibole structure.
• cleavages occur between the TOT strips as the TOT strips in amphiboles are wider than the equivalent ‘I-beams’ in pyroxenes, cleavage is at about 56 and 124 degree.
• The stacking of chains produce (100) layers of chains with apices alternatively pointing up and down, defining the B-axis repeat distance of 17.8 angstrom.
• The coordination of the M4 sites is 8 when occupied by a larger cation such as Ca⁺², but reduces to 6 when occupied by a smaller cation such as Mg⁺² or Fe⁺². The M1, M2 and M3 site are octahedral.
• the OH^– and F^– lies in the center of the hexagonal rings, at the level of the tetrahedral apices.

Amphiboles Classification

General Physical and Optical properties

• Occurrences
  • Amphiboles are generally found in the Igneous and Metamorphic rocks.
  • Greenstone belts named after the green color of amphiboles.
  • Crocidolite occurs in highly siliceous metamorphic rocks.
• Indian Occurrences
  • Rajasthan : Khetri mines
  • West bengal : Purulia district
  • Orissa : Sundargarh district
  • Tamil Nadu : Salem district
• Varities
  • Smargdite : Foliated actinolite
  • Uralite : Secondary tremolite-actinolite
  • Asbestos
    • Amosite : Fibrous anthophyllite
    • Actinolite : Asbestose proper
    • Crocidolite : Fibrous riebeckite
    • Chrysotile : Fibrous serpentine
  • Nephrite : Fine grained tremolite-actinolilte
• Uses
  • Amphiboles are used as many decorative stones.
  • Asbestos was used in older buildings as insulation, roofing material and other fire proof applications.