Amphiboles - W0-1X2Y5Z8O22(OH,F)2

Amphibole, from the Greek amphibolos, meaning "ambiguous", was named by the famous French crystallographer and mineralogist René-Just Haüy (1801) in allusion to the great variety of composition and appearance shown by this mineral group. There are 5 major groups of amphiboles leading to 76 chemically defined end-member amphibole compositions according to the British mineralogist Bernard E. Leake. Because of the wide range of chemical substitutions permissible in the crystal structure, amphiboles can crystallize in igneous and metamorphic rocks with a wide range of bulk chemistries.
The amphiboles differ chemically from the pyroxenes in two major respects: amphiboles have hydroxyl groups in their structure and are hydrous silicates that are stable only in hydrous environments where water can be incorporated into the structure as (OH)-. The second major compositional difference is the presence of the A site in amphiboles that contains the large alkali elements, typically sodium cations and at times potassium cations. The pyroxenes do not have an equivalent site that can accommodate potassium. The presence of hydroxyl groups in the structure of amphiboles decreases their thermal stability relative to the more refractory (heat-resistant) pyroxenes. Amphiboles decompose to anhydrous minerals (mainly pyroxenes) at elevated temperatures.

Amphiboles constitute the most chemically complex group in nature, but they possess the distinctive arrangement of atoms known as the "double silicate chain" (Fig.1). The amphibole structure consists of doubled Si4O11 chains running parallel to c-axis, with the bases of the tetrahedra nearly in the bc plane. 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. The result is an "I-beam" structure like that of pyroxene (Fig.1). "I-beam" structure in amphiboles are approximately twice as wide as the equivalent t-o-t (tetrahedral-octahedral-tetrahedral) strips in pyroxenes, because of the doubling of the chains in the amphiboles, yielding typical near 60°-120° cleavage.

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Fig.1: Double silicate chain.



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Fig.2: Schematic projection of the monoclinic amphibole structure on a plane perpendicular to the c axis



The general formula for amphiboles is W0-1X2Y5Z8O22(OH,F)2, where:

W = Na1+ or K1+ in the A site with 10 to 12 fold coordination.
X = Ca2+, Na1+, Mn2+, Fe2+, Mg2+, Fe3+, in an M4 site with 6 to 8 fold coordination
Y = Mn2+, Fe2+, Mg2+, Fe3+, Al3+ or Ti4+ in an M1, M2, M3 octahedral coordination sites.
Z = Si4+ and Al3+ in the T tetrahedral site.

The principal classification of amphiboles is based on the chemistry of the X cations:

If X sites is occupied by Ca we have calcium amphiboles:
• Tremolite Ca2Mg5Si8O22(OH)2
• Ferroactinolite Ca2Fe5Si8O22(OH)2
• Hornblende Na0-1(Ca,Na)2(Mg,Fe2+,Fe3+, Al)5(Si,Al)8O22(OH)2

If X sites is occupied by Na we have Sodium or alkali amphiboles:
• Glaucophane Na2Mg3Al2Si8O22(OH)2
• Ferroglaucophane Na2Fe2+Al2Si8O22(OH)2
• Riebeckite Na2Fe2+Fe2+Si8O22(OH)2
• Magnesioriebeckite Na2Mg3Fe3+Si8O22(OH)2
• Arfvedsonite Na3Fe2+Fe3Si8O22(OH)2

If X sites is occupied by Ca and Na we have Calcium-Sodium amphiboles:
• Richterite Na(Na,Ca)Mg5Si8O22(OH)2

If X sites is occupied by Fe, Mg, Mn we have Iron-magnesium amphiboles:
• Antophyllite Mg7Si8O22(OH)2
• Cummingtonite Mg7Si8O22(OH)2
• Grunerite Fe7Si8O22(OH)2

All the amphiboles, except anthophyllite are, monoclinic, and all show the excellent prismatic cleavage on (110). The angles between the cleavages, however are 56° and 124° making all amphiboles easy to distinguish from the pyroxenes. Looking at faces that show only a single cleavage trace would show inclined extinction, except in Anthophyllite.

Bibliography


• Klein Cornelis: Mineralogia. Zanichelli 2004
• Optical Mineralogy: The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc


Photo
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Hornblende in a Gabbro. XPL image , 2x (Field of view = 7mm)
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Hornblende in a Gabbro. PPL image , 2x (Field of view = 7mm)
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Hornblende in a Gabbro. XPL image , 2x (Field of view = 7mm)
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Hornblende in a Gabbro. PPL image , 2x (Field of view = 7mm)
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Hornblende in a Gabbro. XPL image , 2x (Field of view = 7mm)
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Hornblende in a Gabbro. PPL image , 2x (Field of view = 7mm)
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Hornblende in a Gabbro. XPL image , 2x (Field of view = 7mm)
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Hornblende in a Gabbro. PPL image , 2x (Field of view = 7mm)
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Hornblende in a Gabbro. XPL image , 2x (Field of view = 7mm)
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Hornblende in a Granite. PPL image , 10x (Field of view = 2mm)
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Hornblende in a Granite. XPL image , 10x (Field of view = 2mm)
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Hornblende in a Granite. XPL image , 10x (Field of view = 2mm)
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Hornblende in a Granite. PPL image , 2x (Field of view = 7mm)
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Hornblende in a Granite. XPL image , 2x (Field of view = 7mm)
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Ribeckite crystal in Alkali feldspar Syenite. PPL image , 2x (Field of view = 7mm)
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Ribeckite crystal in Alkali feldspar Syenite. PPL image , 2x (Field of view = 7mm)
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Ribeckite crystal in Alkali feldspar Syenite. PPL image , 2x (Field of view = 7mm)
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Ribeckite crystal in Alkali feldspar Syenite. PPL image , 2x (Field of view = 7mm)
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Ribeckite crystal in Alkali feldspar Syenite. PPL image , 2x (Field of view = 7mm)
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Ribeckite crystal in Alkali feldspar Syenite. PPL image , 2x (Field of view = 7mm)
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Ribeckite crystal in Alkali feldspar Syenite. XPL image , 2x (Field of view = 7mm)