Download Amphiboles

42
Amphiboles - (Mg,Fe,Ca,Na)2(Mg,Fe,Al)5(Si,Al)8O22(OH)2
Structure:
- Double chain silicates with
(Si,Al)O4 tetrahedra. Fig 53.
- The OH group is an essential
part of the structure.
-there are both orthorhombic
and monoclinic amphiboles.
- The complex structure allows a large number of different
ion substitutions, therefore, you can get amphiboles
occurring a very diverse number of rock types. Eg
hornblende, there is no simple composition and it occurs in
many rock types, hence there are no simple charts that can
be developed to describe properties.
Chemistry
There are several amphibole groups (see handout)
Anthophyllite-Cummingtonite (Ca+Na≅0)
Calcium Amphiboles (Ca>Na)
Alkali Amphiboles (Na>Ca)
Within these groups there are several continuous series.
43
Chemistry cont.
A typical series is:
Tremolite
Ca2Mg5Si8O22(OH)2
→
Actinolite
→
Ca2(Mg,Fe)5Si8O22(OH)2
Ferro-Actinolite
Ca2Fe5Si8O22(OH)2
These are essentially metamorphic minerals and are found in
all types of metamorphic regimes. They are usually found as
fibrous, radiating clusters.
There are many such series for the amphiboles.
There are also mixtures to Hornblende:
(Ca,Na,K)2-3(Mg,Fe+2,Fe+3,Al)5[Si6(Si,Al)2O22](OH,F)2
Higher temperatures allow Al to enter the amphibole
structure and hornblende often occurs. However, depending
on Fe/Mg ration and pressure, you can get an miscibility gap
between actinolite/tremolite and hornblende.
44
Alterations
Anthophyllite (orthorhombic), Tremolite/Actinolite, and
Nephrite alter to Talc. Riebeckite alters to an iron-stained
fibrous quartz known as “tigers-eye”.
Optical and Physical Properties
Orthoamphibole (Eg: Anthophyllite)
Colour: colourless
Form: Long prismatic crystals and columnar to fibrous
Relief: High
Birefringence: Moderate
Cleavage: 2 directions at
~ 60° and 120°
Extinction: Parallel
Clinoamphibole
Eg:
Cummingtonite → Grunerite series
Tremolite → Ferrotremolite series
Hornblende → Basaltic Hornblende series
Glaucophane → Riebeckite series
Colour: Colourless (cummingtonite-grunerite), colourless to
pale green (trem-act), Green to brown to yellowish
brown (hbl- basaltic hbl) and blue to violet (glaucrieb).
45
Optical and Physical Properties cont
Clinoamphibole
Form: Long prismatic crystals (pseudohexagonal crosssections and columnar to fibrous aggregates.
Relief: High to Fairly High
Birefringence: Moderate to Strong (except for Riebeckite
which is very weak).
Cleavage: 2 directions at ~ 60° and 120°
Extinction: Inclined
Changes in the optical properties (optical angle, extinction,
birefringence) are governed by replacement of Mg by Fe.
The Refractive increases with Fe content. Al also substitutes
and the is OH replacement by F.
Paragenesis is very important for identifying the amphiboles.
Amphibole Paragenesis
1) Thermal metamorphism of sediments (common reaction)
5CaMg(CO3)2 + 8SiO2 + H2O → Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2
(dolomite)
(silica)
(tremolite)
(calcite)
If you increase the grade of metamorphism, tremolite breaks
down to diopside and at very high temperatures forsterite can
form. All such reactions liberate CO2 and H2O and are
therefore pressure dependent (Figure 65 on handout).
46
Amphibole Paragenesis
1) Thermal metamorphism of sediments (common reaction)
5CaMg(CO3)2 + 8SiO2 + H2O → Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2
(dolomite)
(silica)
(tremolite)
(calcite)
If you increase the grade of
metamorphism, tremolite
breaks down to diopside and at
very high temperatures
forsterite can form. All such
reactions liberate CO2 and
H2O and are therefore
pressure dependent (Figure 65
on handout).
2) Regional Metamorphism
- tremolite occurs where calcareous sediments dominate.
3) Regional metamorphism of Ultrabasic rocks
- both tremolite and actinolite (common) + talk, carbonates
and chlorite.
47
Amphibole Paragenesis cont.
Other Amphiboles:
4) Hornblende
- occurs in a wide variety of T-P conditions in both igneous
and metamorphic rocks.
- stable in most silicious rocks and very common in
intermediate plutonic rocks.
Gabbros
Mg rich (Mg:Fe = 3:1)
Syenites and
granites
Diorites
Fe rich (Mg:Fe=5:95
Mg:F=1:1
5) Basaltic Hornblende
- only in igneous rocks
- occur as phenocrysts in andesites
6) Kaersutite (Ti bearing amphibole)
- occur as large phenocrysts (glomoporhyritic)
in alkalic volcanic rocks such as trachyte
7) Alkali Amphiboles - Ca (Na,K)
Glaucophane - occur under high pressure, low temperature
conditions such as subduction zones (also large amounts of
Na from seawater). Eg: Glaucophane schists.
Riebeckite - occurs in alkali igneous rocks (plutonic) such as
quartz syenites, granites, trachyte and rhyolite.