Page 53 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS AMPHIBOLE GROUP

*INTERFERENCE Basal section gives an excellent Bxa ( + ve) figure with a small 2V. Note Sillimanite can also occur in high temperature xenoliths found as
FIGURE
that basal sections are usually small, so a very high power objective lens residual products in aluminous rocks after partial melting has taken
will give the best figure ( x 55 or more). place. All the AI 2 Si0, polymorphs have been recognised as detrital
*OTHER FEATURES In high grade regionally metamorphosed rocks the fibrous sillimanite minerals in sedimentary rocks.
(formerly called fibrolite) is usually found associated with biotite,
appearing as long thin fibres growing within the mica crystal. Amphibole group Inosilicates
*OCCURRENCE All three polymorphs can be used as index minerals in metamorphic
Introduction
rocks. They all develop in alumina-rich pelites under different condi-
tions of temperature and pressure (Fig. 2.4.). Andalusite forms at low The amphiboles include orthorhombic and monoclinic minerals. They
possess a double chain silicate structure which allows a large number of
pressures ( < 1.5 kb) and low to moderate temperatures in thermal
elemental substitutions. The double chain has a composition of
aureoles and regional metamorphism of Buchan type (high heat flow,
(Si40,,)n, with some substitution by AP + for silicon. The chains are
low P). At higher temperatures it inverts to sillimanite. Kyanite forms at
joined together by ions occupying various sites within the structure,
medium to high pressures and low to moderate temperatures in regional
and these sites are called A, X and Y. The Y sites are usually occupied
metamorphism of Barrovian type (high heat flow, moderate or high P).
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2
2
by Mg + and Fe +, although Fe' +, AP +, Mn + and Ti + may also enter
2
At higher temperatures kyanite also inverts to sillimanite which occurs
2
2
the Y sites. The X sites are usually filled by Ca + or Ca + and Na +,
over a wide range of pressures and high temperatures. The sequences of
2
2
mineralogical changes in pelites are: although the orthorhombic amphiboles have Mg + or Fe + occupying
the X sites as well as the Y ones. The A sites are always occupied by
(a) Buchan (low P, high heat flow - 60 °C/km): (low grade) micas- Na +, although in the calcium-poor and calcium-rich amphiboles the A
sites usually remain unoccupied.
andalusite ( + cordierite) -sillimanite (high grade).
(b) Barrovian (moderate to high P, high heat flow - 30 °C/km): (low The main amphibole groups include:
grade) micas- staurolite- garnet- kyanite- sillimanite (highest
grade). (a) The Ca-poor amphiboles (Ca + Na nearly zero), which include
the orthorhombic amphiboles and the Ca-poor monoclinic
The P-T diagram (Fig. 2.4) shows the stability relations of the three amphiboles. The minerals included are the anthophyllite-gedrite
polymorphs. The minimum melting curve of granite has been superim- group (which have properties extremely similar to the
posed on to the diagram. To the right (up temperature) side of this curve cummingtonite-grunerite group in the monoclinic amphiboles).
melting has taken place and the polymorphs would therefore occur in The general formula is:
metamorphic rocks which had undergone some melting (e.g. migmatitic
X,Y ,Z.O,, (OH,F)z
rocks).
where X = Mg,Fe, Y = Mg,Fe,AI and Z = Si,AI.
minimum melting ------1 I / /
curve of granite 1 1/ (b) The Ca-rich amphiboles (with Ca > Na) are monoclinic, and
y include the common hornblendes and tremolite- ferroactinolite.
II The general formula is:
KYANtTE /
/ \ I SILLIMAN ITE AX,Y,z .o , (OH,F),
/~ \ with A= Na (or zero in some members), X = Ca, Y = Mg,Fe,AI
/ ........_ I and Z = Si,AI.
/ ........_ \
/ ""- ' (c) The alkali amphiboles are also monoclinic (with Na > Ca), and
ANDALUS ITE ~', the general formula is:
""'',:--,. __
.\lXI ~m sm om 71XJ XIX I AX, Y,Z 8 0 22 (OH,F),
Temperature ( C)
0
where A = Na, X = Na (or Na ,Ca), Y = Mg,Fe,Al and
Figure 2.4 Stability relations of the three AI,SiO, polymorphs. Also shown is Z = Si,AI. The main members are glaucophane-riebeckite,
the melting curve for albite + orthoclase+ quartz+ water, representing granite. richterite and eckermannite-arfvedsonite.
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