Page 98 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS GARNET GROUP
*occuRRENCE A characteristic primary crystallising mineral of alkali igneous rocks. 1 11 AVAGE Poor { 001} fracture.
Nepheline is an essential constituent of silica-deficient nepheline- Ml 1 ll f. l· Moderate (less than 1.54 ).
syenites and may occur in volcanic rocks where it is associated with "' 1 IIMK•N r. It has been suggested that the analcime in some intermediate and basic
high-temperature feldspars. rocks such as dolerite, teschenites and essexites (porphyritic alkali gab-
Nepheline may be metasomatic in origin, formed by the reaction of bros) is primary, but recent work has questioned this. In volcanic rocks
alkali-rich magmatic fluids with country rocks. Nepheline may occur in analcime occurs in some basalts as a primary mineral, but most com-
basic rocks near their contact with carbonate rocks; and alkali dolerites monly as a late stage hydrothermal mineral crystallising in vesicles and
with interstitial nepheline have also been described. found with zeolites -especially thomsonite and stilbite.
Analci me occurs as an authigenic mineral in sandstones, again associ-
Another mineral associated with nepheline is kalsilite (KA1Si0 4 )
which forms a limited solid solution with NaA1Si0 4 . Up to 25% of the ated with the zeolites laumontite and heulandite. Analcime has also
nepheline molecule can be replaced with kalsilite, although the amount been found in pyroclastic rocks.
of solid solution increases with increasing temperature, up to a maxi
mum of 70% at 1070 oc. Kalsilite is not found in plutonic igneous rock~ Garnet group Nesosilicates
but is found in the ground mass of some K-rich lavas.
X, Y2Si,01 2 cubic
Sodalite Na.Al6Si602.Cl2 cubic n D
n = 1.483-1.487 lm mdlne (X = Fe) (Y =AI) 1.830 4.318
D = 2.27-2.88 H = 5 12-6 I' rope (X = Mg) (Y =AI) 1.714 3.582
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c:ru ~ular {X = Ca) (Y =AI) 1.734 3.594
coLouR Colourless, sometimes pale blue or pink. I" nrtlne (X = Mn) (Y = AI) 1.800 4.190
HABIT Usually anhedral but occasional eight-sided anhedral crystals occur. uclr dlte (X = Ca) (Y =Fe) 1.887 3.859
CLEAVAGE Poor {110}.
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AJ>+ occupies theY position in the above minerals, but Cr'+, Fe +, and
RELIEF Moderate (much less than CB).
• Sodalite and analcime are virtually indistinguishable in thin section. Ti•+ as well as REEs may also substitute in garnet minerals in this
The type of rock is as good a guide as any as to which mineral it i . position. Solid solutions covering a wide range are possible within the
garnet group of minerals, with a corresponding wide range of optical
occuRRENCE Found in nepheline-syenites in association with nepheline and fluorite.
properties- RI varies from 1.890 to 1.710, and density from 4.32 to
It occurs in metasomatised calcareous rocks near alkaline intrusions.
3.59. A variety called hydrogrossular, which contains hydroxyl groups,
Nosean Two related minerals, nosean (Na 8 Al 6 Si 6 0 24 S0 4 ) and hauyne, which hn\ may show an RI as low as 1.68 and a density down to 3.1.
1111 otJK Colourless, pale brown or pale pink, dark green or brown.
Hauyne a similar composition to nosean with some S and Ca in the structure,
11 1111 Euhedral crystals of garnet showing six sided or eight sided sections are
Cancrinite have virtually identical optical properties. Nosean is distinguished by
having a dark border around each crystal and usually occurs in silica common.
poor alkaline rocks such as phonolites and leucitophyres. Hauyne i\ •1 11 ~vA<o~• No cleavage exists but crystals often fractured.
indistinguishable from sodalite and also occurs in phonolites and other Mll l l' l High to very high; sections always appear to have a rough surface.
undersaturated rocks, but is usually accompanied by sulphide minerab 1 IIIlA liON Fe-Mg bearing garnets typically alter to chlorite (by hydrothermal
such as pyrite. Cancrinite shows higher birefringence than nephelin~: reaction), with quartz being produced in the reaction.
(first order yellows and reds), is uniaxial negative, and is a characteristi · Garnet is isotropic bot sometimes hydrogrossular in particular may be
mineral in similar rocks to those in which nepheline occurs. Cancrinit • anisotropic and zoned.
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may be an alteration product of nepheline. 11111 N 11 111 KI S Garnet occasionally exhibits compositional zoning, and in metamorphic
medium grade rocks garnet frequently contains inclusions of quartz and
Analcime (Analcite) NaAISi206.H20 cubit• micas. Some metamorphic garnets contain inclusions defining an earlier
fabric (e.g. schistosity) which has been trapped within the garnet.
n - 1.479-1.493
1 1 IIMMr N('1 1 Garnet occurs mainly in metamorphic rocks. Almandine (containing
D = 2.34-3.29 H = 5'h
some Mg) is the main garnet of middle grade metamorphism of pelitic
coLOUR Colourless. rocks and igneous rocks. Almandine forms by progressive metamorphic
HABIT Anhedral crystals filling interstices between mineral grains. reactions involving chlorite, and by a reaction between quartz and
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