Page 140 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS ZEOLITE GROUP

coLOuR Colourless but thick sections may be yellowish or pink. cLEAVAGE {1120} and {1011} poor, often appearing perpendicular to the prism
HABIT Usually occurs as prismatic crystals subhedral to anhedral. zone.
*CLEAVAGE { 001} perfect. RELIEF Moderate.
RELIEF Moderate. BIREFRINGENCE Moderate, colours of second order seen but frequently masked if
*INCLUSioNs Liquid inclusions are common either of water or brewsterlinite ( orig- tourmaline has a strong body colour.
inally thought to be C0 2 ). TWINNING Rare.
• ALTERATION Topaz alters to clay minerals (kaolin) and sericite. One reaction is as ZONING Colour zoning may occur.
follows: occuRRENCE Tourmaline typically occurs in granite pegmatites, pneumatolytic veins
and some granites as the schorl-elbaite type. In the pneumatolytic stage
2AI2SiO.(OH,F)2+ 2H 2 0 + 2Si0 2 ~ AI.Si 4 0, (0H,F) of alteration, tourmaline may form by boron introduction, and the rock
0 8
topaz kaolin luxullianite forms in this environment. In pneumatolytic igneous
assemblages tourmaline is associated with topaz, spodumene,
This is a hydrothermal alteration taking place at a late stage, in the cassiterite, fluorite and apatite. In metamorphic rocks (especially
presence of free silica. metamorphosed limestones) and metasomatic rocks, the dravite type of
BIREFRINGENCE Low. tourmaline occurs; and dravites have been recorded from basic igneous
INTERFERENCE A single optic axis figure is required to obtain sign. rocks. Tourmalines have been found as detrital minerals in sedimentary
FIGURE
rocks, and as authigenic minerals in some limestones.
•occuRRENCE Topaz is a mineral found in late stage acid igneous rocks such as granites,
rhyolites and pegmatites, where it can also occur in cavities. It is associ-
ated with pneumatolytic action and is a constituent of greisen. It occurs Vesuvianite Sorosilicate
with quartz, cassiterite, tourmaline, fluorite and beryl. It has been
Vesuvianite (or idiocrase) tetragonal, cia 0.757
known to form in metamorphosed bauxite deposits. Ca 10(Mg,Fe),Al 4 Si 9 0,.(0H,F) 4
1.708-1.752
n 0
Tourmaline Cyclosilicate n. = 1.700-1.746
8 = 0.001-0.008
Tourmaline Na(Mg,Fe,Mn,Li,Al),AI 6 Si 0, (BO,)J(OH,F) trigonal, cia 0.447 Uniaxial -ve (prism section is length fast)
6 8 4
D = 3.33-3.43 H = 6-7
The composition of tourmaline varies greatly, with many types known.
Thus dravite is the magnesian variety (NaMg, etc.), schorl the ferro- coLOuR Colourless, pale yellow, pale brown.
manganoan variety (Na(Fe,Mn) 3 etc.) and elbaite the lithium-bearing HABIT Prismatic crystals usually occur, but in general crystals are subhedral
type (Na(Li,Al), etc.). with only a few faces present.
CLEAVAGE {110} and {100} pOOL
no = 1.610-1.630+
n. = 1. 635 _1. 655 + RI depends on composition RELIEF High.
BIREFRINGENCE Low, greys of firSt Order.
8 = 0.021-0.026+
Vesuvianite is a difficult mineral to recognise; in relief and birefringence
Uniaxial -ve (all types) (prismatic sections are length fast)
it resembles zoisite (see 'Epidote group'). However, it usually occurs as
D = 2.9-3.2 H = 7-7%
large mineral grains and its occurrence and mineral associations are
•coLOUR Highly variable; colourless, blue, green, yellow. most important.
*PLEOCHROISM Elbaite is usually colourless, but the other varieties are pleochroic: *occuRRENCE Vesuvianite or idocrase occurs in thermally metamorphosed limestones
and in skarns. It is associated with grossular (Ca-bearing) garnet,
dravite 0 dark brown e pale yellow diopside and wollastonite. It has been found in nepheline-syenites and
0 yellow brown e yellow in veins in basic igneous rocks.
schorl 0 dark green e reddish violet
0 blue e pale green, pale yellow
Zeolite group Tektosilicates
*HABIT Tourmaline almost always occurs as large elongate prismatic crystals, The zeolites are hydrated alumina silicates of K, Na and Ca. They occur
often occurring in radiating clusters. in amygdales and vesicles in basic extrusive rocks where they are
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