Page 142 - A Practical Introduction to Optical Mineralogy

ZIRCON
SILICATE MINERALS
Zircon Nesosilicate
deposited by late stage hydrothermal solutions. Analcime is closely
related to this group of minerals, which also includes natrolite tetragonal, cia 0.891
Zircon ZrSi0 4
(Na 2 AI 2 Si 3 0 10 .2H 2 0). The zeolites are widely used as indicator minerals
in th'ick lava piles, such as ocean floor basalts, to determine temperature n 0 1.923-1.960
n. = 1.968-2.015
and depth of burial. A typical sequence from a recent Icelandic lava pile
8 = 0.042-0.065
is:
Uniaxial +ve (prism section is length slow)
D = 4.6-4.7 H = 71/2
top
zeolite-free zone COLOUR Colourless pale brown.
chabazite-thomsonite CaAl 2 Si.0,2.6H20- *HABIT Very small, squat, square prisms occur with terminal faces. Zircons are
[ NaCa2(AI,Si),0, 0 ] 2.6H20 usually found as euhedral crystals.
analcime ( + natrolite) NaAISi 2 0 6 .H 2 0 (for natrolite
CLEAVAGE {110} imperfect; {111} poor.
see above) *RELIEF Extremely high.
mesolite-scolecite [Na 2 Ca 2 Al 2 Si 3 0, 0 ),.8H 2 0-
ALTERATION None.
bottom CaAI 2 Si 3 0, 0 .3H20 *BIREFRINGENCE Very high, a prismatic crystal section will show third or fourth order
interference colours.
Although other areas of extrusive rocks may show slight variations 'in the TWINNING Rare.
zeolites present, the zones described above generally occur. Natrolite ZONING May be present due to outer metamict zones on an unaltered core.
and most other zeolites are colourless in thin section, with Rls very much DISTINGUISHING Tiny euhedral crystals in alkaline or acid plutonic igneous rocks are
lower than the cement. They mostly belong to orthorhombic or mono- FEATURES usually zircon. Sphene is pale brown with usually a diamo~d-~haped
clinic crystal systems (natrolite is orthorhombic), with either straight cross section and is biaxial +ve. Monazite is biaxial +ve. Cassttente and
extinction or small extinction angle. Natrolite is Ieagth slow. Their 2V is rutile are coloured minerals.
usually large +ve or -ve and their birefringence is variable but low. An accessory mineral found in all igneous rocks, but essentially in
*OCCURRENCE
Their occurrence in vesicles and amygdales is the most reliable indicator intermediate to acid varieties, where it is associated with biotite crystals.
for identification. X-ray diffraction techniques are required for positive Haloes frequently occur in the biotite surrounding minute zircon crys-
identification of zeolite type. The main optical properties of the zeolites tals (due to radioactive decay of U and Th damaging the biotit~ structu~e
are as follows: by f3 particle bombardment). Zircon is most commonly found m plut?mc
igneous rocks, particularly granites, granodiorites, diorites, syem~es,
*cOLOUR Colourless. nepheline-syenites and pegmatite veins. Zircon also occurs as a d~tntal
HABIT Apart from analcime (see 'Feldspathoid family'), most zeolites are mineral in sediments, and will survive through many metamorphtc and
elongate fibrous or platy, often occupying cavities or amygdales in melting events.
extrusive igneous rocks.
CLEAVAGE Variable depending upon crystal system. Most fibrous varieties possess
at least one prismatic cleavage.
*RELIEF Low to moderate; RI is less than 1.54 for all minerals.
ALTERATION Rare, but a few zeolites will alter to clay minerals.
*BIREFRINGENCE Generally low to very low. A very few zeolites may show first order
yellow.
INTERFERENCE Variable.
FIGURE
EXTINCTION All fibrous varieties have straight extinction on prism edge except for
scolecite. Platy varieties usually possess inclined extinction.
TWINNING Simple twinning is common in mesolite, laumonite, chabazite and
stilbite. Multiple twinning is common in scolecite, phillipsite and
harmotome.

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