Page 71 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS CLAY MINERALS
form amphibole and plagioclase. In igneous rocks chlorite is usually a I II VMiE { 001} perfect. Another poor possibly prismatic fracture may be present
secondary mineral, forming from the hydrothermal alteration of pyr- which distinguishes chloritoid from micaceous minerals.
oxenes, amphiboles and biotites. Chlorite may be found infilling Kl I lEI· High.
amygdales in lavas with other minerals, and may occur as a primary I II KA liON Chloritoid may alter to muscovite and chlorite, but this is not common.
mineral in some low temperature veins. II MINCI I NC"E Low but masked by greenish colour of mineral, often anomalous blue
Chlorites are common in argillaceous rocks where they frequently colours are seen.
occur with clay minerals, particularly illite, kaolin and mixed-layer clays. II Mil KI. NC"E A bluish green coloured basal section of chloritoid will give a Bxa figure
IIIIIJ RE with a moderate 2V and positive sign.
Chloritoid Nesosilicate I II I li ON Straight to perfect on { 001} cleavage.
111 111 1 KS Zoning occasionally appears as a peculiar hourglass shape seen on
Chloritoid (ottrelite) (Fe,Mg),(Al,Fe'+)Al 3 0 ,[ Si04] ,(OH)4 monoclinic
prismatic sections.
1.725 :1:3.314, ,6 = 101°30'
1 1 I MM I NC"E Chloritoid occurs in regionally metamorphosed pelitic rocks with a high
Fe' +: Fe'+ ratio, at low grades of metamorphism. Chloritoid develops
about the same time as biotite, changing to staurolite at higher grades.
Chloritoid can occur in non-stress environments where it usually shows
triclinic crystal form, particularly in quartz carbonate veins and in
altered lava flows.
Clay minerals Phyllosilicates

The clay minerals are extremely important in weathering processes.
Many primary igneous minerals produce clay minerals as a final
weathering product. Feldspars particularly give rise to clay minerals;
plagioclase feldspar reacts with water to give montmorillonite, and
orthoclase feldspar in a similar way produces illite. If excess water is
present both montmorillonite and illite will eventually change to kaolin,
which is always the final product.

I ulh I urullte) triclinic
I I 0.576: 1:0.830
I I
I I a = 91°48' , ,B = 104°30', y = 90°
I I n,. 1.553-1.565
, I 1.56-1.57
~ 11 /J
,., y 1.56-1.57
n. = 1.713-1.730 8 0.006
n p 1.719-1.734 2V. = 24°-50° - ve
nr 1.723-1.740 AP perpendicular to (010)
1
8 0.010 0 = 2.61- 2.68 H = 2- 2 h
2V r = 45°-68° + ve (normal range). 2V can be highly variable with 2V r
IIIIIIIIK olourless.
36°-90° + ve and 2V. 90°-5SO - ve
11 1111 Similar to mica group, but crystals are extremely tiny.
OAP is parallel to (010)
"IIII I Low.
D = 3.51-3.80 H = 61/2
1 11 1 111 Perfect basal - similar to micas.
coLOuR Colourless, green, blue green. tiMI"I" Nil' Low, greys of first order.
* PLEOCHROISM Common with a pale green, ,B blue andy colourless to pale yellow. 11 HII 111 1111 1 Size of individual crystals is such that interference figures can rarely be
HABIT Closely resembles mica minerals, occurring as pseudo-hexagonal tabu- lllillkl' obtained.
lar crystals. 1 111111 1111111 Straight but occasional slight extinction angle on (010) face.
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