Page 100 - A Practical Introduction to Optical Mineralogy

SiliCATE MINERALS HUMITE GROUP

staurolite which gives garnet and kyanite. In high heat flow, low P n. np n, 0
metamorphism garnet may form from cordierite breakdown if th Ill Ill' glt 1.561-1.567 1.567-1.579 1.587-1.593 0.026
chemistry is correct. In thermal aureoles in, and regional metamorphism I hmulrodlte 1.592-1.643 1.602-1.655 1.619-1.675 0.025-0.037
of, impure limestones, grossular forms; and spessartine forms during the ll umlh 1.607-1.643 1.619-1.653 1.639-1.675 0.028-0.036
metamorphism of Mn-rich rocks. Andradite occurs in thermally c lluuhumJte 1.623-1.702 1.636-1.709 1.651-1.728 0.028-0.045
metamorphosed impure calcareous sediments, and particularly in Variation in RI is caused by Fe + and Ti + entering structure.
2
4
metasomatic skarns.
Pyrope garnet is an essential constituent of some ultrabasic igneous 2V, D
rocks, especially garnet-peridotite and other similar types derived from urlwq~lte 44°-50° +ve 3.15-3.18} H = 61/2
the Earth's upper mantle. High grade metamorphic rocks (very high / 1 hutulrodlte 64°-90° +ve 3.16-3.26
and T) called eclogites have garnet of almandine-pyrope composition as 65°-84° +ve 3.20-3.32}
H= 6
an essential constituent, along with the pyroxene omphacite. 52°-90° +ve 3.21-3.35
Garnet also occurs as a detrital mineral in sands.
lUI OU R Pale yellow or yellow.
IIIIIIIIKOISM Norbergite, chondrodite and humite have a pale yellow, {3 colourless or
Humite group Nesosilicates rarely pale yellow, andy colourless. Clinohumite has a golden yellow,
and {3, y pale yellow.
orthorhombit•
nMg 2SiO.Mg(OH,F) 2
II Alii r Anhedral masses of crystals usually occur. Occasionally large subhedral
. . 0.855 (No)
There are four members of the group; norber- 0 463 1 porphyroblasts can be present.
· · "2.057 (Hu)
gite (with n = 1), chondrodite (n = 2), humite IIIAVMJE Basal { 001} usually present.
(n = 3) and clinohumite (n = 4). monoclinit• Ml ll llF Moderate to high (clinohumite).
2.170:1:1.663 ( h) I II M l iON All the humite minerals alter to serpentine or chlorite, as follows:
0.462: 1: 1.332 ( I)
Chondrodite Mg 2SiO •. Mg(OH,F) 2 + H 2 0 + Si0 2 ~ Mg,Si 2 0 ,(0H,F).
section parallel to 010 Humite
c
(OAP section) Moderate to high (clinohumite) giving maximum upper second order
section parallel 111 interference colours (lower third order, clinohumite).
001 (nordbcrgit~:
is similar) II Mil KI!N E Single optic axis figure yields positive 2V which varies in size in different
II( ORE humite minerals (see above).
I liN( li ON Norbergite and humite have straight extinction on cleavage (both
orthorhombic), whereas chondrodite has a'cl = 3° to 12° and clino-
humite a'cl = oo to 4° (both monoclinic forms).
I INN ING Simple or multiple twinning on { 001} in monoclinic forms.
ION ING Common, shown by colour intensities.
II NIII/1 II ING Similar to olivine except for pale yellow colour with moderate interfer-
II I II RES ence colours. Yellow colour in olivine normally implies alteration to
c
Clinohumite serpentine with low or anomalous interference colours. Olivine 2V is
a section parallel to 010 a = a
very large and usually negative. Staurolite has higher Ris, lower bi-
refringence and occurs in schists. Individual humite group members are
difficult to distinguish from each other.
( o 1/ MKI NC'E The humite minerals have a restricted occurrence, being found in con-
a = ~ tact metamorphosed and metasomatised limestones and dolomites, near
acid or alkaline intrusions.

88 89