Page 182 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES
SULPHATE$
DISTINGUISHING Brucite has anomalous birefringence and is uniaxial positive. Micas and Occurrence Limonite is very common as a weathering product after iron-bearing
FEATURES
talc have higher birefringence and are optically negative, as is gypsum. minerals, especially iron carbonates and iron sulphides. It is associated
Apatite and serpentine are very similar optically, but serpentine is with other hydroxides and oxides in various types of gossans.
usually greenish and both are always length slow. Distinguishing Compared with limonite, hematite is brighter, harder and has only
features scarce internal reflections, and sphalerite is isotropic and usually differs
Limonite FeO.OH.nH,O texturally.
Goethite
Lepidocrocite
Limonite is brown earthy material consisting of goethite ::!:: lepidocrocite 3. 7 Sulphates
with absorbed water.
Crystals Goethite a-HFeO, is orthorhombic, a: b: c = 0.4593: 1:0.3034. Anhydrite Caso. orthorhombic
Lepidocrocite y-FeO.OH is orthorhombic, a :b :c = 0.309:1:0.245. 0.893: 1 : 1.001
c=-y
Both minerals occur as flakes or blades flattened (010) or as fibres I
elongated [ 100]. There is perfect cleavage on { 100}, { 010} and { 001}. I
I
D = 4.28 (goethite); D = 4.0 (lepidQcrocite). I
Thin section Poorly crystallised limonite appears reddish brown and isotropic.
Goethite is yellowish to brownish, pleochroic with absorption
a < /3 < y, and has a smaii2V, -ve. Lepidocrocite is yellow to brownish
red, strongly pleochroic with absorption a < {3 < y, and has
2V = 83°, - ve.
Polished Poorly crystallised limonite is bluish grey with R = 16-19%; anisotropy
section is strong in bluish greys, and deep red to brown internal reflections are
typical. Goethite is grey with R = 17%; anisotropy is distinct in shades
of grey. Lepidocrocite is grey with R = 10-19%; anisotropy is very
strong in slightly bluish light greys. Internal reflections are deep red to
brown in both minerals.
/
Limonite is often inhomogenous, varying in colour or porosity. /
/
Goethite is usually colloform and botryoidal whereas lepidocrocite is /
usually better crystallised. VHN: 772-824 goethite; 690-782 / /
lepidocrocite. / /
a=~
Limonite n. = 1.569-1.574
'heavy mineral' grain of pyrite n p = 1.574-1.579
(white) oxidised and replaced n, = 1.609-1.618
by limonite (grey) a = o.o4
2V, = 42°-44° +ve
OAP is parallel to (100). Crystal is elongated along the a axis and can
be length fast or slow
D = 2.9-3.0 H = 3-3112
coLouR Colourless.
HABIT Prismatic crystals with aggregates common.
CLEAVAGE {010} perfect, {100} and {001} good.
PPL
300 )J.m
I RELIEF Low to moderate.
ALTERATION Just as gypsum can dehydrate to anhydrite, so anhydrite can react with
water to form gypsum.
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