II. Give reasons: (8 Marks)
1.
Granitic rocks are not explored for chromite.
Because chromite (and the
element Cr) is concentrated in the early-stage differentiate such as
ultramafics, whereas granite is a late differentiate felsic rock.
2.
Magnetite is classified as a spinel mineral.
Because it is a
double-oxide made up of two oxides, one is of ferrous Fe (Fe2+) and
the other is ferric Fe (Fe3+).
3.
Galena is not a common placer mineral.
Because galena is not a
stable mineral during weathering and it is commonly altered to cerrusite (PbCO3)
and/or anglesite (PbSO4).
4.
Framboidal pyrite is not always of biogenic origin.
Because it is sometimes
found hosted by VMS ores.
5.
Tetrahedrite is a sulfosalt.
Cu12 Sb4
S13 is the chemical formula of tetrahedrite and Sb atoms partially
replaces part of the sites occupied by the S atoms which is a must for any
sulfosalts.
6.
Cassiterite belongs to the rutile-group minerals.
Chemical formula of
cassiterite is SnO2, i.e. XO2 which is general formula of
rutile-group minerals (TiO2).
7.
Corundum belongs to the hematite-group minerals.
Chemical formula of
corundum is Al2O3, i.e. X2O3 which
is general formula of hematite-group minerals (Fe2O3).
8.
Al Gurryah alkaline granites in NW Saudi Arabia are economically
important.
Because they host large
amounts of Nb-Ta ores.
IV. Compare between each of the
following pairs: (8 Marks)
1)
|
Wadi Fatime Fe ore
|
Wadi Sawawin Fe ore
|
|
-
Jurassic Fe ore (Phanerozoic).
-
Makkah-Jeddah
highway.
-
Common Fe-mineral is
hematite.
-
Oolitic &
pisolitic textures.
|
-
Precambrian Fe ore
(Proterozoic).
-
Wadi Sawawin, NW KSA.
-
Rich in magnetite,
hematite & jasper.
-
Banding &
folding.
|
2)
|
Magnetite
|
Manganite
|
|
-
It is Fe ore
mineral.
-
Oxide mineral (spinel
group).
-
Chemical formula: Fe3O4.
-
Common in Igneous
rocks & BIF.
-
Magnetic mineral.
|
-
It is Mn ore mineral.
-
Hydroxide mineral.
-
Chemical formula:
MnO(OH).
-
Common in sedimentary
rocks.
|
3)
|
Pyrrhotite
|
Troilite
|
|
-
It is Fe-sulphide but
with
Fe deficiency.
-
Chemical formula: Fe1-xS.
-
Sometimes magnetic.
-
Often found in
mafic-ultramafic
rocks intergrown with
pentlandite.
|
-
It is Fe-sulphide but
with no
Fe deficiency.
-
Chemical formula: FeS.
-
It is non-magnetic.
-
Only found in
Fe-meteorites
|
4)
|
Acanthite
|
Argentite
|
|
-
Ag2S poly
morph of argentite.
-
Stable at room
temperature.
|
-
Ag2S
polymorph of acanthite.
-
Forms as cubes at 173
°C.
|
------------------------------------------------------------------------------------------------------
V. Write short accounts on: (8 Marks)
1. Varieties of sphalerite.
Sphalerite, also known as Blende or Zinc Blende, is the major ore of
Zinc. When pure (with little or no iron) it forms clear crystals,
usually
red (known as Ruby Blende) but as iron content increases it forms
dark,
opaque
metallic crystals (known as Marmatite).
2. Economic importance of oxide ore
minerals.
• They include the chief iron ores (e.g. hematite &
magnetite).
• They include common Ti ores (e.g. ilmenite &
rutile).
• Also, they include other important ores that yield
strategic metals for modern industries, e.g.:
(Cr ore: chromite), (Sn ore: cassiterite) and (W ore: wolframite).
3. Peacock ore.
It is a complex ore of
copper, where Cu ore minerals (mostly supergene) are gathered together in the
same sample.
In most cases, it is
composed of bornite, chalcocite, chalcopyrite, covellite and others.
4. Ore
minerals in bauxite deposits