Deposits Formed by Sedimentary and Surficial Processes - Metals and Society: An Introduction to Economic Geology - page 134

Solution: When producing aluminum in Australia the transport costs are
negligible but to refine 1 t of metal from 4 t of bauxite consumes 300 kWh of
coal-produced electricity, which emits 300 t of CO 2 . Transportation to
Iceland emits only 40,000

4gor4tofCO 2 and emissions from
electricity production are negligible. From this back-of-the-envelope calcula-
tion, it is clear that it is ecologically sound to ship ore half-way across the
world to benefit from the low cost - both financial and in terms of CO 2
emission - of electricity in Iceland.

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Just as for iron, there is little chance we will ever exhaust our resources of
aluminium. The reserves of bauxite are enormous, about 25 billion tons, enough to
last more than 300 years even allowing for greatly increased consumption. And
even if bauxite were ever totally mined out, even larger amounts of Al are present in
clay and feldspar, whose abundance is immeasurable. Even now some aluminium is
produced in Russia by mining feldspar in alkaline intrusions.
The world's bauxite deposits are found mainly in equatorial regions with tropical
climates; Guinea, Australia, Brazil and Jamaica. The biggest and richest deposits
form on continental peneplains that are subject to long periods of alternating wet
and dry seasons, conditions that cause the water table to oscillate. As its level of
groundwater rises and falls, the more soluble components are leached out and what
is left is a lateritic soil that retains only the most insoluble components. In parts of
Africa, South and Central America and Australia, lateritization on flat peneplained
surfaces has persisted for up to 100 million years resulting in a thick deeply
weathered lateritic blanket whose thickness may reach 150 m.
The majority of major rock-forming elements - Si, Fe, Mg, Ca, Na and K - are
moderately to highly soluble in the near-neutral (pH 5-9) waters in lateritic
weathering horizons. The oxides of these elements make up about 80% of feld-
spathic rocks such as granite, gneiss or shale and if they were totally removed, the
concentration of Al 2 O 3 increases fourfold, from about 15% (the level in the source
rock) to close to 60%, the level in rich Al ore. The best bauxites are mixtures of Al
hydroxides - gibbsite (Al(OH) 3 ), and two polymorphs with the composition AlO
(OH), boehmite and diaspore. In practice, the removal of the more soluble
components is not complete; some of them are retained in partially altered rock
and others accumulate, often in secondary minerals, at various levels in the lateritic
soil profile.
In Fig. 5.9 we show a profile through two lateritic ore deposits, one containing
bauxite and the other a Ni laterite. The saprolith zone at the base of the profile
consists of highly weathered rock that preserves much of its original texture and
structure. Feldspar and the ferromagnesian minerals are destroyed and the soluble
components are partially removed while Si and Al are retained in clays and some Fe
is retained in goethite and hematite. In the upper part of the saprolith and in the
pedolith all but the most resistant primary minerals are destroyed and the rock in