alkaline earths resemble the alkali metals, but are less strongly
reducing. They occur only in the zero or +2 oxidation states, and
are always found naturally as +2 ions. They are found as soluble
chloride salts in sea water, and in minerals such as sulfates, carbonates,
phosphates, and silicates. Calcite, chalk, limestone, and marble
all are forms of calcium carbonate, .
Dolomite and some marbles are mixtures of
and magnesium carbonate, .
Gypsum is a calcium sulfate. Approximately 4% of the Earth's crust
is Ca and Mg ions. Beryllium, strontium, and barium are much rarer,
the most common source of Be being emeralds and lesser forms of
beryl, a beryllium-aluminum silicate.
Magnesium and calcium usually are obtained by electrolysis of fused
salts such as molten ,
although these +2 salts also can be reduced by metallic Na or K:
Here the chloride ions play a passive role in the reaction, which
involves only a transfer of electrons from sodium to magnesium ion.