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When plant and animal matter gets buried under tons of dirt and rocks
for millions of years, it is sometimes transformed into coal. Heat and
pressure inside the earth turn a type of soft, brown coal called lignite
into harder, more useful coals. There are two different types of coal;
bituminous and anthracite. Bituminous coal is softer and puts more pollution
into the air when it is burned. Anthracite coal is harder, shinier and burns
with less pollution.
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