There are many types of sulfide minerals, but pyrite and marcasite, the two structural forms of FeS2 that are especially common in coal regions, are the major acid-producers.

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Belonging to a class of worlds known as diamond planets, 55 Cancri e is thought to be rich in the element carbon, which can exist in a variety of structural forms such as graphite (the material used as pencil lead), graphene, or diamond.

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It represents a perception that such differences have some social significance-primarily because of perceived cultural differences though these are frequently associated with forms of structural differentiation.

UNESCO: Social and Human Sciences

There are a couple of structural problems with health insurance that are not present in other forms of insurance.

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