Are melting points for mixtures of solids ALWAYS below the melting points of the individual components?
Is this always true?


No. As a general rule of thumb, if two compounds are miscible in each other, that is they exhibit solid solution, then the melting point of a (chemical, not mechanical) mixture will be intermediate. If the two compounds are not miscible and it's only possible to make a mechanical mixture and not a chemical mixture, then you will have a lower melting point.

Let's try to see this with several examples of phase diagrams, taken from FACT.


  ...mixtures of compounds are going to have melting points lower than either of the components pure melting points.


This is called eutectic relationship, for example sodium chloride and sodium sulfate. There is no intermediate compound, and they have a eutectic relationship, where the melting point of any mixture is lower than the melting point of both compounds. Note that if you are not in the pure liquid field, you will have a pure compound together with a liquid that is a mixture.



Silver and gold are a good example for a case when there is complete miscibility. All intermediate compounds can exist, therefore the melting point of the mixture changes continuously with the composition:



The double line is there because there is a very thin temperature range in which a liquid coexists with the solid of a different composition. See the solid solution page for more about that.

These were two simple cases. More complex cases arise when you have discrete compounds rather than a complete solution solution. A rather good example is cerium and nickel:



This system consists of several eutectic and peritectic points.