Why do these minerals:orthoclase and microcline (both KAlSi3O8), have the same names?

Question

I don't understand how two different minerals can have the same name.

Answer 1

Do you mean you don't understand how they can have the same formula? They have the same formula but the elements are arranged differently--they have different structures.

Orthoclase is a K-spar with monoclinic symmetry. MONOCLINIC, requires either 1 two fold axis of rotation or 1 mirror plane. Orthoclase does not show the lamellar twinning that is common in microcline.
Microcline is a K-spar with triclinic symmetry. TRICLINIC, requires either a center or only translational symmetry. Microcline tends to be deeper-colored and may be blue-green.

Answer 2

A little correction first, orthoclase and microcline don't have the same name, they share the same chemical formula. When viewed in temperature/pressure relationships, orthoclase and microcline (along with sanidine, which also has the same chemical formula) form at different regions. If memory serves me correctly, sanidine forms at high temperatures and microcline forms at higher pressures. Orthoclase forms at lower temperatures and pressures.