What does the constancy of interfacial angles refer to? How did ...

[mia]

What does the constancy of interfacial angles refer to? How did Nicholas Steno explain his
  observation? What will be an ideal response?

Question 2

Why do some minerals form regular crystals and some do not? Even the same mineral, like quartz,
  may form beautiful crystals or irregular blobs. What will be an ideal response?

[janeli]

Answer to Question 1

Nicholas Steno in 1669 determined that the angles of intersection of equivalent crystal faces on
different specimens of the same mineral are identical. He postulated that mineral crystals are made up
of very small, identical building blocks and that the arrangement of these building blocks determines
the external form of the crystals.

Answer to Question 2

While all minerals are crystalline solids in which their atoms are arranged in a three-dimensional
framework, some minerals grow to possess planar surfaces, sharp corners, and straight edges because
they have space to grow into their crystal form. Minerals that do not form crystals grow in proximity
with other crystals and do not have space to form a regular crystal.