Two unknown white, lustrous minerals were tested for hardness and streak. Both had a hardness of 4, and had a white streak. What can you conclude from these tests?

Fluorite

Explanation: Minerals are naturally occurring inorganic materials with definite chemical compositions and definite physical properties. Hardness and Streak are both physical characteristics used in identifying minerals. The colour of a mineral’s powder defines its streak (it does not change from mineral to mineral) and a streak of white shows that such mineral has a calcium fluoride making it a fluorite.  To check streak, one simply scrapes the mineral across an unglazed porcelain plate.

Hardness on the one hand, is the resistance to cutting, scratching, abrasion or indentation of minerals.  Mineral hardness is specified by the Mohs hardness scale when working with hand samples, and the scale ranges from 1 (very soft like talc) to 10 (as hard as diamond). A hardness of 4 also put such mineral into the fluorite range indicating that it is harder than calcite, but softer than feldspar.

In conclusion, the tests show that the mineral in question is a fluorite.