The Mohs scale of hardness is a relative ranking of ten reference minerals, numbered 1 (softest) to 10 (hardest), where any mineral higher on the list can scratch every mineral below it. Their standard order is Talc, Gypsum, Calcite, Fluorite, Apatite, Feldspar, Quartz, Topaz, Corundum, Diamond, going from softest to hardest.
- Topaz > Quartz > Calcite > Talc: Topaz sits at position 8, Quartz at position 7, Calcite at position 3, and Talc at position 1. Written hardest to softest, that is 8, 7, 3, 1, a strictly decreasing order, so this sequence is correct.
- Quartz > Topaz > Calcite > Talc: this puts Quartz (7) ahead of Topaz (8), but Topaz is the harder of the two, so the first two entries are swapped.
- Quartz > Topaz > Talc > Calcite: besides the same Quartz-Topaz swap, this also places Talc (1) ahead of Calcite (3), when Calcite is the harder mineral, so two separate errors appear here.
- Topaz > Calcite > Quartz > Talc: this drops Quartz (7) below Calcite (3), even though Quartz is far harder than Calcite, so the middle of the sequence is out of order.
Only the first sequence keeps the mineral hardness numbers in strictly decreasing order all the way through, so Topaz > Quartz > Calcite > Talc is the sequence that matches the Mohs scale.
Let's summarize:
- The Mohs scale numbers, softest to hardest, are Talc 1, Gypsum 2, Calcite 3, Fluorite 4, Apatite 5, Feldspar 6, Quartz 7, Topaz 8, Corundum 9, Diamond 10.
- A correct hardness sequence must always list the higher scale number before the lower one when going from hardest to softest.
So the mineral hardness order Topaz > Quartz > Calcite > Talc is the one consistent with the Mohs scale.