The hardness of a geologic material depends upon the hardness of each of the minerals of which it is composed and also on the relative amounts present of each of those minerals. If the hardnesses of the individual minerals and the percentages of those minerals are known, a numerical "hardness value" (HV) for the material as a whole may be calculated.
Two sets of data are needed:
  • The percentage of each mineral present
  • The Mohs hardness of each mineral
The calculation is simple:
  1. For each mineral, find the product of its percentage multiplied by its Mohs hardness.
  2. Add together all the products
  3. Divide by 100.
The Method Illustrated By A Worked Out Example - The Steps To Follow:
  1. Print out a copy of the 'Hardness Value Calculation Table' (printout # Q-3) (see example on the right).
  2. Enter the name of the Material (e.g., GNEISS) at the top of the Table.
  3. Enter the percentages of the minerals out of which it is made in the 'Mineral Percent' column. (Obtain these percentages from printout #Q-1A, the Mineral Composition Tabulation Sheet).
  4. Enter the hardness of each of the minerals in the 'Mineral Hardness' column. (Obtain the hardness from printout # Q-2, NY City Minerals Hardness Table).
  5. For each mineral, multiply (%M) times (MH) and enter the result in the (%M x MH) column.
  6. Add together all the results of step (5) and enter the sum in the TOTAL box.
  7. Divide the value in the TOTAL box by 100. Round off the result to two decimal places and enter it in the HARDNESS VALUE box.
The Hardness Values of five of the eight NYC geologic materials have been determined for you:
MaterialHardness Value (HV)
L.I. Hill Sediment5.26
L.I. Plain Sediment4.14
Gneiss 6.07
Schist 4.68
Shale 1.44

What You Need To Do:
Determine the Hardness Values of the remaining three NYC geologic materials:
1. Print out three copies of the 'Hardness Value Calculation Table' (printout # Q-3) and use them to determine the Hardness Values of diabase, marble, and sandstone.
2. Click on the 'Hardness Value Checking Table' button to see if your Hardness Values are correct.
3. Record all eight Hardness Values in column (a) of the Erosion-Resistance Index Calculation Table (printout #Q-6).

David J. Leveson