G. Sign of Elongation
Crystals with a prismatic, acicular or elongate habit can be used for sign of elongation. It is a useful diagnostic property especially in the Amphiboles and Pyroxenes. To take a sign of elongation orient the long axis of the crystal in question NE-SW and insert the accessory plate. The colors will either add or subtract. It is commonly expressed as Length-fast (Negative Elongation) or Length-slow (Positive Elongation). Examine a slide containing Tremolite and a slide containing Hornblende and determine the sign of elongation for each mineral.