Is there a difference between Conchoidal, and Sub-conchoidal? If there is what is the difference and is this something we are going to need to know for the Quiz on Wed. 2/14/01 ?

-- Katie L. France (loki2cute@yahoo.com), February 12, 2001

Answers

Sub-conchoidal fractures have a rounbed breakage surface. Cochoidal fractures have a rounded breakage surface with faint concentric striations on it. I looked this up on the minerals lab backgrounder.

-- Hannah VanDam (petunia130@hotmail.com), February 13, 2001.

Conchoidal fracture is a curved breakage surface. It may be very glassy-looking, as with Obsidian. If you look at a sample of obsidian, you can easily see the curved surface. The "striations" can resemble ripples, like the ones you see when a drop of water lands in a puddle. These striations are the very things that seperate conchoidal from sub-conchoidal, as a sub-conchoidal fracture is a rounded breakage surface, but without the little "ripples". An example of sub-conchoidal fracture can be found in a sample of quartz.

-- Heather Derrigo (hderrigo@oswego.edu), February 16, 2001.