Minerals in amygdules, michigan’s keweenaw

The most obvious properties of the amygdule minerals can be used to identify them. The sport of mineral collecting is done by many visitors who use this kind of mineral Identification to find local treasures, including agates and datolites.

Bill Rose

Professor of Petrology

15 November 2010

Where to look for minerals and how to identify them:

One way to find minerals at Isle Royale or in the Keweenaw, you should walk the coastlines, especially those that are well wave-washed.  The waves expose the minerals and pebbles of various minerals, can be found on adjacent beaches.   Using a canoe or small boat, watershoes and plenty of time, walk the shore and watch for veins and amygdaloids.  Observe the interiors of basalt flows where vesicle cylinders, pegmatites, joints and veins may expose these distinctive minerals (see diagram).

Individual minerals are sometimes difficult to identity, even for experts, but certain groups of minerals can be distinguished very easily. All you need is your finger nail. The phyllosilicates chlorite, corrensite and saponite are all of green color and very soft minerals. You can easily scratch them with your finger nail. The other green minerals as pumpellyite or prehnite are much harder and you will not be able to scratch them with your fingernail. In fact, in pebbles along the shore they stand out, since they are not as easily eroded as the surrounding rock. the pink unusual color of  prehnite of Isle Royale often is the result of very tiny inclusions of native copper which makes it similar to the zeolite thomsonite.The zeolite family is in general difficult to identify. But the zeolite laumontite can easily be recognized. It is of white or pink color and if you touch it with your finger nail it will split up into small fibers.

What’s next?  After mastering the mineral identifications in the boulders, students can also look at amygdular minerals to study the order that minerals deposited in those vesicles, what mineralogists call paragenesis.