*  a=abundant   c=common, r=rare and vr=very rare
** v=veins, am=amydules, p=pebbles, ab=altered basalt
***Quartz occurs in many distinct forms: agate (banded or mossy amydaloidal fillings), carnelian (clear, red chalcedony) , jasper (opaque red yellow, green or brown), amethyst (purple crystals) or rock crystal (colorless crystals) are all found on Isle Royale.  Smoky quartz is also present in many beach pebbles.

Isle Royale Minerals

The rocks of Isle Royale are mainly massive, dark colored basalt lavas which are made up primarily of feldspar and pyroxene with lesser amounts of other minerals.  The sandstone of SW Isle Royale contains mainy feldspar, quartz and rock fragments. When the lava flows cooled down about one billion years ago, gases separated within the lava flow and gas bubbles preferentially rose to the top of the lava flows and were frozen in.  After the solidification of the lava these gas bubbles - now empty voids - were filled by hot fluids that circulated throughout the rocks and deposited new minerals in these empty bubbles. The filled bubbles are now called amygdules. The temperature of the hot fluid determined the mineral assemblage or group of minerals. Zeolites, for example, are deposited at lower temperature than prehnite or pumpellyite. The most beautiful and distinctive minerals of the park specimens come from these amygdules in the basalt since the basalt provides all the necessary elements for the formation of these minerals but also from veins (mineral-filled cracks) which cut across the sandstone and lava outcrops. 

The minerals are part of one of the most famous mineral districts of the world, the Lake Superior Copper District, which was the first great metal mining district in North America.  Isle Royale is most prominently known for its greenstone,  a beautiful nodular compact form of pumpellyite that is Michigan state gemstone. The distinctive minerals are listed in the table, with some of their characteristics as well as some hints and tips to identify them.

Where to look for minerals:
If you want to find minerals at Isle Royale, you should walk the coastlines, especially those that are well wavewashed.  The waves expose the minerals and pebbles  and 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 (=the amygdaloidal flow top of a lava flow).  Observe the interiors of basalt flows where vesicle cylinders, pegmatites, joints and veins may expose these distinctive minerals (see diagram).

For more Information:
For more information about the geology of Isle Royale, consult the book by Huber (1983, The Geologic Story of Isle Royale National Park, USGS Bull 1309, 66pp) which is available in the park.   The minerals of this area are exhibited all over the world and you surley can find them in your local museum.  The largest and most extensive exhibit of minerals of the Keweenaw and Isle Royale can be seen at the AE Seaman Mineralogical Museum in Houghton, Michigan, near the IRNP headquarters.  The headquarters itself has a small mineral display.  A new, illustrated book about the Minerals of Michigan by EW Heinrich is in press and will be available in 2001.

Table 1: Distinctive Minerals of Isle Royale

The individual mineral is 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 stick 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. 

Table 1: Distinctive Minerals of Isle Royale

Mineral *	Chemical Formula	Hardness	Color	Occurrence & Notes**	
Barite     c	BaSO4 	3-3.5	White	v	
Calcite    a	CaCO3 	3	colorless,white,pink	v,am	
Chlorite   a	(Mg,Fe)5Al2Si3O10(OH)8	2-2.5	olive-dk green-black	am, ab	
Copper     r	Cu	2.5-3	copper, green-blackcoatings	v,am	
Corrensite  a	(Mg,Fe,Al)9(Si,Al)8O20(OH)10∑n(H2O) 	1-2	Dk green to black	ab, ìswelling chloriteî	
Datolite    r	 CaBSiO4(OH) 	5.5	white or pink, various	v, am  porcelaineous, opaque	
Epidote     c	Ca2(Fe,Al)Al2(SiO4)(Si2O7)O(OH) 	7	pistachio green	am, ab  crystalline, amygdules	
Laumontite  c	CaAl2Si4O12∑4(H2O) 	3.5-4	wh-brown, gray, pink	am  bladed aggregates, weathers powdery white	
Microcline   r	KAlSi3O8 	6	flesh-pink	v, am	
Mesolite     vr	Na2Ca2Al6Si9O30∑8(H2O) 	5	white-grey-yellow	am  needly	
Natrolite     vr	Na2Al2Si3O10∑2(H2O) 	5.5-6	colorless-white	am  radiating fibres 	
Prehnite      r	Ca2Al2Si3O10(OH)2 	6-6.5	Lt Green-pink	v, am, p  globular, pearly	
Pumpellyite  c	Ca2MgAl2Si3O11(OH)2  H2O	5.5	Green	am, p  mosaic pattern	
Quartz***    c	SiO2	7	colorless-varied	am, v, p 	
Saponite   r	(Ca,Na)0.3(Mg,Fe)3(Si,Al)4O10(OH)2 ∑4H2O	1.5-2	Lt green-yellow-gr	am  granular, earthy	
Thomsonite r	 NaCa2Al5Si5O20∑6(H2O) 	5-5.5	pink	am, p  radiating  xtals	
Xonolite   vr	Ca6Si6O17(OH)2	6.5	colorless, gray	v  with and like datolite