ISLE ROYALE NATIONAL PARK
Checklist of Minerals
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Date(s)_______________________________________
Location (s) on Isle Royale ___________________________________________________________________________________________________________
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                                                      drawing here--Greenstone+ agate +prehnite
                                                                   by Susan Robinson

Abundance                                               Occurrence
a=abundant                                                 v=veins,
c=common,                                                  am=amygdules
 r=rare and                                                  p=pebbles 
vr=very rare                                                ab=altered basalt

Note to park visitors:
Minerals, like all natural things in the park cannot be collected and removed.  Enjoy them where you find them!

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.  These are usually fine grained  and it is difficult or impossible to see the minerals. The dense basalt was partly altered by hot waters to various minerals which can be detected either by the dark green color (chlorite) or swelling aspect (corrensite).   The sandstone of SW Isle Royale is made up of feldspar, quartz and rock fragments.  Glacial activity has carried rocks from Canada, some of them hundreds of miles, and deposited them on Isle Royale.  These exotic rocks, granites and gneisses, are only found in the surface gravels or on beaches.  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 - then empty voids - were filled by hot fluids that circulated throughout the rocks and deposited new minerals. 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 The minerals also occur in 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.  Some of the minerals are difficult to identify, even for experts.  The pink prehnite of Isle Royale is very similar in appearance to thomsonite, and it is hard to distinguish among the zeolite minerals natrolite, mesolite and laumontite.

Where to look for minerals and how to identify them:
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 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. 

Table 1: Distinctive Minerals of Isle Royale

Name, abundance	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·nH2O 	1-2	Dk green to black	ab, “swelling chlorite”	
Datolite    r	 CaBSiO4(OH) 	5.5	white or pink, various	v, am  porcelaineous, opaque	
Epidote     c	Ca2(Fe,Al)Al2(SiO4)3(OH) 	7	pistachio green	am, ab  crystalline, amygdules	
Laumontite  c	CaAl2Si4O12·4H2O 	3.5-4	wh-brown, gray, yellow, pink	am  bladed aggregates , weathers powdery white	
Microcline   r	KAlSi3O8 	6	flesh-pink	v, am	
Mesolite     vr	Na2Ca2Al6Si9O30·8H2O 	5	white-grey-yellow	am  needly	
Natrolite     vr	Na2Al2Si3O10·2H2O 	5.5-6	colorless-white	am  radiating fibers 	
Prehnite      r	Ca2Al2Si3O10(OH)2 	6-6.5	Lt Green-pink	v, am, p  globular, pearly	
Pumpellyite  c	Ca2MgAl2(SiO4)(Si2O7)(OH)2 . H2O	5.5	Green	am, p  mosaic pattern	
Quartz**    c	SiO2	7	colorless-varied	am, v, p 	
Saponite   r	(Ca/2,Na)0.3(Mg,Fe)3(Si,Al)4O10(OH)2 ·4H2O	1.5-2	Lt green-yellow-gr	am  granular, earthy	
Thomsonite r	 NaCa2Al5Si5O20·6H2O	5-5.5	pink, white	am, p  radiating  xtals	
Xonotlite   vr	Ca6Si6O17(OH)2	6.5	colorless, gray, pink	v  with and like datolite	

*Hardness here refers to the  Moh’s Scale, a relative scale going from 1 (soft, represented by the mineral talc) to 10 (very hard, represented by diamond).  Some common availble items that can be compared are human fingernails (2.5) a copper coin(3.5) a knife blade or glass (5.5) and a steel file(7.5).  

**Quartz occurs in many distinct forms: agate (banded or mossy amygdaloidal 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.

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 Mineral 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.

Figure  1: A schematic cross section of a basalt lava flow showing the amydules located in the flow top, in segregation cylinders and pegmatites and in pipe amygdule zones at the flow base.  The thickness of flows varies from less than one to more than 200 meters. Veins (not shown) are tabular or sinuous ranging from <1cm to a meter or more in thickness and may cut across the flows in any direction. 

Compiled by William I. Rose, George Robinson and Susanne Schmidt.  The drawing of the minerals on the front page was made by Susan Robinson.  The sketch of the lava flow cross section is based on a drawing by John Green.