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

Abundance
*  a=abundant   c=common, r=rare and vr=very rare
Occurrence
** v=veins, am=amydules, p=pebbles, 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.  The most distinctive minerals of the park are those that can be found in the many veins (mineral-filled cracks) which cut across the sandstone and lava outcrops, in the formerly void bubbles in  tops of lava flows which we call amygdules, and in beach pebbles which are most often formed from the harder amygdules which are rounded by wave action.  These minerals formed more than one billion years ago when hot water flowed through the porous parts of the rock and deposited the multicolored  minerals in what was formerly void spaces in the rock.  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 to Thomsonite, and it is hard to distinguish among the zeolite minerals natrolite, mesolite and laumontite.

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 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).

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

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, “swelling chlorite”	
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, yellow, 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	
Xonotlite   vr	Ca6Si6O17(OH)2	6.5	colorless, gray	v  with and like datolite	

***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.

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.