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iolite

General information on IOLITE

IOLITE [CORDIERITE] (Mg,Fe)2Al4(Si5O18) Iolite is the gem-quality variety of a mineral known as cordierite. Cordierite forms orthorhombic (pseudohexagonal) crystals with short prismatic habit that are typically found in mica-rich gneiss or schist and in rare mafic aluminum-rich igeous rocks known as syenite and anorthosite. The crystal habit is as rounded, pebble-like grains, massive nodules, and/or disseminated grains. Due to alteration, cordierite may be partially replaced by sericite (a tiny white mica) and quartz, with chlorite (massive green mica) reaction rims. It is typically translucent to transparent dark gray, smoky gray, blue to violet anad light-blue crystals. Transparent grains are strongly pleochroic with vitreous (glassy) luster that produces attractive gemstones. The mineral is brittle, has conchoidal fracture, and may have poor cleavage and parting. It ranges in hardness from 7 to 7.5 with specific gravity of 2.55 to 2.75. This means it is hard enough to produce durable gem material and light weight enough that it is unlikely to concentrate in stream placers. Even so, nearly all of the iolite on the market today is mined from streams and rivers in Sri Lanka as a by-product of other gem mining. Cordierite is sometimes referred to as dichorite due to strong pleochroism, which results in transparent to translucent minerals that appear to change shades of color depending on the angle viewed. In one direction, the mineral will appear sapphire-blue. When rotated, it may appear light grayish-blue to gray. It has also been referred to as Water Sapphire. When gem-quality, it is referred to as iolite. Occurrence. Cordierite forms distinct porphyroblasts in muscovite (aluminum-rich) schist & gneiss in some metamorphosed sedimentary rocks known as metapelites. The mineral is sometimes found in these alumino-silicate-rich rocks in amphibolite-grade (rocks formed by moderate pressure and temperature) metamorphic terrains. It is also a product of high temperature crystallization in rare aluminum-rich igneous rocks. When searching for iolite, look for areas on geological maps listed as 'Precambrian rocks', 'mica schists', 'cordierite schists', 'metapelites', 'syenites' and/or 'anorthosites'. The principal deposits in the world are found in the Laramie Mountains of Wyoming. A group of iolite deposits were found in this region from 1995 to 2005 known as Palmer Canyon, Grizzly Creek Sherman Mountain, Horse Creek and Owen Creek. These deposits were summarized in a 2009 book on Wyoming gems and minerals. Other localities include Canada and Sri Lanka. Nearly all iolite (the gem variety of cordierite) is mined as a secondary product in gem placers in Sri Lanka resulting in a very inconsistent supply of this gemstone. As a result, don’t expect to find this attractive gemstone in your local jewelry store.


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Identification tips:
Iolite is strongly pleochroic (changes colors as rotated in daylight). A dark blue specimen will appear to change to light blue to gray when rotated.

Additional helpful resources:
[include links to websites, book titles, maps, clubs]
Websites http://gemhunter.webs.com/ http://gemstonehunter.blogspot.com/ http://wygemstones.blogspot.com/ http://iolite-wyoming.blogspot.com http://www.rmag.org/rmag_uploads/outcrops/314.pdf http://en.wikipedia.org/wiki/Cordierite
Books & papers on iolite
  • Hausel, W.D., 2009, Gems, Minerals and Rocks of Wyoming. A Guide for Rock Hounds, Prospectors & Collectors. Booksurge, 175 p.
  • Hausel, W.D., and Sutherland, W.M., 2006, World Gemstones: Geology, Mineralogy, Gemology & Exploration: WSGS Mineral Report MR06-1, 363 p.
  • Hausel, W.D., 2006, Minerals & Rocks of Wyoming, A Guide for Collectors, Prospectors and Rock Hounds, WSGS Bulletin 72, 125 p.
  • Hausel, W.D., 2005, Geologists Locate Giant Gemstones: Prospecting and Mining Journal, v. 74, no. 7, p. 7-9.
  • Hausel, W.D., 2004, Geological Reconnaissance of the Grizzly Creek Gemstone deposit – Potential Source for Iolite, Sapphire, Ruby & Kyanite WSGS Open File Report 04-14, 8 p.
  • Hausel, W.D., 2002, Iolite and corundum in Wyoming: Gems & Gemology, v. 37, no. 4, p. 336-337.
  • Hausel, W.D., 2002, A new source of gem-quality cordierite and corundum in the Laramie Range of Southeastern Wyoming: Rocks & Minerals, vol. 76, no. 5, p. 334-339.
  • Hausel, W.D., and Sutherland, W.M., 2000, Gemstones & Other Unique Minerals & Rocks of Wyoming - A Field Guide for Collectors: Wyoming Geological Survey Bulletin 71, 268 p.
  • Hausel, W.D., 1998, Field Reconnaissance of the Palmer Canyon corundum-kyanite-cordierite deposit, Laramie Mountains Wyoming: WSGS Mineral Report MR98-1, 7 p.
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DiamondProspector
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Keyword tags: cordierite hunting for treasure Iolite treasure types of treasure where to find treasure
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DiamondProspector New Section on Iolite Gems 14 Aug 30 2009, 9:08 PM EDT by DiamondProspector
Thread started: Aug 25 2009, 1:42 PM EDT  Watch
Good Day,
We added a new section in Gems & Minerals (listed under Treasure Hunting). See side box to your left on this wiki site where you can 'search this site'). It took me awhile to find the Gems & Minerals section. The new iolite section is located at
http://cash-and-treasures-wiki.travelchannel.com/page/Iolite
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