As readers of this Blog realize, I am always on the lookout for “different” minerals for my modest collection, and for interesting minerals from the Intermountain West and a few Plains states. It is hard to define “different” other than to state a mineral that others might overlook or a mineral I do not recognize (not really that hard) or a mineral that just seems weird to me! So, it was no surprise (at least to me) that after observing a small specimen of laihunite, I nabbed it right off the shelf for a couple of bucks (at Sauktown Sales). It had sort of a South Pacific ring to the name. But then, it turned out to be sort of a double serendipitous moment as the specimen was from El Paso County, Colorado, at Crystal Park. That locality is only a few miles from my home in Colorado Springs. Unfortunately, Crystal Park is on private land and generally inaccessible to the average roaming rockhound. So, I was happy to acquire a specimen from an older collection.
|Photomicrograph of olive-colored fayalite with black laihunite, the weathering product. Width of fayalite crystal is ~2.2 mm. The additional "black" material is probably a mixture of magnetite and laihunite as the specimen is partially magnetic.|
In introductory geology classes instructors always talk about the “mineral” olivine—for many reasons. Olivine is a magnesium-iron silicate [(MgFe)2SiO4] and may be the most common mineral in the earth’s mantle. It usually is associated with basalt, or a few metamorphic rocks, or ultramafic rocks such as dunite or peridotite. However, the kicker is that olivine is not really a mineral but simply gives its name to a solid solution group or series—the Olivine Group. Forsterite is the magnesium-rich end member while fayalite is the iron-rich other member. Olivine is a general term (in introductory geology and in the vocabulary of most rockhounds) indicating a composition somewhere in the middle between these two end members. But that is OK as most rockhounds are more interested in the gemstone segment of olivine, peridot.
Although fairly rare in the mineral world, laihunite is now known to be an iron silicate probably derived from the weathering of fayalite (the iron-rich olivine). In addition, some of the iron in the mineral is ferrous iron while another portion is ferric iron. Eckel and others (1997) noted that laihunite is iron deficient in that it contains only 1.5 atoms of Fe for each Si atom. The formula is written as [(Fe2+Fe3+)2(SiO4)2.
|Additional photomicrograph of fayalite and laihunite (as above)|
As best that I can determine from www.MinDat.org, laihunite is only found at two general localities in the U.S.: the CrystalPark/St. Peters Dome area of Colorado (Pikes Peak), and the Obsidian Cliffs area of Oregon (North Sister Mountain). In addition, there are a few other areas yielding the mineral in scattered parts of the world—but not many. At Crystal Park Eckel and others (1997) reported the laihunite occurs in a quartz-microcline-biotite pegmatite (part of the Precambrian Pikes Peak Batholith).
By the way, the name laihunite has nothing to do with the South Pacific but was named for the Laihe iron deposit in Manchuria, China. But, I suppose that not many rockhounds have a sample of this mineral in their collection!
Eckel, E. B. (and others), 1997, Minerals of Colorado: Friends of Colorado—Colorado Chapter and Denver Museum of Natural History, Denver.