A HUNT FOR COLORADO SAPPHIRE

It was the warmest day of 2013 and an invitation for a field trip was too much to pass up!  So off I went with Mr. Rockounding the Rockies and Diggin’ Bob.  Both of these collectors concentrate on Pikes Peak amazonite and smoky quartz, and both are quite successful.  Our destination at this time was the Calumet Iron Mine near Salida in Chaffee County, a locality famous for crystals of epidote and diopside-actinolite (see blog posting December 8, 2012: Diopside-Actinolite at the Calumet Mine).  However, Calumet is also home to Colorado’s most famous sapphire locality! 

I had noted in my previous blog that after being unable to locate the sapphire during my trip in October 2012, I was going to return and try again.  Diggin’ Bob was after some of the gemmy clear quartz crystals while Mr. Rockhounding was looking for any crystals. The quartz is found as small, loose, but terminated crystals located in pockets of generally decomposed bedrock.  Holmes and Kennedy (1983) noted that at times the crystals do reach six inches in length, often with epidote fibers as inclusions. However, those larger crystals are the exception rather than the rule.

 

Looking west from Calumet Mine across valley of Arkansas River to Sawatch Range.

The day was sunny, warm and beautiful at the mine--elevation of about 8900 feet.  With shimmering snow on the peaks, the Sawatch Range pushed into the blue sky west across the Arkansas River Valley. 

So, I was after the elusive sapphire!  Eckel and others (1997) adequately describe the occurrence of sapphire at Calumet: Corundum, ranging from pale to deep blue in color, occurs as thin, rough, rhombohedral plates…in a feldspar-quartz-muscovite schist…comprises as much as 40% of the corundum bearing ledge, which is only one foot in thickness…extends northward up the mountain side.  One would think that with a description like that, the sapphire could be easily located by a rockhound.  Turns out that was a wrong assumption.  I spent most of the day walking up and down the mountain prospecting for the ledge---didn’t find it!  Finally, in mid-afternoon I decided to check out some of the epidote down in the main area of the mine. 

 

Epidote and calcite crystals. Maximum width 5 cm.

I love green minerals and the pistachio green color of epidote is just beautiful, although the color does range down into a dark green (that almost appears black).  At the Calumet some of the epidote is massive in nature and appears to comprise entire basketball size specimens.  On other rock faces the mineral is more of a “smear” but covers many square feet of surface.  There are also numerous veins of crystalline epidote, some almost “pure” but others mixed with crystals of quartz and calcite.  The veins appear to be fractures or joints that represent infilling by secondary mineralization.  At any rate, from a viewing distance hiking up to the mine, part of the quarry wall appears green in color—a nice sight.

So, epidote covered limestone was everywhere to collect but I also located loose crystals that had weathered out of the country rock; however, these were small compared to Eckel and others (1997) description of individuals two inches in length.

 

Partial crystals of epidote, each ~2.5 cm width.

Epidote [Ca₂(Fe,Al)₃(SiO₄)₃(OH)] is one of those minerals that is quite easy to identify due to its pistachio-to-dark-green color, prismatic and deeply striated crystals. Hardness ranges between 6 and 7 (Mohs) and most crystals have a rather vitreous luster. It is a common mineral wherever metamorphism has reached calcareous rock---like the Leadville Limestone at Calumet.  Epidote forms a solid solution series with clinozoisite [Ca₂Al₃Si₃O₁2(OH)] as Al replaces the iron. This latter mineral is pale green or light brown in color. If manganese is added to the formula then the mineral piemontite is created: (Ca₂)(Al₂Mn)(Si₂O₇)(SiO₄)O(OH). 

 

Leadville Limestone with surface covered by epidote and actinolite crystals.  Calcite crystal in a vug.  US quarter for scale.

While examining the mine face for epidote, I whacked off a chunk of the igneous rock (the Tertiary intrusive), rather than the altered Leadville Limestone and noticed a couple of the specimen faces exposing many tiny crystals.  Even with my numerous color deficiencies (bad color blindness—not good for a geologist) I could use the hand lens and see blue and/or purple and/or rose in some of the crystals.  This was exciting!  I looked for other samples lying around but seemed unsuccessful.  I carefully wrapped up the rock and packed it for examination at home—with the help of a microscope.

In addition to the epidote, actinolite and sapphire, garnet is reported from the mine, often occurring with actinolite. Holmes and Kennedy (1983) believed it to be grossular (variety hessonite) as did Eckel and others (1997), but no variety. I collected a small specimen (~2.5 x 3 cm) of what I think are garnets and actinolite. However, the stones are far from lustrous, as noted in Eckel and others (1997). Perhaps I misidentified the specimen. 

Calumet is an old iron mine and evidence of iron ore is conspicuous.  The major mining mineral was magnetite occupying a massive vertical vein cutting across most rock units.  Mining was accomplished both via underground shafts and open pit.  According to Holmes and Kennedy (1983) the amount of undesirable pyrite increased with depth of the paying vein (and less magnetite).  During the peak mining period in the late 1800’s the ore ran as high as 64% iron and mining evidently ceased when iron content reached ~43%.  Today, small crystals of magnetite are readily available and desirable specimens include clusters of crystals, as well as an occasional larger and loose specimen with well-positioned crystal faces. 

There are a variety of other mineral collected from the Calumet Mine, most uncommon.  If interested, check out www.mindat.org.  In addition, check the blog noted above for information on the stratigraphy and the diopside—actinolite—“uralite” specimens.

The sun was getting lower in the sky so it was time to begin the trek back down the 700 feet to the vehicle—much easier than the climb up but still somewhat unsteady at times due to several pounds of rocks in the backpack! 

 

Portion of quarry in Marble Gulch.

Hand specimen of coarse-grained marble ~ 17 cm length.

However, the collecting day was not quite over as we headed down the road to a report of marble cropping out in the hillside.  Locals seem to refer to the area as Marble Gulch; however, I was unable to find out much information about the mine(s), the geology, and the period of mining.  The marble is a very coarse rock and I suspect, but am uncertain, that the quarried rock was used in landscaping.  I also am uncertain about land ownership so we refrained from collecting---plenty of hand specimens littered the roadway (after falling from quarry trucks).

One of the day’s highlights, other than a fantastic day in the field, was the presence of a dozen Big Horn Sheep (Ovis canadensis) along the road.  Now, that was a great finale. 

 

Ovis canadensis near the Calumet Mine.

After returning home I grabbed the specimens and examined them with a microscope—both digital and stereo zoom.  I am convinced (mostly) that these tiny crystals are sapphire—for several reasons.  The color is unusual, pink, blue, purple, and these colors do not fit other minerals noted from Calumet.  Some of the crystals are broken and a barrel shaped (six-sided) crystal form is notable.  Other crystals are exposed as acicular and prismatic, some with a dipyramidal termination.  However, other authors have noted the sapphire occurs as larger bright-blue nail heads (the barrel shape), and is present in the dark-colored schist above the mine.  So, if readers have suggestions, I always appreciate comments.  Until then I will call them sapphires—and will return again until I locate the garnet bed and the sapphire schist!

On above three photomicrographs note the rose-colored, six- sided crystal cross sections.  Each is ~1.5 mm in width.

Photomicrograph, width ~9 mm.. Blue, almost massive crystals.

Photomicrograph.  Acicular and prismatic crystals.  Largest crystal ~2.5 mm.

REFERENCES CITED

Eckel, E. B. and others, 1997, Minerals of Colorado: Golden, CO, Fulcrum Publishing and Denver Museum of Nature and Science. 

Holmes, R. W. and M. B. Kennedy, 1983, Mines and Minerals of the Great American Rift (Colorado—New Mexico): New York, Van Nostrand Reinhold Company.

LAIHUNITE: EL PASO COUNTY, COLORADO

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 [(Fe^2+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!

REFERENCES CITED

Eckel, E. B. (and others), 1997, Minerals of Colorado: Friends of Colorado—Colorado Chapter and Denver Museum of Natural History, Denver.

 

SKUTTERUDITE: COBALT-NICKEL ARSENIDE

I have been interested in the mineral skutterudite since my mineralogy class a number of years ago.  It was a bright shiny mineral with nice crystals, and the name---who could not appreciate a name like skutterudite?  It just rolls off the tongue!  Several years later I drove through the nickel mining area of Sudbury, Ontario, and was amazed at what fumes from a nickel smelter could do to living organisms.  But, at a museum I noticed a beautiful specimen of cobalt-nickel arsenide—skutterudite—and decided that someday I would have some of those shiny crystals!  That day came on Friday when I visited the Spring Colorado Mineral and Fossil Show in Denver.

Specimen of skutterudite from Morocco with nice octahedral crystals on fresh surface. The majority of the surface, bottom and sides of specimen, is massive and granular.  White mineral is calcite.  Specimen is difficult to photograph as bright metallic luster reflects light.  Length of specimen ~4 cm.

Skutterudite [(Co,Fe,Ni)As_2-3] has a very metallic luster, sort of a silver to tin-white color, and a hardness of 6 (Mohs) or a little less.  If crystals are present they are usually cubic or octahedral (Isometric) but often specimens are massive or granular.  The mineral is a major ore of cobalt and nickel.

The specimen I purchased came from the 12 Irhtem Mine in the Bou Azzer Mining District of the Anti-Atlas Mountains of southern Morocco, an area “famous” for producing the world’s best specimens of skutterite (and erythrite, roselite, talmessite, wendwilsonite, and gersdoffite). The Bou Azzer skutterudite has a fairly high cobalt content (12%-18.5%) with nickel coming in at 7%-7.5% and iron at 1.4%-8.85%.  In addition, the skutterudite has an unusually high gold content of 120 grams per ton.  It is interesting to note that the local population (Berbers) knew about the toxicity of the outcropping arsenates long before the onset of commercial mining—they used it for insect control and rat poison.

Photomicrograph of skutterudite crystals each ~ 2 mm.

Serpentinization (formation of serpentine group minerals by low temperature metamorphism) and weathering processes of the nickel-cobalt ores begin sometime in the Precambrian (prior to 800 Ma) but may have reached a peak during an orogenic volcanic episode about 550 Ma.  The process finally reached an end during an intense deformation and faulting about 250 Ma.  The highest grade ores are always associated with the serpentine group minerals, and found in areas of intense deformation.

Nickel oxidizes and corrodes quite slowly and therefore is commonly used as a plating agent, and in alloys with other metals (commonly with steel as in stainless steel).  Cobalt is also used in alloys, commonly to strengthen steel.  However, the most people know about “cobalt blue” where cobalt silicate imparts a beautiful blue color to glass, ceramics, and paint.

Skutterudite (cobalt-rich) also is in solid solution with choanthite (nickel-rich) and smaltite (intermediate).  It is my understanding that identification of hand specimens by physical appearance is quite difficult.

Information about mining at Bou Azzer was gleaned from: www.blnz.com/news/2008/05/14/Famous_mineral_localities_AZZER_MOROCCO_9115.html.