DIOPSIDE-ACTINOLITE AT CALUMET MINE

THE "CUT" AT THE CALUMET IRON MINE.  THE LARGE BLOCKS COMING DOWN THE HILL ARE PIECES OF THE LATE PALEOZOIC LIMESTONE.  MANY DISPLAY SURFACE AREAS COVERED WITH CRYSTALLINE EPIDOTE AND DIOPSIDE--ACTINOLITE.  NOTE PERSON FOR SCALE (DIGGING FOR QUARTZ CRYSTALS)

 Colorado is fortunate to have a large number of collecting areas available for visiting by rock and mineral clubs.  One of the standard collecting localities for club members is the Calumet Iron Mine down in Chaffee County about 7 miles north-northeast of the city of Salida.  There seems to be a gazillion articles written about field and collecting trips to the Calumet, so I will just add another one!

Fall weather in Colorado 2012 has been spectacular for collecting trips but less so for the skiers.  At this writing in early December the highs are in the low 60’s and several ski resorts remain closed---no snow.  So, when Yam of the West called me about a trip on Saturday, I eagerly accepted.  Where to---a creek walk for petrified wood?  To northeast Colorado for barite crystals?  Or over to Salida for actinolite, epidote and other crystals?  I chose the latter since I had never actually visited the old mine, and I am a sucker for green minerals.  So, off we (Yam of the West, SharonRocks, and Phil the Digger) went the next day and with a couple of coffee stops arrived mid-morning.

The Mine was discovered/located shortly after 1880 and seemed to be a major producer until the end of the century, generating somewhat less than 250k tons of iron ore.  The ore was taken from the large veins of high grade magnetite shot through the area.  Today there is a large open cut (actually “cuts”) on the hillside plus a number of shafts leading underground (to where I don’t have the slightest idea since the tunnels seem quite decrepit and my momma didn’t raise no fools).  According to Modreski (2005) late Paleozoic carbonate rocks were baked and altered by igneous rocks associated with emplacement of the Whitehorn Granodiorite; remnants of the limestone are clearly visible. Wrucke (1974) described the Whitehorn as including a pluton and a couple of small satellite bodies that intrudes the Paleozoic rocks (and also some of Precambrian age) along the east flank of the Arkansas River Valley east and northeast of Salida.  The rocks generally range in composition from quartz monzonite (less quartz than granite and equal amounts of plagioclase and orthoclase feldspars) to granodiorite (similar to granite but with more plagioclase feldspar than orthoclase feldspar) along an outcrop area of ~16 miles by ~5 miles.  The small satellite body at Calumet is a sill where the igneous rocks are tabular or sheet-like in shape. Wrucke (1974) reported radiometric dates of ~70 Ma or Late Cretaceous and associated with the Laramide Orogeny (the mountain building event associated with the Colorado Rocky Mountains).  The Turret Mining District (includes the Calumet) also has produced small amounts of copper, gold, silver, vermiculite, marble, and feldspar. In 2011 Real Aspen (www.realaspen.com) noted that, “earlier this month, Canada-based Rare Earth Industries announced it is exploring tantalum, beryllium and manganese at the formerly abandoned Turret Mines it acquired northeast of Salida in Chaffee County, Colo”.

One of the more interesting aspects of the Calument mine era was the construction of the Calumet Branch of the Denver & Rio Grande Western narrow gauge railroad in 1881.  The Calumet was a spur line off the main line (Tennessee Pass) and ran from Hecla Junction in Browns Canyon about 8 miles to the mine.  The short line brought the ore down from the mine and transferred it to a line that ultimately moved it to the big steel mill, Colorado Fuel & iron (C&FI), in Pueblo.  The short line was especially steep, about 7% grade, and the curves were tight.  The empty ore cars returning to the mine were “pushed” up the canyon with a locomotive in the rear.  The cars coming down the canyon were “pulled” but the train had a brakeman stationed on each car (shoes wore out quickly).  The mine closed in 1899 and the line was wiped out by a flood in 1901 (above from Rio Grande Info, www.DRGW.net).  Today, concrete remnants of the loading docks and rail line are still in place, having survived weathering for over 100 years.

THE DYNAMITE SHACK AT CALUMET.

I suppose the Calumet Mine is best known for a couple of minerals: diopside--actinolite pseudomorphs, and epidote.  The most interesting might be the former.  Diopside is a magnesium-calcium pyroxene, MgCaSi₂O₆, (see blog posting November 25, 2012) and forms a solid solution with end member hedenbergite (iron-calcium silicate) while augite is situated somewhere in the middle with differing amounts of added titanium, aluminum and sodium.  I remember in my intro geology class that somehow I was always confusing augite with hornblende (a complex amphibole) since both minerals are black in color and both are common constituents in igneous rocks.  After letting me suffer for several labs the instructor finally explained about cleavage, ~56^o and 124^o in hornblende while cleavage in augite is nearly “square” at 90^o.  That tidbit of information really helped! 

DIOPSIDE--ACTINOLITE PSEUDOMORPH.

LONG BLOCKY CRYSTALS OF DIOPSIDE-ACTINOLITE.  WIDTH OF SPECIMEN ~3.5 CM.

Diopside is commonly found in mafic igneous rocks (containing high amounts of magnesium and iron such as gabbro) but also in metamorphic rocks, especially where contact metamorphism has heated some carbonates.  Diopside crystals commonly are columnar to prismatic and are close to augite in cleavage angles: ~87^o and 93^o—sort of like elongated rectangles!

DIOPSIDE-ACTINOLITE.  NOTE LARGE BLOCKY CRYSTAL TO RIGHT.  WIDTH OF SPECIMEN ~4.5 CM.

At the Calumet Mine, results of the contact metamorphism included the formation of many crystals of diopside.  However, post-metamorphism the crystals have pseudomorphed, or altered, into actinolite while keeping the same crystal shape of diopside.  Now, most readers realize that I often just fumble around with the complexities of mineralogy and petrology.  And, this is a case of “how did it happen and why”?  I don’t have the slightest idea about the intricacies of this alteration and certainly could use some help from a competent mineralogist.  At one time the diopside-actinolite was given the mineral name, uralite (now discredited).

Actinolite is a complex amphibole silicate, Ca₂(MgFe)₅Si₈O₂₂(OH)₂, and is the  middle member of a solid solution series between tremolite (magnesium-rich end member) and ferro-actinolite (iron-rich end member).  From my observtions I probably cannot tell the difference between actinolite and tremolite and I believe there is much gradation between the two (and also most likely with the ferro-actinolite).  Actinolite often occurs in long prismatic crystals with two planes of cleavage so that a cross section of a single crystal would resemble a diamond (it is an amphibole and resembles the cleavage of hornblende); fracture is often “splintery”. 

PHOTOMICROGRAPH SHOWING "SPLINTERY" SMALL CRYSTALS OF DIOPSIDE--ACTINOLITE.

Some actinolite, the non-pseudomorph type, may have an interesting crystal appearance.  The mineral occurs as blades radiating out from a central point.  I picked up my specimen from an old mine dump on the trail back to the vehicle.

There are a couple of other very interesting types of actinolite out in rock land.  Very fibrous actinolite is classified as a form of asbestos.  I don’t believe that the mineral is mined for such at the present.  Second, a variety called nephrite is one of the two forms of jade (and that mineral is confusing enough), the other being jadeite, a pyroxene.  No wonder I get the amphiboles and pyroxenes confused!  The famous ‘apple-green” jade from Wyoming is nephrite. 

RADIATING CRYSTALS OF ACTINOLITE.  WIDTH OF SPECIMEN ~6 CM.

We also returned to the Springs with a variety of other minerals including garnet, epidote, and quartz.  However, that will be another story.

REFERENCES CITED

Modreski, P. J., 2005, Colorado Mineral Collecting Localities: Rocks and Minerals, Sept.-Oct.

Wrucke, C.T., 1974, The Whitehorn Granodiorite of the Arkansas Valley in Central Colorado.  U. S. Geological Survey Bulletin 1394-H.