PERHAMITE, A RARE PHOSPHATE FROM MAINE

 

Although I enjoy collecting minerals, especially phosphates, in the pegmatites of the Black Hills of South Dakota I often wish the rocks of Maine were a wee bit closer to my home in Colorado! Since the journey is more than a day trip I am left with purchasing specimens found at rock and mineral shows, a good alternative. Interestingly, to an ole softrocker like me who spent most of his professional career looking for fossils in Mesozoic and Cenozoic outcrops, rocks of Maine generally lack exposures of these ages except for scattered small igneous intrusions and dikes of Mesozoic age. Maine rocks are Precambrian and Paleozoic (mostly Cambrian through Devonian) in age with a covering of unconsolidated sediments left behind by Pleistocene glaciers. The latter seems to support a heavy growth of trees and undergrowth that often inhibits the discovery of the underlying bedrock. However, there are many “stone” quarries around the state that have allowed geologists and rockhounds to collect a bonanza of interesting minerals and gems.  Geology.com noted that the first commercial gemstone mine in the U.S. dates back to 1821 when gem tourmaline was collected near Paris in a pegmatite on a hill known as Mt. Mica.  That mining has continued to the present. Most Maine collecting localities are in pegmatites scattered across the state (especially in the counties of Oxford, Androscoggin, and Sagadahoc ---AKA Maine Pegmatite Belt) that offer rockhounds and geologists opportunities in both commercial mines, such as Poland Mining Camps, and in abandoned quarries.  In addition, there is the Maine Mineral and Gem Museum in Bethel containing stunning displays, and also housing the Mineralogy, Petrology, and Pegmatology Research Group. The latter entity “focuses on exploring the pegmatites of Maine and their associated minerals.”

One of the most studied pegmatites in Maine is the Emmons Pegmatite found on Uncle Tom Mountain in Oxford County where beryl and other minerals were found in the early 1900s and first mined for feldspar in the 1930s; however, that venture was rather short-lived due to the long distance of the mine from a processing plant.  Early mineral collecting enthusiasts then started extracting beryl and ultimately collected about 5000 carats of gem morganite (pink beryl); however, specimen mining did not really take off until the late 1900s.; ---see the complete collecting and mineral story in Falster and others (Rocks and Minerals, v.94, no.6).

One of the interesting minerals from the Emmons that popped up in rummaging through my collection (from a purchase several years ago) was the phosphate perhamite [Ca₃Al_7.7Si₃P₄O_23.5(OH)_14.1-8H₂0], a specimen originally collected by Micromounter Hall of Fame member Gene Bearss.  The mineral is a rare calcium aluminum silico-phosphate that usually is found as isolated small spherules in late-stage pegmatite vugs. The Bearss specimen (photo below) shows the common morphology--a spherical aggregate of subvitreous hexagonal plates. Falster and others noted that perhamite from the Emmons pegmatite ranks among the finest examples of this species. Width of the spherule is ~1.0 mm.

SILVER AT MT. SHERMAN TUNNEL MINE

I wasn't born with a silver spoon in my mouth in Kansas City but I was born with a love for nature and the outdoors in a poor central Kansas farming community.

In the Mosquito Range of central Colorado lies one of the state’s most climbable 14ers, Mt. Sherman (14,043 feet). The common way to access the peak hiking trail is a gravel road leading west from near Fairplay to a parking area at the old Leavick Mill. Since Sherman was only the second 14er that I summited in my early hiking days, that was the route I chose.  However, during my next adventure to the peak I wanted to try something different (and more difficult) and that was coming in from the west via Leadville and Iowa Gulch.

For a geologist the upper part of  the Iowa Culch  trail goes near a number of old mines and one mine that may or may not be somewhat active, but still under claim—the Sherman Mine or the Sherman Tunnel Mine.  The mine may be seen from a distance as it is above timberline (12,200+ feet) at the base of a large cirque (Iowa Amphitheater). According to MinDat.org the mine opened in 1968, closed in 1970, reopened in 1975 and then closed again in 1982 and produced around 10 million oz. of silver, and evidently some copper, gold, lead and zinc freed from chalcopyrite, sphalerite, smithsonite, and galena.  Like almost all mines in the area the minerals were hosted in a fault-controlled karst system developed in the Mississippian Leadville Formation with mineralization occurring in the Permian.

The Mine is probably best known for producing a rich array of secondary minerals prized by rockhounds including the gangue mineral golden barite, and the carbonates rosasite (copper and zinc), azurite (copper), smithsonite (zinc), aurichalcite (zinc and copper), and cerussite (lead). Azurite and barite specimens were described in a posting on November 20, 2020.

Now two years later I located a small specimen of rosasite and silver that I had sort of not noticed in my collection. Rosasite [(Cu,Zn)₂(CO₃)(OH)₂] is a rather common, interesting carbonate with the copper-zinc ration about 3:2 and forms in the oxidized zone of ore deposits containing copper and zinc. It usually occurs as a botryoidal or spherulitic crust with colors ranging from blue to blue-green to green. The color and the spherulitic nature of the crust make rosasite somewhat easy to identify. The accompanying native silver is a silver white color with some tarnishing to a black (just like “silverware” utensils).

Spherulitic crust of rosasite with a small piece of silver-white native silver along with tarnished (nondescript) gray masses. Wirth FOV ~4mm.

My mountain climbing days are over, but I shall always cherish the numerous expeditions to the many mines in the area and the exhilaration of hiking the nearby 13ers:  Mt. Sheridan (13,748), Peerless Mountain (13,348), Horseshoe Mountain (13,898), and Gemini Peak (13,951).

Everyone wants to live on the top of the mountain, but all the happiness and growth occurs while you're climbing it.  Andy Rooney

MUSCOVITE: BACK TO THE BLOGOSPHERE

 Well, it appears that I have been off the Blogosphere for several weeks and need to get my writing in gear. Not really nifty excuses but: 1) the house/home needed some repairs in order to prepare for cold weather and winter; 2) do you know how many colorful maple leaves are on three mature trees (and they all need to pitter-patter down to the lawn); 3) receiving a letter from the state department of revenue with their interpretation (not mine) of my 2021 income tax; 4) construction of three power point programs (each an hour long) for meeting/conference presentations (done) is time consuming. Fun, but a real suck of time.

So, I just finished reading a Blog Posting by Hollis over at In the Company of Plants and Rocks and it was a barn burner about the geology of the House Range in the West Desert of Utah.  I could tell that the author put in tens of hours of observing, reading, and writing to construct a fine piece of work. But I am going to start with something very simple. Two interesting photos of muscovite, one from Brazil and the second from South Dakota!

                                        Brazil. Width FOV ~4.0 cm.

                                                South Dakota. Diamond Mica Mine. Width FOV ~2.5 cm.                              

Muscovite [KAl₂(AlSi₃O₁₀)(OH)₂] is one of those minerals that was easy to identify in my beginning geology course. It was the soft (~2.5 Mohs),  “light colored” (pale yellow to pale light brown to pale green to silver white) mineral that could be split into very thin, transparent, elastic sheets; it had perfect basal cleavage. A similar dark colored (iron rich) mineral was always labeled biotite.  These two minerals, commonly referred to as mica, were essentially freebies on the lab tests. However, at times the class examined very fine-grained river sand, or perhaps a piece of sandstone or gneiss and it became tougher to identify the shiny, shimmering flakes as muscovite.

                             

A specimen almost completely composed of muscovite books, quarter for scale (lower).  Collected a few miles west of Custer at an abandoned quarry.

In areas of igneous and metamorphic outcrops these weathered and eroded flakes in river sands shimmer with a slow current and clear water and are a major cause of “gold fever.”  Tourists and other visitors to Colorado see these shimmering flakes when the kids bound out of the car and head to the stream and believe they have hit the jackpot. Alas, no shiny gold but just tiny flakes of muscovite.

Muscovite is easily identified “in the field” by its formation of massive crystalline books found in many types of igneous and metamorphic rocks but especially prevalent in granite and granitic pegmatites.  The larger the mineral grains, as in pegmatites, the larger the muscovite books. Roberts and Rapp (1965) described many large muscovite crystals from the Black Hills of South Dakota, “one of the most productive [mica] districts in the United States”: 1) books of muscovite as much as three feet across from the Crown Mica Mine; 2) a book of muscovite three feet wide, four feet long and eight inches in thickness from the White Spar Mica Mine; and 3) a crystal of mica weighing 36 pounds from the Lake Mica Prospect.  MinDat listed 37 different varieties of muscovite, most were due to different cations appearing in the crystal structure. Some of these cations impart “colors” to muscovite such as in fuchsite, a greenish chromium-bearing variety.

Muscovite is also a member of the Mica Group, a “term for the sheet silicates that can be parted into flexible or brittle sheets”---as defined by MinDat. The data base also listed 70 members of the Group although not all are “officially” valid minerals.  For example, zinnwaldite, a somewhat common “mineral” found in rocks associated with the Pikes Peak Batholith is now discredited and placed as a dark mica containing lithium found in the siderophyllite-polylithionite series (see Blog posting November 24, 2013). Lepidolite, a pink to light purple Mica Group member has been discredited and is now assigned to the polylithionite-trilithionite series (see MinDat). Rock/mineral shops, and purveyors of rock/mineral trinkets, found across the U.S. have lepidolite “for sale.”  In fact, “lepidolite” collected from the Black Hills was the first mineral specimen I purchased as a kid. 

Lavender "lepidolite" collected from Bob Ingersol Mine, Black Hills. Color probably due to manganese.  Width about 5 cm.

Rose or pink or lithium muscovite collected from the Harding Mine, Taos County, New mexico. As Rob Lavinsky noted on Mindat, "on close inspection, you can see these muscovite crystals are translucent to transparent. They are actually masses of sheety crystals stacked densely together." The color is probably not due to lithium but to manganese and iron. Width of crystal mass ~1.9 cm.

OK, back to my simple photos of muscovite.  Most “books” of muscovite that are commonly found by rockhounds have a hexagonal shape in a cross-sectional view. That, along with its tabular form, elasticity, and transparent cleavage sheets, is usually enough to identify the mineral.  However, one of the more collectable micas in the Black Hills comes from the Diamond Mica Mine in the Keystone Mining District. Here the single crystals are, you guessed it, diamond shaped.

Muscovite crystals collected from the Diamond Mica Mine, Width of each crystal ~3.2 cm.

Muscovite crystal collected from the Diamond Mica Mine (Courtesy of Dakota Matrix). Note sharp crystal face boundaries.

Galleries.com noted that muscovite tabular crystals have a prominent pinacoid termination, that is the crystal has a pair of opposite parallel faces. The diamond shaped books have four dominant crystal faces with two pairs of opposite parallel faces. If another pair of parallel crystal faces form, then the books have a hexagonal shape. However, the diamond shaped muscovite from the Diamond Mine varies in shape and pinacoid terminations. Notice in the lower figure above, lifted from DakotaMatrix.com, the sharp corners of the diamond. Compare that with my specimens from the mine where an additional two parallel faces occur on the diamond.  Although the specimens are coarsely diamond shaped, the crystal has six faces. As I understand the situation (a stretch), muscovite is Monoclinic; however, the symmetry is not easily observed. The diamonds simulate Orthorhombic symmetry with prism faces meeting at ~ 60 degrees, The six-sided forms have prism angles of ~120 degrees and appear to have Hexagonal symmetry. I have never pretended to be a crystallographer and my low pay grade does not allow me to speculate on the mechanics of those designs! Crystal Systems and Crystal Classes are one of the reasons I became a paleontologist!

So, just when I think that muscovite is “easy” to identify I notice that the Handbook of Mineralogy throws the proverbially wrench into the equation:  muscovite is also described as “stellate aggregates, plumose, globular, scaly, granular, compact and massive.” One needs to consult MinDat to locate photos of these different habits.

Muscovite with twined crystals forming yellow five-pointed (or less) stars. Most star specimens come from pegmatites in the Jenipapo District, Minas Gerais, Brazil. FOV width ~4.0 cm.

Of these forms, the stellate aggregates, or star muscovite, are perhaps the most collectable, appealing, and attractive of the many habits of muscovite. Star muscovite involves twinning of the crystals in a manner that remains confusing to me.  I spent several hours reading the literature and my noggin still does not comprehend:  Twins in micas are difficultly identified due to mica’s hexagonal pseudosymmetry. In this paper, we present an electron backscattered diffraction analysis to identify twins in the muscovite… A trilling twin with twin law <310>/{110} is common in the muscovite. A six-couplet twin consisting of two trilling twins related by twin laws <110>/{130} and <001>/{001}(or <100>/{100}) has been discovered (Zhao and others, 2019). Remember I am a softrocker!

Regardless of my snarky comments I learned much from this little exercise, not the least of which is that a tremendous amount of literature is available for such a common, rather nondescript, easy-to-identify mineral.  I certainly did not realize that muscovite is such a complex mineral. As such I fell into the habit of pursuing the literature over a ten-day period and therefore did not reach my goal of just getting something into the Blogosphere. Oh well, life is interesting.

One item I did not miss out on during this final exam study (at least it felt like such) was observing the full moon, known as the Beaver Moon, on the evening of November 7. Officially the full moon, the peak illumination, was very early on the morning of November 8.  Unlike many moon watchers I was sound asleep at 4:00 am and as such missed the lunar eclipse and the Blood Moon phase. This reddish hue happens when the moon is completely in the earth’s shadow and the rays from the sun are blocked.  Over the course of several hours the full moon goes from a bright white ball to the copper hued object.  I probably should have got my derriere out of the sack and observed the event as the next full lunar eclipse is something like three years distant. However, I caught the eclipse several times on the magic of the Internet and did watch the previous lunar eclipse in May 2022 (the Super Blood Moon as pushed by the media) although it bounced in and out of the clouds in later stages. At any rate, lunar eclipses are great astronomical events.

As I daydream through my life and think of these astronomical events, I remain fascinated by members of the ancient Mesoamerican civilization of Central America. They built their pyramids, ball courts, palaces, temples, and other structures in accordance with astronomical events and positions of the sun, moon, planets, and stars over the course of a year. Of course, as a person of Scandinavian descent I am partial to the views of the Norsemen who believed that a large wolf, Fenrir, was constantly chasing the sun and at times got lucky and caught the orb and the lights go out--a lunar eclipse.

Bound of Fenrir. Dorothy Hearthy (1909). Public Domain.

REFERENCES CITED

Roberts, W.L. and G. Rapp Jr., 1965, Mineralogy of the Black Hills: South Dakota School of Mines and Technology, Bull. No. 18.

Zhao, Shan-Rong, Chang XU and Chuan Li, 2019, Identification of twins in muscovite” an electron backscattered diffraction study: Zeitschrift fur Kristallographie—Crystalline Materials 234(5). DOI: 10.1515/ZKRI-2018-2139