This gallery of beautiful silver jewelry, coinage, and art works presents works from around the world, and spanning the ages from the 26th Dynasty of Egypt (664-525 BC) to the Art Deco Era (1909-1941 AD) [Ref 1, 2]. Works have been chosen to demonstrate the artisan’s methods of forming shapes in silver [Ref 3] by casting, engraving, repousse’, embossing, and using silver inlay to adorn other metal objects [Ref 4]
This is the first of two Blogs on native silver. In the first I will introduce the mineral, including a gallery of specimens, and in the following blog, “Ancient Silver Jewelry” I’ll present examples of ancient silver jewelry, coinage, and art works, which demonstrate the innovative artisanship of early silversmiths from varied locations around the world.
Among the native elements, silver, [Ref 1], is a favorite among mineral collectors, as it is for me. Silver offers variations in color from metallic silver-white to the optical interference colors of a thin coating film, (such as on copper sulfide minerals – see my earlier Copper Blog), to the sooty black of a thick coating. From locations around the world it occurs in a number of aesthetic and geometrically interesting forms, ranging from groupings of single crystals (Figures 1-2), spectacular fern-like dendritic arrays of crystals (Figures 3 & 4), and striking wire and sheet forms (Figure 5 & 6). The relative arrangement of crystals in, and the shape of, the dendrites has been found to depend on the conditions of the surrounding silver-bearing solution during deposition of the silver, [Ref 2]. Specimens of wires attached to the silver sulfide acanthite, (Ag2S), grow on oxidation of the sulfide mineral, which liberates the silver, as described below, both by roasting, [Ref 3], or by chemical reactions in solutions within both the oxidized and replacement zones of an ore body, (Slide 9 of [Ref 4]). The wires grow at the interface between the acanthite and silver by continuing the face centered cubic lattice shared by both the acanthite and the silver, [Ref 1]. The sulfur of the acanthite occupies the interstices between the silver atoms. X-Ray diffraction and microscopy have demonstrated the crystallinity of a native silver wire.
In order to share with you these beautiful and intriguing forms of native silver, I’ve included a comprehensive gallery of these forms from around the world, (Figures 3-17). I’ve also taken the liberty of including a favorite specimen from my silver collection in the gallery, (Figure 10).
Because the lore of lost precious metal mines, particularly those in Arizona, New Mexico, and Nevada fascinate many of us; I’ll begin referencing descriptions and histories of these mines and provide brief excerpts from the references. In this blog, the emphasis will be on lost silver mines and in future blogs on silver minerals. Future blogs on Gold and Gold minerals will also include lore & history of lost Gold mines.
Silver Crystal Forms
Silver belongs to the isometric crystal system, [Ref 1], and crystallizes in cubic and octahedral forms as shown in Figures 1 and 2. The forms reflect the symmetry of the isometric crystal system. Silver crystals form twins on the octahedral surfaces of two crystals resulting in a Spinel-Twin, [Ref 6], with the remainders of each of the octahedrons visible, as seen in Figure 3.
GALLERY OF NATIVE SILVER SPECIMENS
LOST SILVER MINES
The location of the Lost Duppa Silver Mine in Arizona, [Ref 8], lies in the numerous mines and ore deposits of the heavily mineralized Bradshaw Mountains, (Figures 11-16). When discovered, the deposit was a ledge of silver-bearing quartz located in one of the many steep canyons located on the east side, of the northern Bradshaw Mountains. The ore was native silver. After his initial find, Duppa failed to relocate his original path to the deposit and never found it again.
The Lost Silver Lode of Carbonate Creek, New Mexico, [Ref 11]
The discovery of lode was in the Kingston Mining District, located in the southern region of the Black Range in Southwestern New Mexico. Located within the range are the Chloride, Kingston, and Hermosa Silver Distracts which have been rich producers of the metal. The lost lode lies in the Kingston Silver Mining District shown in [Ref 12], which eventually produced silver amounting to over Six Million in USD. The lost lode was discovered along Carbonate Creek near the town of Kingston as surface float of acanthite (silver sulfide). The weights of pieces of the float ranged up to 250 pounds. Ultimately float yielding over 80,000 ounces was found, but the source of the float was never discovered.
In this blog, the subject is the Garnet Family and their six members, (See Figure 1), which are gemstones and mineral specimens highly prized by collectors, as well as by those that wear them, because of their colors. Their colors, as seen in Figure 1, span the rainbow, and some unexpectedly undergo a change in color, depending on the wavelength of the light, (daylight or incandescent light), that illuminates them, as seen in Figure 2.
I will discuss the relationships between the colors of the garnets and their chemical compositions and how their colors are perceived, due to absorption of light by specific colors and by included chemical species. Such as a tomato absorbing mostly green light and reflecting red light. As an example of another optical effect, (and other gemstones can exhibit this effect also), I’ll discuss asterism, the cause of the formation of a star-shaped figure, seen on the surface of some Star Almandine garnets.
GARNETS AS GEMSTONES
Since antiquity garnets have been admired and worn as gemstones in jewelry such as: the 7th century Anglo Saxon sword pommel found as part of the Staffordshire Hoard in July 2009, [Fig 3]; And in Figure 4, the 1st -2nd century Roman necklace; and in Figure 5 the Roman ear rings; and in Figure 6, the 30 – 323 BCE Egyptian ring; the Byzantine pendant shown in Figure 7; and the Victorian necklace shown in Figure 8. References for each piece of jewelry accompany its picture.
CHEMISTRY OF THE GARNET FAMILY
The chemical relationships between the six, end members, of the garnet family are summarized in the two, phase diagrams, shown below in Figure 9, [Ref 11]. The shaded regions of each diagram show compositions where the metal ions, calcium (Ca), magnesium (Mg), and iron (Fe), can substitute for each other. For example, increasing substitution of Mg2+ for Fe2+ in Almandine leads to a garnet increasingly approaching, and ultimately equal to, Pyrope in composition. Appreciation of the shaded regions of mixed composition is important in relating colors of the gemstone to the relevant metals presented in its ionic form, as shown in Tables 1 and 2.
The formulas of the end members of each of the families are also shown in Table 1. The formula cited for the end member Uvarovite, in Figure 9, is erroneous in this old version, (The only one I could dig up), of its compositional group. Only the metals calcium (Ca) and chromium (Cr) are present in this garnet.
The three garnets of first group, the Pyralspites, contain the metals magnesium (Mg), Manganese (Mn), and iron (Fe), and aluminum (Al). Each garnet of the other group, the Ugrandites, contains calcium. The other metals are aluminum (Al), iron (Fe), and chromium (Cr). In common, all six members of the garnet family contain the element silicon (Si), bound with Oxygen (O), and are known as silicate minerals. These phase diagrams are from an older unreferenced source with an erroneous formula for Uvarovite garnet – Its correct formula can be seen in Table 1.
The formulas for each of the members of the garnet family are listed in Table 1. In the formulas, the charge of the ion of the metal is shown for each metal. For example, the divalent iron ion is written as Fe2+ and the trivalent chromium ion as Cr3+.
SOURCES OF COLORS IN GARNETS
Of the garnet family, Almandine and Andradite owe their colors to the iron present in their formulas, Spessartine to manganese, Uvarovite to chromium, all in ionic form. Pyrope and Grossular, in pure form, are colorless. Various combinations of iron, manganese, titanium, and vanadium ions are the causal agents of color in both Pyrope and Grossular. The formulas for each of the members of the garnet family are summarized in Table 1. The colors; the responsible metal; and its ion forms, which are present in garnets, are given in Table 2.
HOW GEMSTONE COLORS ARE PERCEIVED
The colors of the light we see, either transmitted or reflected by a gemstone, stem from that part of the spectrum of the incident light which is not absorbed within the gemstone. The light, incident on the gemstone, is ambient light. Depending on the source of the incident light, its intensity over the blue to red range of the spectrum can be weighted more in the blue than in its red regions. As examples, light from a halogen lamp, or a white light diode have a spectrum richer in the blue region than an incandescent lamp which has a spectrum richer in the red region.
The effects of the spectral content of the incident light and its absorption at some wavelengths and not at others on perceived color can be demonstrated in a study done on color change in a Pyrope garnet [Ref 13]. Changes in the colors of a gemstone with illumination lend drama to the gemstone. A study was undertaken of a type of Pyrope color-change garnet from Tanzania so that the thickness of a cut gem was optimized in a way that the color change, with a change in illumination, could be maximized.
STAR GARNETS: ASTERISM IN GARNETS A BEAUTIFUL EFFECT
Some gemstones, such as the Almandine garnet, Moonstone, Spinel, Rose Quartz, Citrine, Diopside, Emerald, Sapphire and Ruby may exhibit asterism in displaying a rayed star, best viewed when centered on the dome of a cabochon-cut gem, as in Figure 12. The star effect is caused by the scattering of light from nano-sized crystals of the mineral rutile, oriented in parallel fashion to each other [Ref 14]. In Almandine garnet the star may be either 4 or 6-rayed according to which directions in the crystallographic lattice the rutile crystals are located [Ref 15]. Star garnets are typically a purple shade, as seen in Figures 12 & 13. Idaho and India are the major, if not the only suppliers, of starred garnets [Ref 16].
In the presentation of photos of specimens of various garnets the source of the photo is referenced. For more or supporting images, Google the name of the mineral and the word image to see a gallery of additional images.
Specimens from the classic localities of Wrangall Island, Alaska and Tyrol, Austria as well as those of very large size from the Salida Mine, Salida, Chaffee County, Colorado have long attracted collectors.
Rhodolite garnet, the raspberry red to purplish red variety of Pyrope, as shown in figure 20, was first discovered and described from Cowee Valley, Macon County, North Carolina. Now considered a classic locality. Bohemian garnets from the Czech Republic, with their glossy red color, as shown in Figure 21, were first marketed in the 17th century. Newer finds in Madagascar, Brazil, and Arizona are current sources of Pyrope garnets.
Orange Spessartine Garnets associated with Smokey quartz from China are greatly desired by collectors. The vividly orange-colored Spessartine, single crystal \pessartine garnets, many displaying perfect forms, from both Tanzania and Nigeria are also prized. Crystals of red Spessartine Garnets, from Brazil, with their complex multi-faced faces present unique crystal forms.
The gem varieties of Andradite garnets, the green demantoid, the honey-colored topazolite, and black melanite, respectively from Madagascar, Canada, and California are not only prized as gemstones, but also by collectors. Also coveted are the beautiful green Andradite specimens from Stanley Butte in Arizona as well as the brown andradite garnets from Mali and Greece.
The classic Grossular garnet specimens, from the Jeffery Mine in Quebec, with their pristine crystals and colors, which range from colorless through pink, honey-brown, and green, are highly regarded by specimen and gemstone collectors. The pink ,manganese-rich, Grossular garnets and their iron-rich companions from Coahuila, Mexico, are also prized by collectors. The startling green Tsavorite gem variety of Grossular garnets from Tanzania also appeal to both specimen and gemstone collectors.
The two hallmark localities for Uvarovite specimens are located in Russia and Finland, and are associated with chromite, the oxide mineral of chromium. Specimens from Russia typically present dense fields of small perfect crystals on a chromite matrix. Some specimens also contain a drusy (dense array of small crystals) made up of the lavender mineral Amesite. Specimens with larger crystals occur in Finland.
This short tutorial on how to operate a slab saw is directed toward the new user and can be a refresher for the seasoned slab-miester.
Generally slab saws are heavy, so find a covered area indoors to place it, (or outside if you have to). You probably won’t be moving it unless yours has wheels. Because your saw uses oil as a coolant/lubricant take that into consideration when deciding where to place it. The area around the saw will be messy no matter what you do. A nearby source of electricity, good overhead lighting and a workbench next to the saw are essential for obtaining the best results.
Safety is number one when working with any type of saw. Your electric cord should be a grounded three prong plug, no bare wires showing. Depending on the size of the blade your saw uses, 12”, 14”, 16”, 18” on up to 36”, your motor size will vary accordingly. From a ½ hp to 1 ¾ hp electric motor, so if you need to use a drop cord, the bare minimum to use is a SJTW 16awg cord, the older saw motors, unless they were upgraded, will use a lot of amps just turning them on. Sometimes a saw-cut can take an hour or better to complete so using a cord that is too small for the amperage will overheat the motor and wear it out prematurely. Never leave a running saw unattended, even if it has an automatic shut-off, the moment you walk away bad things tend to happen, maybe you didn’t secure that large rock in the saw vice well enough and it moves, binding that $400.00 blade in a running motor, so it’s best to stay within 10 feet or so to prevent expensive mishaps.
So now you want to cut one of those prize rocks you found last weekend, gotta know what’s inside right ! Your saw uses oil as a lubricant or should, using water is insufficent to lubricate and keep the blade cool during cutting, I recommend NOT using water. What I do recommend is using an appropriate oil available through slab saw dealers, which will give you many options to choose from. I use a Shell Oil product, Amber Neutral Oil 100 – available in 5 gallon containers. So how much oil will I need? Just enough to cover the diamonds on the rim of the blade, on my 18” that’s 3/8 of an inch. Adding more than what covers the rim will cause unnecessary drag making the motor work harder.
My saw is a 1960’s Great Western 18” which uses about 4 gallons of oil to cover the diamonds on the rim 3/8” and a combination hydraulic feed and 15 lb weight, to pull the rock through the blade. The speed at which the rock moves through the blade is important, slow is your friend, giving a smooth cut, too fast a feed and you get hop marks making more work for your finished piece.
Depending on what type of material you are cutting, mud build-up in the pan will signal when it’s time to drain the oil. Your motor may run slower, overheat and shut off due to mud building up in the bottom of the pan. This can happen to such an extent that the saw blades’ diamond rim are in the mud. Soft materials like Onyx, Serpentine, and Howlite are ground away as the rock passes through the saw blade, creating a lot of mud.
When it’s time for an oil change, look forward to getting filthy. At this time the entire machine interior should be cleaned. My ritual is to let the machine sit idle for at least 3 days after my last cut to let the mud settle out. I next use a 5 gallon bucket, with a wire strainer I made to fit inside, and a paper grocery bag resting on that. I then drain the oil into the bag. So now there is a layer of mud at the bottom of the pan and oil above that. I pull the drain plug and let the oil drain out, the mud is heavy enough that the oil will drain right over it – now I have reclaimed 75% of the oil leaving the mud and some oil. The fun begins when it’s time to push the mud through that drain hole and do the same process with the 5 gallon bucket strainer and grocery bag. It takes 3 separate buckets and strainers on my machine, as the oil remaining suspended in the mud will gravitate out into the bucket over several weeks time, retrieving a little more oil.
Linda and I brought some finshed Cabachons to compare to some rough uncut pieces provided by our President and Vice President to our May 18th meeting.
There was a lot of interest though just 8 members were up early on Saturday to meet Linda and I at the Safeway in Williams, the gateway to Perkinsville, and a good place for snacks and rest room stop before heading out for a 40 mile trip to the collecting site, weather was perfect, clear skies, temperatures in the 70’s and a slight breeze made for a very comfortable day in the hills.
The Perkinsvill Agate location has always been a productive source of nice agate with interesting inclusions and color. Our May field trip proved to be just as fruitful, with some surface collecting and digging out larger buried specimens for future cutting and cabbing.
In this blog post I’ll talk about the ore minerals of copper in Porphyry Copper deposits; major sources of copper; Porphyry Copper Deposits in Arizona; the formation and geology of Porphyry Copper deposits; and am including a gallery of Copper ore minerals; and lastly, discuss Peacock Copper Ore.
Porphyry Copper Deposits
Porphyry Copper deposits are the world’s largest source of copper [Ref 1] and are distributed globally (Figure 1).
World wide, production by the ten largest producers amounted to 15.4 million tons in 1917 [Ref 2]; of these, the United States ranked fourth in production at 1.27 million tons, of which 68% was produced by mines in Arizona [Ref 3].
Numerous deposits are located in the geological Basin and Range Province of the Southwest, as shown in Figure 2. Among the currently active mines in Arizona are those operated by Freeport-McMoRan Inc, the Morenci, Bagdad, Safford, Sierrita and Miami mines [Ref 4] and those operated by ASARCO LLC, which are the Silver Bell, Mission complex mines, and the Ray complex mines [Ref 5].
Formation of Porphyry Copper Deposits and Ore Minerals
The ore bodies of Porphyry Copper deposits are formed by the intrusion of hydrothermal fluids emanating from a magma chamber several kilometers below the earth’s surface and the deposition of ore minerals as veins in pressure-induced fractures within a granitic porphyry (See figures and text in Ref 6). Chalcopyrite is the major copper mineral deposited [Ref 7, Page 4]. This initial mineralization results in grades of 0.3 to 0.9% copper and almost always less than 1% [Ref 6]. It is by Supergene Enrichment (Slides 6, 7, 8 in Ref 8], a secondary enrichment process, that the deposition and the accumulation of copper ore minerals above and below the water table increases the ore grade. As shown in Slide 8 of [Ref 9], oxidizing conditions in the ore body above the water table result in deposition of copper minerals such as azurite, malachite, and chrysocolla, and the sulfide minerals, chalcocite and bornite, form under a lesser concentration of oxygen below the water table.
The Ore Minerals
Many young mineral collectors, drawn by the brilliant spectrum of colors on their surfaces, have collected specimens of either oxidized Chalcopyrite or Bornite (Figures 10 and 12); these specimens typically are labeled as “Peacock Ore” or ‘Peacock Copper”. The color stems from a thin film formed by the oxidation of the mineral surface. The colors are caused by an optical effect due to light waves reflected by both the underlying surface of the mineral and the surface of the film, which reinforce each other. Reinforcement of the waves depends on the thickness of the film and the wavelength of the light [Ref 8, Equation 4(41)]. In thinner film the reflected light tends to the blue end of the spectrum and in thicker film, to the red end of the visible light spectrum [Ref 8, Equation 4(41)]. The copper sulfides Chalcocite and Covellite can also demonstrate blue to red reflections as seen in Figures 5 and 6.
This was a popular field trip with 19 attendees. Marty & Linda rendezvoused with everyone, including two gals all the way from New York, who were passing through and saw the field trip listed on our website. (The both joined the club and are looking forward to coming back soon for more fun in our Arizona sun).
From the McDonalds in Cordews we jumped onto I-17 heading south to exit 259. Driving down the long, winding, dusty, washboard road we followed the Bradleys to our collecting site, a couple miles west of Cleator.
This site is known for collecting Schorl, the most common type of Tourmaline. The massive Schorl is in a matrix of white Quartz, with the finer specimens in Rutile form, embedded in the quartz, which is mostly found in float. Brad worked an outcrop inside a wash that was productive and several of us went to a site with tailings of malachite and chrysocolla, across the road from the Schorl site. That site was pretty well picked over, but we all managed to find something of interest.
We also ran into three rockhounds from Minnesota, who came by to see what we were doing and they offered us advice on some interesting Minnesota sites for us to visit.
Our next scheduled field trip is to Perkinsville, May 19th, following our monthly Friday club meeting. We’ll be collecting Perkinsville agate there.
Marty & Linda, our Field Trip Co-chairs, hosted 12 club members at their home in Paulden, this past weekend. Marty generously invited us into his workshop & showed us how to use his equipment (with help from Allan Mills).
We were each able to have a slab cut from larger rocks we brought and Marty provided each of us with a pre-cut & drawn cab slice for us to use to learn how to grind & polish – so each of us left with our very own finished cabochon.
The food was delicious – Marty & Linda provided burgers & dogs with everyone else contributing a side. Roaming dust devils provided entertainment – particularly the one that went straight through Marty’s workshop. Fortunately we were all busy eating at the time.
A wonderful time it was & as we packed up to leave Marty & Linda were already planning to invite us all back. So if you missed out this time, not to worry, there will be more Rock Parties to come.