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Calcite I

Despite its wide distribution in limestone and as a common gangue mineral in ore deposits, the rainbow of colors and beautiful crystal forms of calcite, from locations around the world, have made it a favorite of collectors. One such favorite is the group of calcite crystals, tipped by hematite, from the Egremont Mine shown in Figure 1. In massive form it has provided lapidists and other artisans a beautiful material for the creation of jewelry and other artworks, such as the sculpture done in Utah calcite, shown in Figure 2.

In this blog I will briefly describe the mechanical properties of calcite, its crystallography, optical properties, and sources of its colors. In Calcite II, I will follow with a gallery of calcite specimens sought by collectors from world-wide locations.

Figure 1. Calcite with a partial hematite coating, Egremont Mine,
Cumberaland, England [Ref 1]
Figure 2. Abstract calcite sculpture “Sundance” carved from Utah calcite [Ref 2].

Mechanical Properties [Ref 3]

The hardness of calcite on the Mohs Scale is 3. It’s brittle because of its perfect cleavage along rhombohedral planes. It parts readily, along twin planes formed by stresses. It can also exhibit conchoidal fracture.

Crystallography [Ref 3, 4]

Calcite crystallizes in the Trigonal System with crystallographic axes, and the often-seen rhombohedral, (Left), and scalenohedral, (Right), forms, shown in Figure 1 [Ref 4]. The system possesses three a1,2,3-axes at 120 degrees with respect to each other in the horizontal plane and the perpendicular c-axis. Other crystal forms can be seen in Ref 3.

Any plane through the trigonal lattice is represented by four numbers (hkmi). These are the Miller Indices which are the reciprocal values of the intercepts respectively on the a1, a2, a3, and c-axis. A family of planes is indicated by the notion {hkmi}.

Calcite forms twinned crystals according to four twin laws [Figure 4 in Ref 5] as shown in Figure 2. The twin forms and the associated family of planes are given in Table I. The angles between the vertical c-axes for each twin form are respectively to the nearest degree of 180, 127, 90, and 53. Calcite specimens exhibiting the twin laws are shown in Figures 5-8.

Among minerals calcite can be considered to be the best one to demonstrate cleavage because of its perfect cleavage along the rhombehedral family of planes {10-11}. The rhombehedral shape of the specimen demonstrating birefringence (double refraction) is evident in Figure 10.

Figure 3. Trigonal Crystallographic System [Ref 4]

Figure 4. Four twin laws of calcite shown with scalenohedral forms [Ref 5]

                                             TABLE 1

                        Twin Form                 Family of Twin planes
                               a                                {0001}
                              b                               {10-11}
                               c                               {01-12}
                               d                               {02-21}
Figure 5. Calcite crystal twinned on the basal (0001) plane,
Elmwood Mine, Carthage County, Tennessee [Ref 6].
Figure 6. Calcite crystal twinned on the rhombahedral plane
(10-11) [Ref 7].
Figure 7. Calcite crystal twinned on the pyramidal plane (01-12) location not stated [Ref 8].
Figure 8. Calcite crystal twinned on the pyramidal plane (02-21)
Brushy Creek mine, Reynolds County, Missouri [Ref 9].

Optical Properties 

Calcite exhibits a range of lusters between vitreous to pearly and its transparency ranges between transparent to translucent. Calcite also exhibits birefringence in its refraction of light within the crystal [Ref 3]. Because the refractive index of calcite varies with the direction of light within a calcite crystal the light entering the crystal is doubly refracted into two different directions, as shown schematically in Figure 3 and demonstrated by the double image of the lines seen in Figure 4.

Figure 9. Double refraction in calcite showing two light paths [Ref 10].
Figure 10. Doubly refracted light in a calcite rhombahedron [Ref 11]

Sources of Color in Calcite 

In its description of the properties of calcite, the mineral reference site mindat lists besides white, a rainbow of colors: yellow, red, orange, blue, green, brown, grey, etc. for calcite.

Calcite, which chemically is calcium carbonate with the formula CaCO3, is colorless when pure, as shown by the crystals on matrix shown in Figure 7 [Ref 12].  The rich colors of calcite specimens arise from different sources, such as substitution of low levels of one of the transition metals its divalent ionic form M+2for the calcium ion Ca+2in the lattice of the crystal, low densities of radiation-induced defects in the lattice of the crystal, or inclusions of a pigmented mineral. 

Colors due to transition metal impurity ions [Ref A]

The colors present with iron and chromium, manganese, cobalt, or chromium present in, respectively ferroan, manganoan, cobaltoan, and chromian calcites are summarized in Table II. The yellow, pink and green colors due to these transmission metals can be seen also in many other minerals as can be seen in a search on the Web. 

                                                                        TABLE II

                             MINERAL                               COLOR
                         Ferroan Calcite                               Yellow
                       Cobaltoan Calcite                                 Pink
                     Manganoan Calcite                                 Pink
                        Chromian Calcite                                Green

Colors arising from radiation damage in the crystal lattice 

The search on the web for what colors of calcite might stem from radiation damage disclosed two references in which blue and amber-colored calcite were associated with radiation damage. Results of the study of References suggested that radiation-induced color centers involving the negative ion CO-3and the presence of stress-induced twInning and lattice disloctions account for the coloring mechanism.

The Mineral Spectroscopy Server of the Divisions of Geological and Planetary Sciences of the California Institute of Technology states that natural radiation induces an amber color in the Calcites from the lead-zinc Tri-State Mining District [Ref B, C]

Colors arising from colored inclusions 

Colorful calcite is also produced by the presence of pigmented inclusions. Clear and transparent crystals of calcite, when included by such strongly colored minerals as malachite, pyrite, hematite, native calcite and others, make highly aesthetic specimens. The inclusions may be dispersed in the cystal (Figure 16) or reside on an included phantom crystal within the specimen crystal (Crystal 17).

Calcite, Bigrigg Mine, Egremont area, West Cumberland Mining District, Cumbria, England [Ref 12]
Figure 12. Ferroan calcite, Boral Limited quarry, Bundoora, City of Whittlesea, Victoria, Australia [Ref 13]
Figure 13. Manganoan calcite, 2ndSovietsky Mine, Dal’ngorsk, Far Eastern Russia [Ref 14]
Figure 14. Cobaltoan calcite with green kolwezite, Kambove Mine, Katanga Copper district, Democratic Republic of Congo [Ref 15].
Figure15. Bladed crystals of chromian calcite, Santa Eulalia District, Chihuahua, Mexico [Ref 16].

Color due to an included pigmented mineral 

Figure 16. Calcite with conichalcite inclusions, Mapimi district, Durango, Mexico [Ref 17].
Figure 17. Calcite crystals containing phantom crystals covered with marcasite, Linwood Mine, Buffalo, Scott County, Iowa [Ref 18].

Dec 2018 Trip to Dobell Ranch

Our December 8th field trip to Dobell Ranch began with a chilly morning meet at Silver Saddle Rd. A scant, lucky, seven members ready to scout the open pits and grounds strewn with multi-colored Arizona Petrified Wood.

Our host, Rhonda Dobell and grandchildren, helped by pointing out some of the better material. It was as if we were shopping in a candy store with so many potential specimens to choose from, you ended up having to be very selective as to what you were bringing home. I think we all found what we wanted and then some. We all would like to have the tree trunks decorating our yards, but decided to be reasonable!

Lunch was prepared for all of us and we had fun with the younger grandkids. Then, when everyone was leaving, there were hugs for all around from the kids.

A very successful field trip that I’m sure we will make a repeat visit to, early next year.

Martin & Linda Dougherty

Gold III – Lost Gold Mines in the Southwest

Tales of the discovery and loss of rich gold mines such as The Lost Dutchman in the Superstition Mountains of Arizona and the El Naranjal lost gold mine in the Sierra Mountains of  Mexico, as popularized by folklorist, J. Frank Dobie, in “Apache Gold and Yaqui Silver” have fascinated many of us. An extensive list of both lost gold and silver mines of the Southwest , each with extensive and entertaining descriptions of their histories, can be found in “Lost Treasure Tales” on the GeoZone Site [Ref 1].

Perhaps the lost mine tale that most of us are aware of, is that of the Lost Dutchman Mine [Ref 2]. One, of 62, hand drawn maps of its supposed location, made available in Reference 3, is shown in Figure 1 and orients it with respect to the prominent geological landmark, Weavers Needle, shown in Figure 2.

Figure 1. Map of the location of the Lot Dutchman Gold Mine [Ref 3]

Figure 2. Weavers Needle, the landmark for the location of the Lost Dutchman Gold Mine [Ref 2].

Two roots for the of name of the lost El Naranjal Mine have been attributed to its location near a grove of trees with oranges (naranjas) or to the orange color of the gold in its ore [Ref 3]. It is supposedly located at the bottom of canyon (Barranca) beside a river and near an abandoned hacienda.

Among many of the discussions about this mine, Treasurenet suggests that proof of the its existence lies in an old road sign naming the road to the mine in Sinaloa and in records found in Guadalajara, which were found by a British consul, describing production in the millions in the 17thCentury [Ref 4]. In another posting,

TreasureNet, [Ref 5], suggests its location fits that of an 1800’s lost, and very rich gold mine, in the region of the lost Tayopa silver mine [Ref 6], and in another post [Ref 7] that its location lies in the state of Durango.

Ref 1. http://www.thegeozone.com/treasure/arizona/index.jsp

Ref 2.  http://treasure-hunting-information.com/?page_id=2641

Ref 3. https://en.wikipedia.org/wiki/Naranjal_mine

Ref 4. http://www.treasurenet.com/forums/tayopa/36414-el-naranjal.html

Ref 5. http://www.treasurenet.com/forums/tayopa/273860-can-el-naranjal-possibly-found-tayopa-complex.html

Ref 6. http://www.treasurenet.com/forums/tayopa/36414-el-naranjal.html

Ref 7. http://www.treasurenet.com/forums/treasure-legends/487274-update-mine-el-naranjal.html

November Field Trip to Meteor Crater

November’s club field trip saw 10 members drive out to Arizona’sfamousMeteor Crater participating in a guided tour lasting about an hour.here is a brief description of the Craters history copied fromWikipediafor those that have not visited the Crater.
The crater was created about 50,000 years ago during the Pleistocene epoch when the local climate on the Colorado Plateau was much cooler and damper. The area was an open grassland dotted with woodlands inhabited by woolly mammoths and giant ground sloths.

The object that excavated the crater was a nickel-ironmeteorite about 50 meters, (160feet), across. The speed of the impact has been a subject of some debate. Modeling initially suggested that the meteorite struck at up to 20 kilometers per second, (12 miles per second), but more recent research suggests the impact was substantially slower, at 12.8 kilometers per second, (8.0 miles per second). It is believed that about half of the impactor’s bulk was vaporized during its descent through the atmosphere.

For additional information click on this link to wikipedia.

GOLD II – Artwork

Having greatly enjoyed researching and writing about ancient and recent jewelry in my blog on Garnets (Glorious Garnets) and on silver jewelry in Silver II, I decided to present in this blog, “A Gallery of Ancient Gold Jewelry”, gold coinage and art works from around the world. Despite gold’s tendency to tarnish – its metallic beauty and its workability, stemming from its Mohs softness of 2.5-3, [Ref 1] it has long been a favorite with artisans using techniques such as casting of three-dimensional objects [Ref 2], chasing and repousse’, for the shaping of sheet gold, [Ref 3] and its alloys [Ref 4], and the forming of shapes using filagree and decorating with granulation [Ref 5], as well as the embossing coinage [Ref 6], and as an inlay [Ref 7]. Alloys of gold span the color spectrum offering a palette of colors to the artisan, ranging from purple to red, as well as white. Gold objects fabricated using these techniques comprise this gallery.


Art Work References:

Ref 8. https://www.artsy.net/article/artsy-editorial-history-gold-art-ancient-egyptian-burial-masks-jeff-koons

 Ref 9. https://en.wikipedia.org/wiki/Inca_Empire

 Ref 10. https://masksoftheworld.com/pre-columbian-burial-mask/

Ref 11. https://www.christies.com/lotfinder/Lot/a-gold-and-silver-inlaid-bronze-censer-fangding-5805296-details.aspx

Ref 12. http://new.uniquejapan.com/an-example-of-a-custom-koshirae-with-edo-period-piece-mountings/

Ref 13https://www.pinterest.com/pin/51298883231615583/

 Ref 14https://en.wikipedia.org/wiki/Shakud%C5%8D

Ref 15. https://www.metmuseum.org/art/collection/search/34416

Ref 16. https://en.wikipedia.org/wiki/Shibuichi

September Field Trip to Gray Mountain

September’s field trip was well attended and productive, eight club members departed from the Market at Silver Saddle and State Route 89  for Gray Mountain. Upon our arrival to the area, Andrea Eubanks led the way to our primary collecting site, winding through geologic sandstone formations to a plateau above the Little Colorado River . There we fanned out across a wide area finding  broken pieces of Petrified Wood left behind by a massive flood eon’s ago. Our recent rains helped uncover some cut-able prizes.

Members present, included Andrea, Gordon, Alan, Loewn, John & Beth and Marty & Linda.

Native Gold – Part 1

This is the first of two Blogs on native gold. In the first I introduce the mineral and its properties, including a gallery of specimens exhibiting the crystal forms of gold and the effects of deposition conditions on forms of gold. Having just found out about it, I’ll also describe the deposition of gold by bacteria in alluvial deposits placers. In the following blog, I’ll present examples of ancient gold jewelry and art works from various locations around the world which demonstrate the innovative artisanship of early goldsmiths.

Among the native elements, Gold [Ref 1] because of its beautiful golden color, its rarity, and its aura of wealth and power, is a favorite among collectors and museum-goers [Ref 2] [Ref 3] [Ref 4]. Specimens of electrum, the lighter colored of alloy of gold, containing silver, [Ref 2] are also favorites. Specimens of both native gold and electrum from around the world occur in a variety of aesthetic and interesting forms ranging from single crystals and their groupings (Figs 5-9), twinned crystals (Figures 10-14), intricate dendrites, which are fern-like single crystals (Figures 14-15), and in spectacular sheet forms (Figure 16).

Some Properties of Gold [Ref 1] 

With a Mohs hardness in the range 2.5-3 gold is malleable which makes it easy to work into decorative forms by a goldsmiths. It also doesn’t oxidize, which facilitates melting it for casting, and soldering. In thick form, gold exhibits a rich yellow color due to its high reflectance of light in the yellow-red spectral range. In sufficiently thin form, as gold leaf, it transmits blue and green light. Its high specific gravity measures in the range 15-19.3 grams/(cubic centimeter), which allows efficient recovery of gold in placer deposits such as gold panning.

Basic Gold Crystal Forms [Ref 1]

Gold, (and Electrum), crystallize in the isometric crystal system in it’s typical forms, shown in Figures 1-2. Gold forms twinned crystals about an octahedral plane as shown inFigures 3-5 [Ref 5 ]. Dendritic crystals result from repeated Spinel-twinning in a branched structure with branches at 60 degrees relative to each other. A native gold specimen referred to as wire gold, is not a wire in the sense of the native silver wire, (See the Native Silver Blog), but is an extended single group of multiple Spinel-Twinned crystals as shown in the specimens of Figures 12-13. Gold deposited in fractures with the host mineral, when exposed, possesses leaf-like forms as in Figure 15.

Figure 1. Gold Octrahedron



Figure 2. Gold cube

Figure 3. Two crystals forming a Spinel-Twin [Ref 5]
The Spinel-Twin is formed by a rotation of the lattices of each of the two crystals about an axis perpendicular to the octahedral plane, as demonstrated by the model of an octahedral crystal in Figures 4-5 [Ref 5].

Figure 4. Rotation of 180 degrees of the right-lower-most segment of the octahedron about an axis perpendicular to the cut along an octahedral plane in the model results in a Spinel-Twin [Ref 5]











Gallery of Native Gold Specimens

In many gold specimens the crystals do not display the perfection of form typical of some other minerals such as pyrite, but are skeletal with depressions, or [Ref 6], are also referred to as being hoppered [Ref 7]. Gold crystals can also exhibit interesting, complex twinned and dendritic forms. These departures from ideal forms stem from conditions of rapid deposition in absence of thermodynamic equilibrium [Ref 8]. Gold specimens featuring octahedral, cubic, wire, dendritic. and leaf forms, the latter, which in many instances is formed in interstices in fractured quartz [Ref 9].

Deposition of Gold By Bacteria

In searching the web, I found recent studies which, surprisingly to me, demonstrated the deposition of particulate gold by the action of specialized bacteria in alluvial deposits [Ref 10] [Ref 11]. In the studies, the presence of toxic gold complexes was shown to initiate the formation of a population of bacteria which excretes enzymes that catalyze the formation of nanoparticles of gold, [Ref 12]. Aggregation of nanoparticles of silver and gold have been shown to participate in crystallization of these metals, which allow growth of gold on a nugget [Ref 13]. Bacteria resident in a biofilm [Ref 12], on the surface of a gold nugget could function as a source of gold which forms a gold coat on the crystal with sustained activity of the bacteria. A scanning electron microscope image of the bacteria in the biofilm on a gold nugget is shown in Figure16. The deposition process takes place for years to decades in order to accrue on a gold grain, suggesting that if the process can be speeded up, bacterial deposition could improve ore-processing [Ref 14].

Figure 16. Enlarged view of gold-depositing bacteria in the enclosing biofilm on a gold nugget. The length of the scale for size comparison is 0.000197 inches.












Ref 1. https://www.mindat.org/min-1720.html

Ref 2. https://www.amnh.org/exhibitions/gold/

Ref 3. https://en.wikipedia.org/wiki/Gold_Museum,_Bogota

Ref 4. https://adrianhepworth.photoshelter.com/image/I0000f0UBlFQgu3I

Ref 5. https://www.mineral-forum.com/message-board/viewtopic.php?t=3044

Ref 6. https://www.mindat.org/glossary/skeletal_crystal

Ref 7.https://en.wikipedia.org/wiki/Hopper_crystal

Ref 8. https://en.wikipedia.org/wiki/Crystal_growth

Ref 9. https://pubs.geoscienceworld.org/gsa/geology/article-abstract/16/6/551/190624

Ref 10. https://phys.org/news/2009-10-bacterium-formation-gold.html

Ref 11. http://www.mdpi.com/2075-163X/3/4/367/htm

Ref 12. https://en.wikipedia.org/wiki/Cupriavidus_metallidurans

Ref 13. http://iopscience.iop.org/article/10.1088/0957-4484/17/23/021/meta

Ref 14. https://phys.org/news/2017-04-role-microorganisms-industrial-gold.html

Silver Part II – A Gallery of Silver Jewelry

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]


Ref 1. https://en.wikipedia.org/wiki/Twenty-sixth_Dynasty_of_Egypt

Ref 2. https://artdeco.org/what-is-art-deco/early-20th-century-timeline

Ref 3. https://en.wikipedia.org/wiki/Silversmith

Ref 4.https://en.wikipedia.org/wiki/Inlay

Ref 5. http://www.busaccagallery.com/catalog.php?catid=141&itemid=5619

Ref 6. http://www.ancientresource.com/lots/ancient_jewelry/diannesommelet/diannesommelet2.html

Ref 7. https://www.secretenergy.com/illustrations/cultures/mesopotamia/

Ref 8. http://www.getty.edu/art/exhibitions/ancient_luxury/

Ref 9. http://www.historyandcivilization.com/Picture-Gallery—Early-Mesopotamia-from-Sumer-to-Assyria—Artifacts–Objects—Sculpture.html

Ref 10. https://www.pinterest.com/pin/354799276868988412/?lp=true

Ref 11. http://www.antiques.com/classified/1112715/Antique-Huge-Roman-Silver-Ring-w–Portrait#

Ref 12. https://www.ngccoin.com/news/article/6078/ancient-coins/

Ref 13. http://www.britishmuseum.org/research/collection_online/collection_object_details.aspx?assetId=973486001&objectId=154939&partId=1

Ref 14. https://www.archaeology.org/issues/149-1409/artifact/2388-denmark-viking-figurine

Ref 15. https://art.thewalters.org/detail/40080/signet-ring-2/

Ref 16. http://www.getty.edu/art/exhibitions/ancient_luxury/

Ref 17. https://boylerpf.com/products/antique-victorian-silver-italian-coral-bracelet

Ref 18.https://boylerpf.com/products/vintage-italian-silver-carnelian-art-deco-bracelet