Pegmatite Garnets

By TGMS Educational Instructor - Bill Shelton

The previous article dealt with rhyolites garnets and here you will find pegmatite garnets that are quite similar in that the two species almandine and spessartine are the dominant species.  According to Wright, 1038, the average proportions of the five major garnets in pegmatite are almandine at 41.8 and spessartine at 47.1.  We ought to expect to find chemical analyses that support this idea; further it should be found that both species are rather common in pegmatites. 

Middletown, Connecticut at Dripps Road has been studied and the content is Sp 50 and Al 50!  The granite aplite from Italy is Al 59.1 and Sp 28.7.  Deer Creek, Colorado is Al 60 to 70 and Sp 30 to 40.   The aplite from Portugal is Al 60.6 and Sp 28.7.  Finally, the deposit at Alabashka, Urals, Russia is Al 60 to 66 and Sp 28 to 34.  So, we should call all of these examples’ almandine.  Elsewhere, we find Nuevo, California at Al 53 to 70 and Sp at 30 to 47.  From Himalaya, California, we see Al at 73 and Sp at 24.  Other examples from here are Sp dominant, however.  Russell, Massachusetts sits at Al 54.43 and Sp 28.90 in spite of many claims to it being a spessartine.  The RRUFF data includes a Al dominant garnet from the Rutherford mine in Virginia as well as a spessartine dominant example from the same locality.  Becker, Case and Strickland quarries in Connecticut all are listed as almandine localities.  The Lakeview Mt., pegmatite is also an almandine locality.  Of course, there are numerous others and perhaps they are more common than we think.  Some collectors are inclined to assume nice examples must be spessartine.  It is very important to realize that a few famous localities are noted to produce both species.

Spessartine data looks quite different in that a number of locations are noted for nearly pure spessartine.  This is interesting and, as a guide in deciding what to call a specimen, the pure examples are virtually all orange.  See the list below.
 

Spessartine     Almandine       Locality
95                    5                      Hercules dike, California
96.4                 0.8                   Kenya
50 to 99.5        0.5 to 49          Himalaya, California
50 to 95           5 to 50             Little Three mine, California
50 to 95           3 to 43             Rutherford Mine, Virginia
58 to 80                                   Malkhane field, Russia
51.6                 32.9                 Avondale, Pennsylvania

Additional areas include Jail Hill, Strickland Quarry and Middletown in Connecticut, Little Cahuilla Mountains and the Tourmaline Queen mine in California, Morefield in Virginia, as well as Brazil and Pakistan.  We also see localities in Bohemia, Kola (Russia), Ireland, Poland and Rhodesia.  The data for these ranges from just over Sp 50 to Sp 80.  So, we see a lot of possible localities and a wide data range; perhaps appearing different from the almandine data seen above.

If you buy an example, it may or may not be labeled as spessartine.  The safe label is simply garnet, but we may not settle for that.  The name on a label can be correct but the similarity of members in the middle ranges can and do look essentially the same.  You can rest assured that some areas routinely produce both species and some are likely to only produce one or the other.  Input caption text here. Use the block's Settings tab to change the caption position and set other styles.

RHYOLITE GARNET AREAS

Rhyolite Garnet Areas - By Bill Shelton

Almandine

We as collectors may have visited a few areas where the garnets are found in rhyolites.  One that we may find of interest is the Aquarius Mountains in Arizona.  According to some sources, the garnets might be either almandine or spessartine.  While we should not necessarily rule out the possibility of both species occurring here, it may an error in terms of identification.  The lack of good evidence leaves me wondering what the correct answer is.  In Garnet (2008), the author says the garnets from here may be either spessartine or almandine.

Ash Creek, near Hayden, Arizona is an area where the garnets are usually labeled as spessartine; I have not been able to find conclusive evidence regarding them so will accept the species as given for the time being.  Dana’s New Mineralogy says the garnets form here are spessartine. 

The last two areas are perhaps the most widely known and collected for garnets form rhyolites.  Garnet Hill, Ely, Nevada is a rich source for specimens and very well-known to us.  The material from here has been sold as spessartine and almandine.  Chemical study shows the correct label should say these garnets are almandine.  It is unfortunate that the Sinkankas text (1976) claimed they were spessartine.  Dana’s New Mineralogy, for example indicates Alm 73 Spess 27 for these garnets.  Pabst (1938) shows 69.3 % Alm and 28.5 Spess.

Ruby Mountain, Nathrop, Colorado has always been considered to be a location for spessartine and the chemical data confirms this to be true.  Early studies and recent work corroborate this answer.  Yet, Dana’s New Mineralogy (1999) says they are almandine.  Lauf, (2012) claims the specimens are spessartine.  So, we see two relatively recent sources that do not agree.   An older source, Pabst (1938) shows 65.4 % Spess and 31.3 Alm.

Spessartine

Perhaps one lesson we may learn from this is apparent: sources may not agree on the proper species for a garnet from rhyolites.  It is clear, at least to me, that both can occur but whether almandine and spessartine will be found at a specific location is a different problem.   I think the consensus now is spessartine is correct for Ruby Mountain and almandine is correct fro Garnet Hill. 

I hope to visit some other less popular areas and see if I can get analysis done on similar garnets.  Perhaps we can provide the results to you in the future.

Bismuth and Silver

Article reprinted with permission and originally appeared in the May Issue of the NYMC Bulletin - http://www.newyorkmineralogicalclub.org/

 “It’s Elemental” is a series of columns by Bill Shelton written this in year in recognition of the United Nations’ International Year of the Periodic Table of Chemical Elements.

Here we will find two metals that are especially easy to find if you wish to purchase a sample for an element collection. Now, silver as bullion coins or bars are instantly available if you think you want to own one. Bismuth is very easy to find as man-made hopper-like crystals with rainbow colors. I see them all the time at major shows and expect you could find them on the internet. I used Google and found bismuth for sale as both ingots and crystals. That’s good news for element collectors.

 On the periodic table, silver is number 47 and it is located near the middle of period five. In rank order, silver is number 67 meaning 66 out of 92 naturally occurring elements are more common than silver. Contrast this with bismuth which is element number 83. It is located in period six next to lead. You will find it right of center and near the bottom of the main part of the periodic table. It is number 64 in rank order so we see that elemental rarity is very similar for this pair. About three quarters of the 92 natural elements are more common in the earth’s crust.

You might wonder why the symbols for some elements clearly match their names, i.e., Bi for bismuth while a few others such as Ag for silver do not. Well, silver is part of a group of elements that have been known for a very long time. In this case, the name is based on the Latin term argentum; hence the symbol Ag. Bismuth was first noted around 1,500 years ago while silver is present in ancient artifacts and mining is known from 3,000 years ago. Ancient Sumer is thought to be the source of the oldest silver artifact dating to about 4,000 BC. Sumer is located in modern-day Iraq. So, in terms of the 92 natural elements bismuth ranks among the first fifteen to be recognized while silver is among the first ten – all known before 1,000 BC. In fact, silver may have been known as long ago as 5,000 BC.

 Mineral collectors are likely to come across bismuth and silver because a few species with these elements present as major components are relatively common both on the internet and at mineral shows like the next NY shows on March 2nd and 3rd or June 22nd and 23rd in 2019. Dana’s Textbook, 1966, tells us there are 22 species of note for bismuth and emphasizes bismuthinite and native bismuth as perhaps the most frequent examples one might encounter. Also, silver has 55 species and the most common ones are argentite and native silver. Acanthite is also important and you may wish to seek more data regarding this species because it may be used to refer to species once called argentite. Using data from Mindat.org, it appears that we can find 187 species that contain silver and 233 with bismuth present. As a note to collectors, many of these are not likely to be interesting to you.

 My experience makes me suspect silver species are far more desired and sought out by collectors than bismuth examples. Some additional silver-rick species that I notice in museum collections, personal collections and the mineral marketplace are listed below.

Proustite, pyrargyrite, polybasite and stephanite are four additional examples that seem to get a fair amount of attention from collectors. To a lesser extent, I find amalgam, dyscrasite, stromeyerite, andorite and cerargyrite. Since natural examples exist for bismuth and silver, you can add them to a collection of native elements. There are about twenty different possibilities that I believe you can locate with little effort. In aesthetic terms, silver may be among the five or so examples that are most popular – I base this on what is present on internet sales sites, museum collections, private collections and the illustrations in books and magazines.

Good Neighbors

Article reprinted with permission and originally appeared in the April Issue of the NYMC Bulletin - http://www.newyorkmineralogicalclub.org/

 “It’s Elemental” is a series of columns by Bill Shelton written this in year in recognition of the United Nations’ International Year of the Periodic Table of Chemical Elements.

This month, we will focus on three elements that are located next to each other. Starting with Cr, #24, we proceed right to Mn, #25 and Fe, #26. The idea that the periodic table groups similar elements in vertical columns is at the base of why we call it periodic [= repeating]. In some cases, such as this group of three, similarities can exist among neighboring elements. Minerals illustrate numerous examples where iron can be substituted for manganese and vice versa. One common example is rhodochrosite [MnCO3] and siderite [FeCO3]. Chromium is far less common but can be seen to follow a similar procedure in chromite. Iron will replace a percentage of the chromium and this leads to ferrian chromite as we see in Deer et al, 2011.

On the periodic table, we can sometimes find information that may be useful in mineral studies. First, the valence helps explain likely substitutions in minerals. When the valence is equal, two elements of similar size are likely to replace each other. Mn 2+ and Fe 2+ are commonly seen; Fe 3+ and Cr 3+ also occur but less frequently. The ionic radius for Mn 2+ is 0.80 and Fe 2+ is 0.74. Fe 3+ is 0.64 while Cr 3+ is 0.63. See Klein and Hurlbut, Jr, 1985. Essentially the same crystal structure is assigned to each element. The common presence of iron, with 1,150 species, and manganese with 581 looks very different from chromium with only 97 species. If we rank order elements, iron is number 4, manganese is number 9 and chromium is number 40 with regard to the number of mineral species.

 Solid solution suggests two or more elements will share and replace one another in a mineral so that we may find an iron-rich species [an end member] and a manganese-rick species such as rhodochrosite and siderite. Analytical results range from pure Mn and no Fe for rhodochrosite to examples where Mn is essentially equal to Fe. The values given are Mn = 1.003 and Fe =0.946. I have seen a piece from Kazakhstan that looks to be brown but the underneath is a bright red core – maybe the crystal started as rhodochrosite and then ended up as siderite. Regarding siderite, nearly pure Fe with a small amount of Mn [0.01] is known and other examples range to where the ratio is 0.68 Mn to 0.98 Fe. Not quite equal but approaching that value, it should become clear that natural samples can and do cover a wide range of values. This situation may be referred to as a complete solid solution series.

 Chromium, being a lot rarer, provides less examples but consider chromite. Here, Fe 3+ replaces Cr 3+ and grades toward ferrian chromite (as we see in Deer et al, 2011.) Here, the chromium values range from 7.8 to 14.3 and the iron values are from 0.1 to 1.7. The wt % shows quite pure chromite with 58 % Cr and 0.5 % Fe. This grades to examples with 45% Cr and 8.5% Fe. I would call this a partial solid solution series.

 If we compare uvarovite garnets, based on molecular end members, they range from 91.2 to 36.3 UV. The GR values range from 0 to 41 while PY ranges from 0.2 to 34.1 SP values are between 0 and 1.9; AN ranges from 0 to 18. AN reflects iron and SP reflects manganese present; UV represents chromium here. A rather more complex example as you can see but it is common to find several elements present in certain minerals as we see here.

 The same three elements affect the color of spinel. V and Co also can play a role but Cr is important in most red examples. Mn is linked to some green crystals and the presence of iron is noted in all colors! See Schmetzer, 1987. Green may be related to zinc or chromium-rich examples. Cr can be the culprit for more that one color. Strange as it may seem, Cr causes red or pink in spinel but very high levels, over 15 %, actually produce green crystals. Olivine group members can have chromium present; it is usually a small amount such as 0.70 % or less. The manganese content can be zero but also has been measured up to nearly 8 %. They share a common structural position and we may find small amounts of Ca, Ni, Al, Ti, K and Na present as well. So, we might decide that the neighbors seem to get along well.

Coin Metals

Article reprinted with permission and originally appeared in the March Issue of the NYMC Bulletin - http://www.newyorkmineralogicalclub.org/

 “It’s Elemental” is a series of columns by Bill Shelton written this in year in recognition of the United Nations’ International Year of the Periodic Table of Chemical Elements.

There are only four metals that are usually considered when we discuss coins, especially before 1964 when the USA stopped making coins for circulation that contained much metal value. The same four metals also play a role in the mineral collector’s world because they provide mineral specimens for us to enjoy. On the standard periodic table, we find column 1B contains copper, silver and gold. Together, they are the three most common metals present in coins since the first coins were actually used in commerce.

Platinum actually does not fall in the same column but is the neighbor to gold and has seen limited use in the manufacture of coins. On one occasion, Russia made over 1.5million coins during the 1820’s and that may be the only case where we find much use until the recent interest in bullion coins has created a market filled by several different nations. None of these new issues were ever intended for circulation or general use. I am surprised to see a report that Egyptian artifacts made from platinum were found that date back to the 7th century BC. Also, Colombian remains from 2000 years ago are claimed to be made of platinum.

Regarding coins, I am using the term to refer to oval to round, flattened discs of metal and do not include other objects that may be used for trade, etc. in the distant past. Since we can rule out platinum here as well, the three remaining metals are the primary components of the coins we will discuss. A trip on the internet will easily take you to coins for sale that are in fact quite old. I tried this out and found a couple nice examples. A Roman bronze coin from 395 – 408 BC and a Macedon bronze from 336 – 323 BC serve as two among many. There are older ones that you may find as well as hundreds of items offered for sale.

The oldest coins might be from the 5th or 6th century and may be labeled as Lydian. For a mineral collector it gets interesting here because they are supposed to be made from electrum, a natural alloy of gold and silver. Many of us have seen specimens labeled this way for sale at mineral shows. A lot of other ancient coins were made from bronze, an alloy of copper with some tin. We can find stories about even older copper beads that are perhaps 10,000 years old. The coins are more likely to be from the year 700 BC or less.

Mineral collectors are aware of certain elements on the periodic table that are often included under the heading of native elements. It is routine to attend a mineral show or view an internet site and find all of the four metals mentioned above as natural specimens. We might see gold nuggets from Australia, silver wires from Mexico, copper crystals from Michigan or platinum nuggets from Russia. All of these seem to me to be readily available, despite the fact that they can be costly.

 Ever more common are certain compounds, especially those that contain copper. The most common member of this group of four, copper supplies hundreds of examples at any mineral show. Here we can find ever-popular species such as azurite, cuprite and malachite. I would not believe you can’t find chalcopyrite; we often see little inexpensive samples labeled as bornite when they are really treated pieces of chalcopyrite with a paper-thin skin of bornite. Besides being far from natural, they are almost entirely made up of chalcopyrite. Trya destructive test and chip the sample to see whether the inside is anything like the colorful skin on the surface.

 Searching for silver minerals will prove to be a little more difficult since they are far less available but you can expect to find pyrargyrite, proustite, stephanite and acanthite. They are the most likely ones to be found for sale. Even native silver appears to be a bit elusive compared to gold or copper. You can find dealers thatsell almost exclusively gold or copper but I never heard of a silver dealer in the mineral context.

 Platinum is rare in many ways – compared to the others here, I believe it is the rarest of the lot. So, even native platinum will not be ever-present in a search. Also, the next most likely example, the compound sperrylite will also be less common as a choice at a sale event. Some sources will describe all of the platinum minerals as rare and it is true, especially by comparison. While there are other choices, many collectors will not see examples they are eager to add to a collection.

“The Mighty Quarter - America has made quarters since 1796; mostly there has been but little change in size and composition till 1965. Starting out at 6.74 grams and 27.5 mm in diameter, they ended up at 6.25 grams and 24.3 mm in diameter. The silver content started at 89% but went up to 90% shortly thereafter and stayed there for a very long time. Now, as we see since 1965, a quarter is 5.67 grams and 24.3 mm in diameter with a silver content of zero. The silver value for any quarter is a lot more than 25 cents – just for the silver content you can expect to get about $2.60 cents at today’s values of approximately $15 per ounce [Oct. 2018]. As a collector’s item, the value can be much more depending on various numismatic factors.”

Gold is reported from 24,350 localities and can be found in about 30mineral species. The record for the most different species belongs to the Kochbulak mine in Tajikistan with 11 species. Added to the previous comments, some collectors also like auricupride, nagyagite or petzite but we have so few choices! Most widely present in collections as native gold.

Silver is reported from 4,757 localities and can be found in about 140 mineral species. The record for the most different species belongs to the Lengenbach quarry in Switzerland with 24 species. Added to the previous comments, some collectors also like andorite, boleite, eugenite, miersite and polybasite amongst over 100 others. Most widely present in collections as argentite or native silver.

Copper is reported from 3,152 localities and can be found in about 560 mineral species. The record for the most different species belongs to the Clara mine in Germany with 101 species. Added to the previous comments, some collectors like ajoite, dioptase, kinoite, libethenite, spangolite and turquoise amongst over 500 other. Most widely present in collections as malachite and native copper.

Platinum is reported from 424 localities and can be found in about 25 mineral species. The record for the most different species belongs to the Driekop mine in South Africa with 12 species. Added to the previous comments, some collectors like cooperate, isoferroplatinum or niggliite but we have so few choices! Most widely present in collections as native platinum.

Three Elements

Article reprinted with permission and originally appeared in the February Issue of the NYMC Bulletin - http://www.newyorkmineralogicalclub.org/

 “It’s Elemental” is a series of columns by Bill Shelton written this in year in recognition of the United Nations’ International Year of the Periodic Table of Chemical Elements.

 We all know oxygen, silicon, and aluminum are common elements in the Earth’s crust. They are also the three most common but in terms of the number of minerals that contain them, we find a somewhat different accounting. Oxygen is the most numerous in the number of species, silicon is third and aluminum is seventh. They all have more than 1,000 known species which is also the case for five other elements. (Those other elements are hydrogen, calcium, iron, sulfur and sodium.)

These elements can occur in an uncombined state. Oxygen is present as an important component of our atmosphere (about 21%). We cannot consider it as a mineral however, because it is a gas; minerals are defined as solids. Silicon has a presence in the Earth’s crust as a native element just as we find in the list of Back, 2018. One unusual place where we see this is in fulgarites that form when lightning strikes the earth’s surface. Aluminum, perhaps surprising us, occurs in two elemental forms – one we call aluminum while the other is called steinhardtite. (Dr. Paul Steinhardt lectured to the NYMC in January 2014 about quasicrystals.) I would readily consider the native elements for silicon and aluminum as greatly restricted in occurrence; specimens are mainly very small and probably of little interest to many mineral collectors.

The combined occurrences for these three elements are very well known. Oxygen and silicon alone produce quartz, opal and a host of varietals like agate and jasper. The essential materials for all of these species are the same. Their widespread presence, and interest to collectors, makes them important to us. Far less interest is noted for tridymite and coesite who also share identical chemistry. If one or two other elements are included, there are many more possible species.

Oxygen and aluminum produce the species corundum which includes the varieties ruby and sapphire. It is very important as a gem material and will be likely to be present in a huge number of mineral and/or gem collections. Diaspore comes close but requires hydrogen with the other elements listed here. Adding another element or two greatly expands the number of possible species as we see with diaspore.

For all three together, we find the mineral kyanite. This is very popular amongst collectors especially as bright blue crystalline examples. The occasional gem usage appears to be quite limited but the very best faceted stones are quite appealing. Sillimanite and andalusite belong here but seem to be less common in some collections. All three species can be used to suggest the degree of metamorphism in rocks.

If we add one or two more elements, we have the bulk of the rock-forming minerals. Feldspars, micas, pyroxenes, amphiboles and so forth are composed mainly of oxygen, silicon, aluminum and one or two more elements. We see, therefore, that despite the very common presence of these elements in mineral species, there are really very few examples made up of only these three elements. Perhaps this is not what you would have guessed. When you look at lists for say, aluminum species, you find the various combinations noted in nature exceeds 1,000.

2019 International Year of the Periodic Table

The UN Proclaims 2019 the International Year of the Periodic Table of Chemical Elements

 On 20 December 2017, during its 74th Plenary Meeting, the United Nations (UN) General Assembly 72nd Session has proclaimed 2019 as the International Year of the Periodic Table of Chemical Elements (IYPT 2019). In proclaiming an International Year focusing on the Periodic Table of Chemical Elements and its applications, the United Nations has recognized the importance of raising global awareness of how chemistry promotes sustainable development and provides solutions to global challenges in energy, education, agriculture and health. Indeed, the resolution was adopted as part of a more general Agenda item on Science and technology for development. This International Year will bring together many different stakeholders including UNESCO, scientific societies and unions, educational and research institutions, technology platforms, non-profit organizations and private sector partners to promote and celebrate the significance of the Periodic Table of Elements and its applications to society during 2019.

The development of the Periodic Table of the Elements is one of the most significant achievements in science and a uniting scientific concept, with broad implications in Astronomy, Chemistry, Physics, Biology and other natural sciences. The International Year of the Periodic Table of Chemical Elements in 2019 will coincide with the 150th anniversary of the discovery of the Periodic System by Dmitry Mendeleev in 1869. It is a unique tool enabling scientists to predict the appearance and properties of matter on Earth and in the Universe. Many chemical elements are crucial to enhance the value and performance of products necessary for humankind, our planet, and industrial endeavors. The four most recent elements (113, 115, 117 and 118) were fully added into the Periodic Table, with the approval of their names and symbols, on 28 November 2016.

The International Year of the Periodic Table of the Chemical Elements will coincide with the Centenary of IUPAC (IUPAC100). The events of IUPAC100 and of IYPT will enhance the understanding and appreciation of the Periodic Table and chemistry in general among the public. The 100th Anniversary of IUPAC will be on the UNESCO Calendar of Anniversaries on 28th July 2019.

“As the global organization that provides objective scientific expertise and develops the essential tools for the application and communication of chemical knowledge for the benefit of humankind, the International Union of Pure and Applied Chemistry is pleased and honored to make this announcement concerning the International Year of the Periodic Table of Chemical Elements” said IUPAC President, Professor Natalia Tarasova.

 Chemical elements play a vital role in our daily lives and are crucial for humankind and our planet, and for industry. The International Year of the Periodic Table of Chemical Elements will give an opportunity to show how they are central to linking cultural, economic and political aspects of the global society through a common language, whilst also celebrating the genesis and development of the periodic table over the last 150 years. It is critical that the brightest young minds continue to be attracted to chemistry and physics in order to ensure the next generation of scientists, engineers, and innovators in this field. Particular areas where the Periodic Table and its understanding have had a revolutionary impact are in nuclear medicine, the study of chemical elements and compounds in space and the prediction of novel materials.

 The IYPT is endorsed by a number of international Scientific Unions and the International Council for Science (ICSU). The IYPT will be administered by an International Steering Committee in collaboration with the UNESCO International Basic Sciences Programme and an International Secretariat, to start operating in early 2018. In addition to IUPAC, IYPT is supported by the International Union of Pure and Applied Physics (IUPAP), the European Chemical Sciences (EuCheMS), the International Astronomical Union (IAU) and the International Union of History and Philosophy of Science and Technology (IUHPST).

Cubic Crystals

By Bill Shelton

CUBIC CRYSTALS

It seems as though it might be a simple matter to deal with this topic.  Well, perhaps it is but I have the idea that mineral collectors may not exactly know what is involved.  Any source reveals that there are seven systems and 32 crystal classes where the system is subdivided based on characteristic symmetry for that system.  Here, we shall look at the cubic system and the five classes within the system.  Each class is named for a certain form that characterizes the class; also, we find a customary reference to the class where numbers and letters will also describe the same class.  This is also called the point group.   The full symbol and an abbreviated symbol are commonly used for the same purpose.  Below, I have arbitrarily listed examples that I consider to be more or less important for collectors.  Species that are not included are the ones I think many collectors do not seem interested in as well as rarely found samples that will be hard to collect and/or purchase.

The first class is the tetrahedral pentagonal dodecahedral; also noted as 23.  We find the name tetartohedral used as well.  References may call this class the ullmannite type.  Some older sources say cobaltite belongs here.  Actually, it has been moved to the orthorhombic, pseudcubic division where we can see crystals that look like pyritohedra but are not. Kostov and Kostov (1999) indicate that it can and does resemble pyrite which is in a higher symmetry class, 2m3.  Sadly, this is the most notable member of this class.  So, who’s left?  Well, we have ullmannite and I believe it is of little interest so this class is of minor concern in my opinion for we collectors.  Ullmannite fits in P23 space group and will display octahedral, pyritohedral and tetrahedral crystal forms as well as twins.

 The next class (2m3 or m3) is the di(akis) dodecahedral which is also labeled pentagonal hemihedral, dipliodal, didodecahedral and pyritohedral class.  References may call it the pyrite type.  I choose to refer to four species as the prime examples for us to consider.  This class is important and the members are likely to be familiar and present in your collections.  Sperrylite, Pa3 space group, resembles pyrite especially regarding morphology and variation of crystal habits.  It is less common among collector material than the others listed here.  Smaltite, a name found in older sources has been discredited and is considered as a variety of skutterudite.  The Handbook of Mineralogy (1990) shows analytical data for CoAs2 (more like the material previously classified as smaltite and data for CoAs3 that is akin to the original skutterudite.  Space group Im3 is for this mineral; cubic, octahedral and cubo-octahedral forms are commonly present.  Bixbyite, Im3 space group, exhibits cubic forms and often has twins.  Pyrite, the namesake here, is a member of Pa3 space group.  Pyritohedral, dodecahedral, cubic, and octahedral forms are common as are twins.  It is the most prevalent example for this class.

 Third among the five classes, we find bar43m.  This is the hexa(kis) tetrahedral, hextetrahedral, of tetrahedral hemihedral class. The names tetrahedral class and tetrahedrite type are also used in some sources.  Members more or less noted in collections and numerous by comparison to some other classes, include sphalerite.  It is Fbar43m space group and an exhibit tetrahedral, cubo-octahedral, dodecahedral forms and twins.  This very common mineral is no doubt well-known to you.  Tetrahedrite, Ibar43m space group, shows tetrahedral forms and twins.  Tennantite, also Ibar43m space group, looks essentially the same in general as tetrahedrite.  Last, we find helvite, space group Pbar43m, with tetrahedral forms and twins. 

 Class 432 also noted as 43 is the plagiohedral.  I have chosen to say no more - it has no mineral representatives anyway.  Previously, cuprite was placed here.  The Handbook says it is m3m and space group Pn3m – so look for it in the next class.

 Finally, the major class, called the normal class or galena type, is represented as 4m3m2 which is abbreviated as m3m.  The name hexoctahedral or holohedral may be found.  A lot of collector type minerals are placed here.  I include thirteen examples below but there are nearly 30 very well-known species in this class.

 Species           Space Group                         Forms                                                              twins?

Copper            Fm3m             cube, octahedral, dodecahedra,tetrahedra.                          twins

Gold                Fm3m             octahedral, dodecahedra, cube                                             twins

Platinum          Fm3m         cube (often isoferroplatinum and then twins, Pm3m.              twins?

Halite               Fm3m             cubic                                                  

Galena            Fm3m             cube, octahedral, cobo-octahedra                                         twins

Fluorite            Fm3m             cube, octahedral, dodecahedra                                            twins

Spinel              Fd3m               octahedral, cube, dodecahedra                                            twins

Diamond         Fd3m               octahedral, dodecahedra, cube                                            twins

Grossular        Ia3d                 dodecahedra, trapezohedra (garnet grp)               

Sodalite           P43n                dodecahedra                                                                        twins

Analcime         Ia3d                 trapezohedra                                                                        twins

Microlite          Fd3m               octahedral                                          

Pollucite          Ia3d                 cube, dodecahedra, trap. (rare)

 Well, now we should list a few others.  Silver, lead, iron, alabandite, magnetite, argentite (acanthite pseudos), gahnite, franklinite, chromite, pyrochlore, and uraninite all belong here.  Also, all the garnet group like almandine, andradite, pyrope, spessartine and uvarovite are best included here.  So, the best group for further study is the so-called normal group since we find so many minerals we all enjoy and probably have in our collections.  Based on the source I chose to use, I think the data here is more or less correct and up to date. 

Color in Beryl

By Bill Shelton

Color in Beryl

We should consider the fact that a pure beryl has no color-causing element in its formula.  Beryllium, aluminum and silicon do not turn out to be among the various elements that usually impart color to a mineral.  In nature, we can occasionally find examples where beryl is identified as white, colorless of nearly colorless.  The examination of some of these samples shows the absence of a few elements that are typically present in beryl which does exhibit color.  We should be especially aware of iron, manganese and chromium because these elements do impart color to the vast majority of specimens we see in collections and dealer stock. 

Let’s start out with the variety emerald – the color we usually see is attributed to a trace of chromium in the traditional sense and more recently to vanadium or even a combination or the two.   In one analysis, an emerald from Chivor, Columbia was noted to have 0.14% (by weight) chromium.  Another example, from Rhodesia, was recorded as having 0.28 to 0.59% chromium which can be compared to a further sample from the Ural mountains in Russia where results state .04 to .38% chromium were present.  So, we should conclude there is quite a range of values reported but what the absolute limits may be are not clear and may be subject to change with more reports being issued all the time.  Consider the unusual data for a piece from Zambia:  it has a value of .01 for vanadium, .07 for chromium and .73 for iron.  In spite of the mixture present, the color is said to be a nice emerald green.  Analytical results suggest the values for chromium in synthetic emeralds may be higher than the values seen in natural examples. 

 Red beryl, which has been assigned a few other names, is a rare variety known from a handful of localities.  Crystals are small and can be pale to intense in color.  The data available indicates the color is due to the presence of trace amounts of manganese.  One report lists the value at .08% which can be compared to another variety, morganite where the values are often lower.  Chemical testing shows red beryl may contain traces of titanium, zinc, tin, chromium, cesium, lithium, rubidium, boron, zirconium, niobium, lead and others.  I dare say beryl from most other places does not have a list like this; it also is odd that red beryl has essentially no water while most all pieces of other types will have water from .76 to over 2.5%.  The most noted location seems to be the Wah Wah Mountains, Utah but even the Topaz Mountain area has been noted as a source of similar pieces.  The common associates in the area include bixbyite, hematite, calcite, fluorite, garnet, hyalite, pseudobrookite, quartz and topaz. 

Morganite, a pink variety of beryl, is noted to contain small amounts of manganese and the color is attributed to it.  Besides containing manganese, morganite often also has alkali rich chemistry and this does not seem to affect color but may influence the habit (tabular) that is common in this variety.  While an orange colored type exists, it may be a bit unstable and reports say that these pieces will slowly alter to pink with exposure to light.  When a yellow tinge is present in morganite, heat treatment may remove it and improve the color to a better pink. 

Aquamarine may be the most desired variety for gemstones due to acceptance by customers, availability and price.  Here, color is attributed to iron; blue shades are caused by iron 2 in the channel site and this is the classic color for aquamarine.  Some sources also include green in the aquamarine sector and then we find color is due to iron also.  The difference is that both iron 2 and iron 3 are present.  This green color seems to be especially common in both ordinary beryl and gem grade material. Heat treatment may improve the green color to a better blue because the iron 3 is driven off and only blue will remain.  You may not be aware of it, but heat treatment is far more common that one may think. 

 If a beryl contains only iron 3, it will be yellow and perhaps called heliodor.  We find this variety to be less common than aquamarine and less desirable for gem use.  The color may range from golden to brownish yellow and the rarely seen deep shades are very pretty but still not widely offered as gems.  Some greenish samples are also sold as heliodor and here we can quibble about exactly what colors are acceptable under this variety – it seems to me that as one might expect, the definition for any variety may be a bit flexible. 

I referred to colorless and white beryl in the beginning – the varietal name goshenite is used here and it implies there is no trace element chemistry causing color in the example.  Well, as usual, it may be a bit more involved since iron 2 in the octahedral site and iron 3 in the channel site do not impart color to the mineral.  The presence of alkalis and other trace elements also do not add color.  So, a clever individual will see that the so-called pure beryl may just be lacking chromophores and not be technically pure.  An indirect piece of evidence is hidden in fluorescent data for beryl.  Here, mostly we find all types are inert or usually so.  Some goshenite, some emerald and a few morganites do fluoresce.  These are the very ones that are likely to be relatively iron-free.  When we consider the varieties aquamarine, heliodor and red beryl, they are all inert to fluorescent light.  Green beryl seems to behave in the same manner.  As you may know, iron is often described as quenching fluorescence in a great many mineral species. 

Colors in Garnets

By Bill Shelton

Colors in Garnets

Perhaps, we can find 10 major colors in garnets that belong to the six main species recognized by many sources.  What are the colors and which species are noted to occur in those colors?  Well, it appears that as one might guess that red is most common and will be seen in five out of six species.

The six species we refer to are almandine, andradite, grossular, pyrope, spessartine and uvarovite.

Red is never seen in uvarovite but will occur in all the others.  Pink is noted for three species – they are andradite, grossular and pyrope.   Orange, a nice color for garnet, is present in grossular and spessartine.  Yellow will likely occur in andradite and grossular.  Green, a personal favorite, is noted in andradite, grossular and uvarovite where it is also the only color to be seen.  Purple is unique to pyrope; however there are hints in certain almandines.  Brown, not a collector favorite, can occur in andradite, grossular and spessartine.  Black is mainly noted for andradite but a few grossulars may be this color.  Colorless garnet would be a rare thing in a general sense; pure grossular does exhibit this color now and then.  White rarely is found and when it is, expect the species pyrope and grossular to be present.  So, the winner is grossular with a total of 15 different shades.   I suppose uvarovite is the big loser with only one shade.  Almandine has three, andradite has eight, spessartine has six and pyrope has four.

The exact cause of the various colors is suggested to be due to chemicals present in the garnet.   Iron may impart red and yellow-green; yellow, orange and black (often with titanium); also purple when chromium is also present as well.   Chromium is also thought to cause green and notably in uvarovite where this is the only color we see.  Vanadium will be credited with yellow-green; manganese is responsible for pink and orange.  You may note that more than one cause is given for some colors.   The odd alexandrite effect may be due to chromium, vanadium and manganese together.

Various sources list the following colors for grossular.  They are pink, red-brown, orange, yellow, pale green, emerald green, greenish-blue, colorless (or nearly so), olive green, cherry red, dark green, honey brown brown orange-brown and black. I find the colorless and white garnets to be a peculiar thing – the explanation is said to be due to the lack of chromophores and is limited to pyrope and grossular when they are pure.  There is no iron or manganese; also no chromium or vanadium which all can be suspected causes of color in garnets.