Turquoise Group Minerals

This can be a broad and difficult subject when we add all the science and all the opinions. I have attempted to group the information in a way that we all can learn a little more about several of the minerals being sold as turquoise and that would sometimes test as a turquoise. The pictures that I have used are just a few samples.

Definitions
Mineral-“a naturally occurring inorganic solid, with a definite chemical composition, and an ordered atomic arrangement.” Oxford Languages
Is turquoise a mineral? Yes

Gemstone-“a precious or semiprecious stone, especially one cut, polished and used in a piece of jewelry.” Oxford Languages
Is turquoise a gemstone? Yes

Chemistry Periodic Table of the Elements-“a table of the chemical elements arranged in order of atomic number, usually in rows, so that elements with similar atomic structure appear in vertical columns.” Oxford Languages

Chemical Formula-“a set of chemical symbols showing the elements present in a compound and their relative proportions, and in some cases the structure of the compound.” Oxford languages

What is turquoise’s standard chemical formula?
CuAl6(PO4)4(OH)84H2O

What is a chemical formula variation for turquoise with intrusive iron (Fe)?

Cu(Fe3+, Al)6(PO4)4(OH)8 ∙4H2O

Crystalline Structure-“is any structure of ions, molecules, or atoms that are held together in an ordered, three-dimensional arrangement.” Study.com
Turquoise has a triclinic crystalline structure.

Formations
Turquoise is a secondary formation that is deposited into existing spaces of open pockets or crevices within a hillside. The various recipes necessary to create turquoise is a complex composition of copper, aluminum, phosphates, water and oftentimes other available intrusive substances. The acidic water indiscriminately collects all available substances as it flows downward; it dissipates and evaporates depositing the new chemistry into existing seams, cracks, and open spaces within the hillside.

Chemistry
“Any student of mineralogy is well acquainted with what are called “series” of minerals, chemically related substances within which atoms of two different metals can easily substitute for each other in the structure. Such series members are very similar in properties, and often occur close to each other in a deposit as solutions tend to vary somewhat within certain boundaries of that deposit. Such series of related minerals are possible because certain metal ions have similar sizes and other characteristics, so they can interchange.” Bob Jones
“When three mineral species are closely related, they are then shown in textbooks as the three corners of a triangle. The lines connecting those corners represent a gradually changing chemistry as one metal ion substitutes for another in regular percentage amounts. The result is a “family” or “series” of species and varieties.” Bob Jones

The original chemical equation for turquoise defines a pure blue formation with no inclusions or intrusive substances. If we keep this “perfect” sample test in the forefront of this blog post, it will help bring clarity to other turquoise formation varieties, chemistry and comparative mineralogy.

Ferrian
Ferrian is the technical term that refers to most green-colored turquoise. Iron (Fe) is a strong and abundant chromophore that generally becomes dominant when it is brought into contact with a turquoise solution deposit. The Iron (Fe) can replace the Aluminum (Al) within the chemical formula.
This progression of Iron (Fe) will show in analytical tests as chalcosiderite. The solid solution series occurs when more than one element occupies the same space in the structure and substitutes another.

Turquoise Group Minerals
The Turquoise Group has several minerals in its series. Each of these minerals can be similar to one another in chemical formula, but they are not the same mineral, one element can replace another, making a different chemistry.

At each end of the Turquoise Group Minerals spectrum you have specific gems such as Turquoise, Faustite and Chalcosiderite. Turquoise, Faustite and Chalcosiderite in their purest chemical formulas are also visually distinctive and can be specifically identified using good eyesight. I identify formations and cabochons using the visual test for most of these minerals. Turquoise within the lightest blues and greens to the darkest. Faustite cabochons in all the apple green colors. Chalcosiderites in greens, off whites, yellows and some milky white blues. Faustites and chalcosiderites have their own rarity and beauty and are continuing to gain their own grading system and favor among more and more buyers.

Turquoise CuAl6(PO4)4(OH)8∙4H2O
Faustite (Zn,Cu)Al6(PO4)4(OH)8∙4H2O
Chalcosiderite CuFe6(PO4)4(OH)8∙4H2O
Aheylite FeAl6(PO4)4(OH)8∙4H2O
Planerite Al6(PO4)2(PO3OH)2∙4H2O

Since the water indiscriminately collects and deposits a formation’s mineralogy; a variety of chemical formulas can exist within a single formation. When there is a varied collection of mineralogy it can become complicated to specifically identify the formation into a single mineral class. This is one reason there is so many opinions when identifying Turquoise Group Minerals. These sedimentary formed minerals can have similar chemistry and some visual attributes as turquoise. Especially when these minerals are between chemistry spectrum. In these types of formations, even the world of science has a difficult time specifically analyzing which of the two or three minerals a formation could be identified. One of the most interesting and sometimes rare minerals is when a visual identification can specifically identify two minerals in the same formation.

Analytical Testing and Identification

Minerals with similar structure and somewhat different chemistry can be difficult to identify as one specific mineral without spending some money. The correct scientific tests and the correct interpretation of the tests can add to the expense. The equipment and procedures can be high-tech and specific; identifying every chemistry that exists within one formation. Many times it is possible to identify at least 2 or 3 minerals in one formation when reading the final test results.

1. Only a specific pin-pointed area is tested from a formation. A different area of the formation could be tested with a different result.
2. Different mineral interpretations can be made depending on the qualifications of the person who is analyzing the test results.
3. Sometimes “people” have been known to send in one formation for testing and then using that evaluation paper for a totally different formation.

Why do so many people identify other minerals as turquoise?

We should always ask a salesperson, TV personality, or miner what their experience or opinions are. They probably have a good reason for how they identify the gems they offer for sale.

The following are some of my theories on why most people group so many different minerals as turquoise.

1. Turquoise is the most famous gem in the category of semi-precious gemstones and the Turquoise Mineral Group. Many people do not take the time to explain or learn about the different minerals.
2. Turquoise has so many colors, variety of hardness, and host rock matrix that turquoise itself includes a wide range of properties, therefore many people just identify a few more colors and minerals as turquoise.
3. Turquoise in the American Southwest is absorbed into the amazing Native American history, mystique, and arts. So much so, that if any stone is set in silver Native American jewelry, it is expected that it must be a turquoise.
The tourist often asks: Is this red turquoise?… when looking at coral.
4. Through the last fifty years, turquoise has become rarer and more expensive. Many other minerals have helped fill the demand as comparatively affordable and colorful gems for the market.
5. Most artists are going to use the best available materials to build and use in their art and a lot of the quality turquoise sources have been depleted.
6. Most people are visual buyers. They buy what they like and many of the other minerals are not only beautiful, but they are also rare.

Other Minerals
There are many other minerals that can be visually similar to turquoise. These include some of the Turquoise Group Minerals, copper ores, variscites, dolomites,
chrysocollas, azurites, jades and even some larimars along with other minerals. The picture below is just a few colors of variscites.

Conclusion
Buy what you like and if you would like to know if your salesperson has any extra information about the mineral or gemstone that you are purchasing; ask them. There is some really good and interesting information out there about so many unique and beautiful minerals and gemstones. Such as….
1. Faustite was named as a mineral in 1953
2. Variscite not only has a different chemical formula than turquoise, it also has a different crystalline structure. (Al(PO4)∙2H2O)-Orthorhombic
3. A diamond and turquoise are each Triclinic in their crystalline structures.

Joe Dan Lowry ©June 8, 2020

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