QUALITATIVE ANALYSIS FOR THE MINERAL COLLECTOR

In these modern times when mineralogists routinely use X-ray diffraction, the electron microprobe, and spectroscopic techniques for the identification of a mineral species, qualitative analysis harkens back to a much simpler time. In the very early days of mineralogy, simple chemical methods were used to determine the presence of key elements in a mineral specimen. Information on the physical nature of a mineral specimen, when combined with confirmation of elemental composition, provided positive identification of a mineral species. Chemical assay techniques could then be employed to provide quantitative data on the composition of a mineral specimen. For the mineral collector, the primary interest is in positive identification of a mineral specimen. Clearly for the vast majority of cases physical characteristics of a mineral specimen can be used to identify the mineral species. However, in some cases the use of qualitative analytical techniques can provide positive identification where physical characteristics alone do not afford a positive identification. In addition these qualitative analytical techniques can deepen an understanding of mineral chemistry, are a lot of fun and can provide an extra measure of enjoyment to mineral collecting.

To better understand the techniques of qualitative analysis for minerals, we need to first take a trip back in time to the mid 1700s in Sweden. In these early days of mineralogy, minerals were largely described by physical characteristics alone. Many scientists of the day began to apply the science of chemistry to the study of minerals. The Swedish chemist and mineralogist Baron Axel Fredric Croenstedt made a discovery that would change the young science of mineralogy forever. Croenstedt discovered that when intense heat was directed at a small sample of a mineral, certain elements in the mineral reacted in characteristic ways. Temperatures in the range of 1500 to 2000°C could be obtained with a simple device called a blowpipe. This device was nothing more than a curved tapering brass tube that was used to direct a blast of air into a candle or lamp flame. The extra oxygen supplied to the flame increased its temperature, and the movement of air through the flame changed its shape and direction. Specific elements, when excited by the high flame temperature, would color the flame in a way that was characteristic of that element. In some cases, the vapors created and the oxides or metallic substances left behind were also characteristic of a specific element. Artisans of the time had been using variations on the blowpipe for some time to work with metals, but Croenstedt was the first to apply this tool to mineral analysis in a systematic way. The makers of stained glass had known for some time that certain metals could be introduced in small amounts into a glass melt to impart color to the glass. Borrowing a page from the stained glass makers, Croenstedt also discovered the ability of specific elements to impart a characteristic color to the glass created when reactive fluxes such as borax where heated in the blowpipe flame. Enough information was gained about minerals through this new blowpipe technique that Cronstedt felt justified in suggesting that minerals be classified not only according to their appearance but also according to their chemical composition. Cronstedt wrote a book detailing this new form of classification that was published in 1758. Although Cronstedt is primarily credited with the discovery of the element nickel, he is also considered the father of systematic blowpipe analysis.

Now we move forward in time to the mid 1800s to Yale University in the United States. A giant in the science of mineralogy was a professor of Physics at Yale University, James Dwight Dana. Earlier discoveries in mineralogy were combined with considerable research of his own and contributions from most notably G.V. Brush and S.L. Penfield to create the Manual of Mineralogy published in 1843. In this classic work, Dana combined an understanding of crystallography, physical characteristics of minerals, and mineral chemistry to produce a system of mineral analysis and identification that is still used today. Brush and Penfield also published a separate work on using the blowpipe in the analysis of minerals. This publication is considered the seminal work on the subject. James Dana’s son, Edward Salisbury Dana followed in his father’s footsteps to publish Minerals and How to Study Them in 1895. This was the first book specifically targeted to the non-professional. This classic book has been updated by Hurlburt and Sharp and re- issued in 1998 as Dana’s Minerals and How to Study Them. At a cost of only $40.00, it is highly recommended for the bookshelf of every serious student of mineralogy.

This presentation will examine some of the techniques used by mineralogists in the days before electronic instrumentation and how the mineral collector can enhance the study of minerals and increase the enjoyment of the mineral hobby by using some of these time honored analytical techniques. The most fundamental analytical techniques for minerals are their physical characteristics. It is important that a complete understanding of the following mineral physical characteristics is obtained before moving on to more involved chemical techniques. Very often a preliminary identification of a mineral species can be made from the physical characteristics alone, leaving chemical techniques to be used in a confirmatory role.

THE IMPORTANT PHYSICAL CHARACTERISITCS OF MINERALS: Crystal Form Cleavage Parting Fracture Hardness Tenacity Specific Gravity Color Streak Luster Fluorescence Taste and Odor Magnetic Characteristics

A detailed discussion of these physical characteristics is beyond the scope of this presentation. Our purpose here is to examine some of the chemical techniques that can be used by the mineral collector. Perhaps the most readily available technique is Blowpipe Analysis. The equipment necessary for this technique is readily available, and it contributes immensely to the enjoyment of the mineral hobby.

There are four groups of blowpipe tests:
Although brass blowpipes are readily available, the advent of the small fine point butane torch has begun to displace the classical blowpipe. This is particularly the case with field mineral exploration and prospectors.

To demonstrate the use of the blowpipe, I will recount the analysis of a mineral specimen found at a construction site just off of Peachtree Industrial Boulevard in Duluth, Georgia. The construction site had cut into the northeastern flank of the Wolf Creek Formation. The host rock was a graphite-bearing schist with pyrite and minor chalcopyrite found in the upper portion of the exposure. The mineral occurred filling a fracture cutting across the fabric of the host rock.


The mineral specimen was tested for fusibility and was subjected to a borax bead test and charcoal block test. Before we begin a word about safety is in order. When conducting any of these mineral tests, always wear SAFETY GLASSES. WARNING: Always conduct tests involving open flame in a fireproof area and always have a fire extinguisher in the immediate area. The results of blowpipe tests show a strong indication for the presence of the element copper in the mineral specimen. Based on this information and observations of its physical characteristics I tentatively identified the mineral specimen as chrysocolla. Now that we have a good idea of some chemical elements in the mineral and a tentative identification, the next step is to use some wet chemical tests to confirm our observations. A number of entrepreneurs have developed mineral testing “kits” for the mineral prospector and mineral collector. These kits are relatively inexpensive and include enough materials for quite a few tests. A note about chemicals is in order here. In today’s world, most chemical supply houses will not sell chemicals to an individual not connected with an education institution or industry. This is particularly true for acids used in mineral analysis. Hydrochloric acid can be purchased from most building supply stores as muriatic acid for swimming pools. In most cases this is the only acid you will need, and then it should be in dilute form. A word of WARNING here; When diluting acid, always add acid to the water. NEVER add water to the acid. Adding water to acid will cause the water to react violently resulting in splashing and acid burns. You should use a solution that is 50% hydrochloric acid and 50% distilled water. Nitric acid and sulfuric acid are difficult to obtain as just a private citizen. If you can find a source for these acids, try to obtain them in dilute form. You will not need very much as there are only a few tests that require these two acids. Acids are dangerous chemicals and MUST be handled with great care. Wear protective clothing and gloves when working with acids or other chemicals. Make sure to wash your hands after performing tests in which chemicals are used.

When conducting wet chemical tests for determinative mineralogy, two approaches are commonly used. The first approach is to use a testing protocol that looks for the presence of one or more of the forty elements commonly found in minerals. In this approach, a series of 36 tests are performed and the results of each test recorded. The data is then collected and a determination of elements present is made. Typically, the tests must be conducted in a specific order because the results of each test are used in conducting the next test. It should be noted here that you do not need large amounts of specimen material for this testing protocol. Each test uses a tiny amount of specimen material. This is the approach used with some of the testing "kits" found in today's market place. The second approach is to use selective tests indicated by the physical characteristics of the mineral, initial blowpipe observations and its mineral associations. For our example we used the second approach. Since we have tentatively identified the mineral as chrysocolla, we will look for the presence of silica and copper with the wet chemical tests.

TEST FOR SILICA:
Since we know that chrysocolla is a Hydrated copper silicate (CuSiO3 – nH2O) a test for the presence of silica was performed: A small amount of the powdered mineral was mixed with 5 parts of sodium carbonate and roasted on the charcoal block. The fused mass was pulverized and ground to a powder. A small amount of this powder was placed in the bottom of a test tube to which 2 cc of dilute hydrochloric acid (HCl) was added. The solution was heated to just boiling and poured into an evaporation dish. As the solution evaporated, a bit of insoluble gelatinous material was left behind. The results of this test are characteristic for silica.

TEST FOR COPPER:
A small amount of the powdered mineral was placed in the bottom of a test tube and about 2 cc of dilute hydrochloric acid (HCl) was added. After a few minutes the solution was a pale green color. The addition of 6 cc of ammonia changes the color of the solution to blue. This is a positive test for copper. Since both tests are positive, we have confirmed the mineral identification as chrysocolla. The mineral specimen is a little harder than is typically the case for chrysocolla. This specimen is most probably tending more toward a copper- bearing Chalcedony than true chrysocolla. Examination of a number of individual specimens showed an apparent gradation from chalcedony to copper-bearing chalcedony to true chrysocolla. Not only did we positively identify the mineral species, we have had a lot of fun and enjoyment in the process. For the mineral collector, using the analytical techniques of old can provide another rewarding aspect to mineral collecting.

POSTSCRIPT:
This specimen is one of the few reported occurrences of the mineral chrysocolla in Gwinnett County, Georgia. The speculation is that the copper is derived from the dissolution of the chalcopyrite found in the host rocks. The silica most likely derives from dissolution of silicates in the host rock. Microscope examination shows tiny gypsum crystals on top of some of the Chrysocolla. Oxidation of both Pyrite and Chalcopyrite found in the formation may have provided source material for the gypsum crystals.

SOURCES OF MATERIALS:
Mineral Test Kit: A Field test kit is available for $122.50 from:
Delos Toole Gold Books 5564 Lloyd CT SE Salem, Oregon 97301
This is a fairly complete kit. However, I recommend purchasing separately a good brass blowpipe, charcoal blocks and platinum wire for bead tests, as those supplied in this kit are not very useful. Acids, hydrochloric, nitric and sulfuric and the base ammonia are listed as reagents but are not included in the kit. (Dropper bottles for these reagents are included however.) No information is given on the recommended strength of the reagents. Some chemicals are supplied in powder form to be reconstituted with distilled water. Instructions are provided for purchase of additional powdered chemicals. The kit also includes a copy of “Duke’s Short Course in Prospecting and Mineral Identification”. This is a multi-step testing protocol using the blowpipe and chemical tests. This qualitative testing protocol is well written; it emphasizes performing the tests under field conditions.

Mineral Testing Supplies: A good brass blowpipe, charcoal blocks, materials for borax bead tests, and other supplies, other than chemicals, can be purchased from:
Miners Incorporated 35 Pollock Road P.O. Box 1301 Riggins, Idaho 83530 – 1301
(ed: A book with a good discussion of these chemical tests is: A Field Guide to Rocks and Minerals by Frederick H. Pough.)