Introduction to Microscopical Geo-Sourcing: Part 1—Minerals

Preface by Richard E. Bisbing, McCrone Associates, Inc.

In the Locard tradition, forensic scientists most often use microscopic trace evidence as a means to connect victims and suspects with each other or the scene of the crime. The principal means to make that association is by direct comparison of microtraces on their clothing or objects at the crime scene with possible sources. For example, foreign fibers found on the victim’s clothing are compared directly with fibers comprising the suspect’s clothing, or paint chips on a screw driver are compared directly with paint from the window sill.

In a few cases, sourcing is attempted before suspects are identified and comparison samples obtained, most commonly in hit and run accidents where paint from the run-away vehicle remains at the scene of the accident. The paint is analyzed and compared with databases, often allowing the laboratory to predict the make, model and year of manufacture of the run-away vehicle. There have also been notable cases where carpet fibers were found on the victim’s clothing and, through cooperation with major car manufacturers, the carpet fibers were identified and determined to have been installed in a single model over just a couple of years. A model case using leaf litter appeared recently in the Journal of Forensic Sciences as a Letter to the Editor (Volume 48, Number 3, May 2003, page 696). Although, sometimes painstakingly difficult and time consuming, these cases illustrate the potential value of microscopical geo-sourcing.

John Delly will begin a short series on the principles and practices of microscopical geo-sourcing, starting with minerals. We invite our readers to contribute articles about other materials that can likewise be geo-sourced, such as pollen, seeds, sand, etc. As usual, we encourage the contributions to be accompanied with color images. We are not interested in news and views on the other geo-sourcing, that is, outsourcing IT services to some other part of the world. On the other hand, we would be interested in such projects as “Geo-Sourcing of Controlled Substances by High Field Deuterium NMR Spectroscopy and Stable Isotope Ratio Mass Spectrometry, ” or “Geo-Sourcing of Cocaine Using Isotopic Ratio Mass Spectrometry (IRMS) and Trace Alkaloid Analysis” as a means for the U.S. Drug Enforcement Administration to determine the geographical origin of controlled substances, (http://www.emsl.pnl.gov/new/highlights/030100.shtml; http://www.carleton.ca/~bhollebo/Chem2303/AlkaloidAnalysis.htm)

Part 1 – Minerals

Each microscopic dust particle bears clues as to its identity; its provenance; its history (which may be a time/temperature history in the case of combustion products); its use, if any; and, ultimately, its origin. All we have to do is know how to interpret what each particle is trying to tell us. Take, for example, the two most common mineral particles, quartz and calcite. The identifying physical and optical characteristics of both particle types are well known, and have been for hundreds of years. Look up either mineral name in, for example, Winchell and Winchell’s Microscopical Characters of Artificial Inorganic Solid Substances, and you will find all of the optical and physical data necessary to positively identify each. There will be specific information on the all- important refractive indices, birefringence, optic sign, crystal system, hardness, density, even the strongest lines in the powder x-ray diffraction spectrum. But this is only the beginning; knowing that the particles are quartz or calcite is only the tip of the iceberg. It is taken for granted that microscopic particles of this kind can be identified after only a week’s training in polarized-light microscopy, together with access to literature reference sources, such as The Particle Atlas, Winchell’s books, or almost any determinative optical mineralogy text. To go beyond this starting point, we have to look very carefully and minutely at the particle’s microscopical morphology, its surface characteristics, its internal features, and its association with other particle types (either adhering to its surface, intimately bonded with it, or loosely found with it). Let’s take the two common minerals, calcite and quartz, one at a time, and look at them more closely.

Calcite

Let us say that we start with a nice, well-formed rhombohedral crystal of calcite, about an inch long, of the type that is commonly found in mineral sets, or that may be purchased at any museum or rock shop. The crystals are found this way in many parts of the world; often they are quite large, and may be known by their locality, such as Iceland Spar. Hand specimens of, say, 1-5 inches have played an extremely important role in the history of optical crystallography, and in our understanding of crystals in general, because of the double refraction effects that are so commonly observed when we place one of these crystals on some printed matter and rotate the crystal. You will see two images of the printed matter. One of the two images will be seen to rotate about the other as the crystal is rotated. If you look at the two images through a polarizer, only one of the images will be seen; when the polarizer is rotated 90°, the other image will be seen. Thus, one can determine the path of light through the crystal and the polarization orientation of each of the two beams. These observations were made many years ago (Erasmus Bartholinus discovered double refraction in calcite in 1669), and led, almost 200 years ago, to the use of these crystals, cut in specific ways, as polarizer and analyzer in the first polarizing microscopes (William Nicol reported on his invention of a polarizing prism made from two calcite components in 1828; and William Henry Fox Talbot applied Nicol’s prism to a microscope in 1834; the first commercial polarizing microscopes were designed in 1838-1855). Microscopes outfitted in this way then became the means by which the optical properties of other crystals could be studied systematically.

If we take such a well-formed rhombohedron of calcite and crush it, say with a hammer or in a hammer mill, we will reduce the nice crystal to a powder. But if we now make a microscope slide mount of some of this powder, for example, by placing it in refractive index liquid 1.66 and covering with a coverglass, and then look at it at about 100X, we will observe among the crushed fragments a great many perfectly formed rhombohedra just like the large crystal was to begin with, but in miniature. This is because of cleavage, the tendency for crystals to split or break along certain specific planes. And there is no lower size limit to these well-formed rhombohedra, until we get down to the lattice unit cell itself.

Most of the other microscopic crystals will not be well-formed; there will be some flaw; some mechanically damaged end to an otherwise well-formed crystal. There will even be some that are mechanically damaged on all sides, so that they will no longer show any rhombohedral shape. They will all have sharp edges, however, because they have been freshly crushed, or “comminuted”. Pass such a pulverized sample through a nest of sieves so as to isolate the 100 mesh portion – about 75 µm – and this is what you have in the Cargille Reference Set of Minerals. This is the sample that was used for the second edition of The Particle Atlas (#133), and is being used in the new, online Atlas of Microscopic Particles. Another thing about calcite is that the microscopic particles so often show evidence of parallel lamellar twinning: between crossed polars find well-formed crystals, and you will often see a series of parallel lines running through the crystals. This is evidence of twinning. It seems like no matter how many times you hit a crystal of calcite with a hammer, there are always some beautiful, tiny, perfect rhombohedral particles – miniatures of the parent crystal. Finding sharp-edged, highly birefringent (0.18) particles, with some or many showing evidence of well-formed rhombohedral faces is a strong indication of comminuted calcite. To nail it down, rotate the particles in plane-polarized light, while determining the Becke line relative refractive index using monochromatic light of 589 nm, i.e., sodium light. One of the refractive indices of calcite is 1.658, which is close enough to the 1.66 of the mounting medium liquid that the particle will almost disappear. On rotating the stage 90° from where the particle disappears, the refractive index is much lower – 1.48 – and the relief will be strong.

The refractive index determination is very important for this reason: calcite, a calcium carbonate, was originally formed from solution in which calcium ions and carbonate ions were present. If, instead of calcium ions, magnesium ions were present, the compound formed would be magnesite, and would have different optical properties (ω = 1.700; ε = 1.509) than calcite (ω = 1.658; ε = 1.486). If both calcium and magnesium ions are present, still a third compound may be formed, dolomite, with optical properties somewhere between calcite and magnesite (ω = 1.679; ε = 1.502); calcite and magnesite are not, however, end members of a solid solution series.

Limestone is a kind of compressed mud made up of mixed carbonates, all of which have high birefringence and different refractive indices. Well-formed calcite crystals are not found in limestone. There are many limestone quarries in the world. The limestone is quarried for use as large building stones, large and small gravel, and for the making of various mortars and cements. These quarries are often abandoned; some are deliberately flooded so as to create artificial lakes for SCUBA diving or recreational swimming. When this happens, the limestone goes into solution, and the water becomes “hard.” There are many forms of plant and animal life that tolerate hard water. The submersed plant Chara is one of these. The common name for Chara is “Stonewort,” so called because if you rub the stems and leaves between your fingers, it feels gritty, like tiny sharp-edged stones being rubbed together. Fresh Chara is a remarkably beautiful plant when viewed microscopically. The plant utilizes the water with its calcium ions and carbonate ions, and recrystallizes the calcium carbonate within its cell walls. Each cell of the plant contains many crystals of many sizes. Usually, there are so many crystals within the cell, that the cell wall itself is distorted from the overgrown crystals and the stems or leaves microscopically are not smooth, and may be quite sharp to the touch. The plant must be looked at freshly prepared and mounted in its own hard water to see this; conventional preparations that utilize acids of one kind or another will destroy these lovely crystals. Chara is especially beautiful when viewed between crossed polarizers. When the plant dies, the organic portions disintegrate, and the crystals slowly settle to the bottom of the quarry, where, over time, they accumulate to form a calcareous bottom ooze. A portion of the crystals go back into solution, and the process starts over again.

In the oceans, many life forms utilize the dissolved calcium, magnesium, strontium, and other ions, together with silica and the dissolved carbon dioxide to help in the making of their shells. Sometimes, they are primarily siliceous, such as diatoms; sometimes calcareous, such as foraminiferans, sponge spicules, holothurian plates, and coral. Sometimes, the calcium combines differently so as to form aragonite instead of calcite, as in the clam’s shell. When these various plants and animals die, their carbonate or siliceous remains settle to the bottom of the body of water they are in to form a benthic ooze. Over geologic time, these mixed calcareous and siliceous remains become compressed, altered, and may, eventually, be heaved up into land masses, such as the White Cliffs of Dover, or the diatomite (diatomaceous earth) deposits of Lompoc, California. When one looks microscopically at a sample of the chalk from a calcareous deposit, the carbonate particles do not have the well-formed faces of a freshly-comminuted calcite, but, rather, look like agglomerates of tiny carbonate beads. They may not show extinction on rotation of the stage, and may appear to have a distorted black cross within each tiny microcrystal of the agglomerate. There will be no sharp edges, and the agglomerates will be more or less rounded. Foraminiferans may be found. Some recent foraminiferans may show, orthoscopically, a black cross within concentric colored rings, resembling a uniaxial interference figure, indicating the radiating orientation of their fundamental acicular crystallites very much like spherulites; but the foraminifera that have been compressed in the chalk are more likely to be jammed full of the randomly-oriented tiny, rounded calcareous particles. We call biomineralized calcareous particles like this “biological calcite” to distinguish it from the morphologically different comminuted mineral calcite.

The appearance of this “biological calcite” is closely approximated by freshly precipitated calcium carbonate. Make up a calcium chloride solution and blow bubbles in it using a straw. The solution will become cloudy with precipitated calcium carbonate (the carbon dioxide coming from your exhaled breath). You can also simply add to the calcium chloride solution a solution of soluble carbonate, say sodium carbonate, washing soda, and get the same effect – but it isn’t nearly so much fun as blowing bubbles with a straw! However the precipitated calcium carbonate is formed, look at it with the polarizing microscope. Again, no beautiful, well-formed rhombohedral crystals, as in the comminuted calcite crystal, but tiny, highly birefringent, generally rounded particles and agglomerates. This kind of calcium carbonate is used as the abrasive in many toothpastes. Look at some toothpaste microscopically to see this; you may also find crystals of flavoring agents. While you are at it, take a look at “Sensodyne™,” the toothpaste for people with sensitive teeth. Here, you will have a surprise because the abrasive consists of diatoms – the siliceous frustules of single-celled aquatic plants, and it is these critters that you brush against your teeth to clean them.

The precipitated forms of calcium carbonate are used for many products, either alone or mixed: paper coatings, paint fillers, fillers in plastics and rubber, pharmaceutical enclosures, mortars and cements of various kinds, automobile and other polishes. Look in the Kirk-Othmer Encyclopedia of Chemical Technology for hundreds of other uses.

Carbonates are of marine origin, so does finding them indicate that they came from an ocean? Yes, but not necessarily in the present. The Midwestern part of the United States is largely carbonates, and at one time, in the geologic past, there was an ocean present, but not now. Then how do you know if a carbonate is from a modern ocean area or not? Look at any calcareous samples from any present ocean area. You will find the rounded agglomerates of “biological calcite,” but you will also find tiny salt particles – these may not be perfect cubes of sodium chloride, though these are found as well, especially from sea spray; they are usually isotropic fragments of irregular outline. Fragments of algae of various kinds are also always found, along with rather larger pieces of well-rounded fragments of various shells. Everything will tend to be rounded because of the polishing action of water attrition. All of the non-calcareous particles and especially quartz will also be rounded from the never-ending motion of the waves and the gentle polishing action of particle against particle with water as the polishing lubricant.

Calcium carbonate that has been calcined, or heated to a very high temperature, as for making cement also has the appearance of agglomerates of tiny rounded particles with no extinction and irregular black crosses. But, of course, in this instance, because of the high heat involved, there will be nothing combustible (e.g., algae fragments) left.

One could go on and on about the various forms of calcium carbonate, but at this point it would be better to look microscopically at knowns. Survey The Particle Atlas and spend some time looking at pink eraser dust, toothpastes, freshly crushed calcite crystal, chalk dust, beach sands from ocean beaches, etc., concentrating only on the calcite and the various other forms of calcium carbonate.

One might think that because calcite is so abundant (it comprises about 4% by weight of the Earth’s crust), and because it is formed in so many different geological environments, that it would not be useful in geo-sourcing. However, calcite is highly varied, in addition to being widely distributed. There are more than 300 crystal forms of calcite that combine to produce thousands of different crystal variations – the famous nine-volume Atlas der Krystallformen (1913) by Victor Goldschmidt contains 2544 drawings of calcite alone! Calcite also produces many twin varieties, phantoms, color varieties, included crystals, pseudomorphs, and unique associations. There are numerous minerals associated with calcite, including these classics: quartz, fluorite, galena, sphalerite, barite, celestite, sulfur, copper, gold, emerald, apatite, biotite, and zeolites; also, several metal sulfides, other than galena and sphalerite; other carbonates, borates, and numerous other minerals.

Then, too, calcite from different locations may be fluorescent, phosphorescent, thermoluminescent (luminescence due to friction), or triboluminescent (phosphorescence developed in a previously excited substance following gentle heating). The variations are endless, and in a lifetime you will not see them all. For geo-sourcing purposes it is of the utmost importance to study and recognize calcite’s variations and associations.

Quartz

Now let’s consider quartz in a similar way. Obtain a quartz crystal. These are very easy to find at any museum store, rock and mineral shop, or any mall where crystal jewelry is sold. Quartz is in the hexagonal crystal system, and you will find it easy to locate a small well-formed crystal, with a pyramidal termination. Again, crush it by hitting with a hammer, preferably in a hammer mill. Optionally, sieve it (100 mesh fraction) so as to isolate the ~75 µm portion. This is how the sample of quartz in the Cargille Mineral Reference Set is made. See Particle Atlas (second edition) sample #183, which is identical to what you will see when you crush your quartz crystal and mount it in refractive index liquid 1.66, or the online Atlas of Microscopic Particles.

Unlike the calcite, when you look at the crushed particles of quartz, you will not see well-formed miniature hexagonal crystals with pyramidal terminations like the crystals you started with, because quartz does not have the same tendency to cleave along certain planes the same way that calcite does. Quartz fractures like glass, and, indeed, by ordinary light the crushed fragments of quartz look exactly like crushed glass; the particles have sharp, irregular edges and outline, and even show conchoidal fracture (a series of concentric curved lines) just like glass. Quartz, however, is a hexagonal crystal with two refractive indices, so that it is immediately distinguished from glass by viewing between crossed polarizers. Glass would be invisible, while the quartz will appear brightly lighted showing brilliant first, second, and some third order colors in the Newtonian series.

As with the calcite, quartz has specific optical and physical properties that distinguish it from other minerals. Its refractive indices are ω = 1.553 and ε = 1.544. The birefringence is low 0.009. It would be instructive at this point to compare quartz to white beryl. The morphology is identical. The refractive indices and birefringence are similar, but on obtaining the interference figure of a particle showing a very low retardation color on complete rotation, with virtually no extinction, the quartz will be seen to be optically positive, while the beryl shows a negative sign of double refraction (optic sign).

Again, the identification of quartz to this point is assumed, and should be able to be done after a one-week polarized-light microscopy (PLM) course or a microscopical Mineral Identification course. Look at a quartz sample and compare your observations to the description in printed or CD-ROM versions of The Particle Atlas (#183), or online in the Atlas of Microscopic Particles. But now again, go back to the same quartz sample, say from the Cargille set of minerals, rotate to your 40X-60X objective and now take all the time in the world to look inside the quartz particles. Be patient, and observe very carefully. Probably the first thing you will notice is that there are inclusions in the particles. Most are black, but these are not opaque particles – although they may be – they are generally rounded, or oval, or drawn out. They contain entrapped gases, which have very low refractive index compared to the quartz, so appear dark. But keep looking, and eventually you may find inclusions which contain liquids! You will recognize these, because where the entrapped liquids contain a gas bubble, the bubble will be seen “dancing.” You will see a tiny shivering bubble. The shivering of the bubble will vary with the liquid volume, the gas bubble volume, and the amount of heating that takes place from your light source. These are fascinating things, and truly awe-inspiring, when you realize that the little shimmering bubbles have been entombed in their glassy matrix since the time of the original formation of the quartz. They are time machines that allow you to look back millions of years. And yes, both the gases and liquids have been analyzed: one way is to grind and polish the matrix quartz until there is only a thin window to the gas and liquid. Then the particle is placed in an ion microprobe, and the window is sputtered away, while the mass spec is turned on. Immediately the window is broken through, the spectrum changes and the contents are analyzed. The only comparable sensation of microscopical time travel is viewing insects entrapped in amber, or looking at a section of petrified wood, or fossil pollen and spores, or ice cores. The difference being that with the quartz inclusion, there is movement. It is all very exciting.

There may be crystalline inclusions as well as entrapped gases and liquids. Beautiful crystals of rutile are common in some quartz samples. This would be a good time to take a look at Gϋblein’s book on Inclusions in Gem Stones, or the later edition with Koivula. No gemstone is absolutely flawless. All have some imperfection, usually inclusions of various kinds, and photomicrographs of these inclusions in valuable gemstones act in the same way fingerprints do to individualize such gems for insurance and other purposes. Gϋblein’s extensive collection of gemstone inclusions demonstrates the extent to which inclusions occur. For our present discussion, notice the vast number of inclusions found in quartz minerals. And if you are ever in Chicago, go to the Hall of Minerals at the Field Museum and the Hall of Gems to see numerous examples of mineral inclusions in quality gems, or to the Smithsonian in Washington, D.C., or to any other curated mineral collection anywhere in the world.

Now back to our sample. You may notice that there is a pattern to the distribution of the inclusions. The bubbles may be in a row, or elongated in a particular direction; or, in the case of crystalline inclusions, the crystals may be all arranged parallel to one another; maybe not; they may form small groups; they may be random.

But what does it mean? It means that “quartz” is not quartz. There are quartz particles with parallel, acicular rutile inclusions, and quartz with no rutile, but oriented gas bubbles, and quartz with neither, or other. Quartz from different locations has different internal characteristics, and these internal characteristics are vital in any attempt to geo-source a sample.

And it isn’t just the internal features, but the external features. There are many of these. Looking at the freshly comminuted quartz, the particles all show smooth, clean fracture surfaces; sharp, irregular edges and outline; possibly conchoidal fracture. But suppose we now throw our comminuted quartz sample into the nearest river, creek, lake, or ocean, and come back to look at it over long time intervals. How will it be different? Internally, it will remain the same, but externally there will be many changes. The most obvious change will be in the particle morphology. The particles will be rounded. How rounded? It depends on how fast the water is moving, the chemical components in the water, and the length of time of exposure. As the comminuted particles move against and over one another because of current, wave action, or movement from fish or bottom dwellers, the sharp edges and corners will be knocked off and the particles will start to round off. The rounding will be smooth because water is a good lubricant. There will be no deep scratches or gouges; only gentle smoothing. The longer the exposure to the tumbling action, the smoother and rounder the particles become; this is called “water-worn” or “water attrition.”

But now look at the surface of the grain, and concentrate only on that – not on the overall morphology; not on the inclusions. Optimize the aperture diaphragm and the magnification (50X-80X objective) to elucidate the surface of the grain. There are several things you may see, one of which is etching. Depending on the chemical characteristics of the water, you will see different kinds of surface etching. Notice the pattern of the etching, and the extent, i.e., the depth and coarseness of the etch boundaries. Are they crenulated and reminiscent of animal fiber scale margins? Do they show suggestions of a triangular etch pattern? Is the etching present or absent? If present, describe the extent and pattern.

Often with or instead of etching, a surface deposit will be observed. Usually, this surface deposit will be in the form of one of the iron oxides. Never a hematite if recovered under water, but always one of the hydrated forms of iron oxide – limonite, goethite, or lepidocrocite (look these up in The Particle Atlas). Sometimes, these usually yellow to yellow-orange particles are submicrometer in size, rounded, and in agglomerates; sometimes, they are in the form of tiny, stacked flakes. The iron oxide coating may be extensive, moderate, slight, spotty, or absent. The kind and nature of the coating should be observed and recorded. Clays are another particle type that coat quartz grains. These will be submicrometer and may give a color cast to the grains by ordinary transmitted light.

With luck, diatoms will be found adhering to the grains. There are over 70,000 species of these “golden algae,” each requiring specific pH, oxygen levels, light levels, nutrients, etc., so that they are quite specific to locale, and have been fairly well documented. They make excellent geo-sourcing indicators.

Now let’s go back to our freshly comminuted sample of quartz again, and this time, instead of placing it in running or standing water, let us put it in one of the world’s deserts or dunes. How will it be different from the water sample? Well, in the first place, the wind will blow the particles about, and so when our particles hit other grains on one of their sharp edges, the edge will be chipped – perhaps completely off, and the particle will become rounded, just as it did in moving water. The difference is that the fluid lubricant now is not water, but air. Air is a terrible lubricant, so the grains, when they strike one another, will leave scratches and gouges that are not smoothly polished, but coarsely rounded, and rough polished with surface scratches. This is called “wind worn” or “wind attrition”. Compare the water-worn sand and the wind-worn sand in The Particle Atlas or the Cargille Mineral Set.

Now let us cast some metal, say for an automobile part. To cast the metal into a particular shape, a mold is needed. The mold needs to be made of a material that will withstand molten metal. Sand (silicon dioxide) has a melting point well above common molten metals. How do you get the sand grains to hold the shape of the part? You coat them with a clay or resin and mold directly, or bake the mold to harden the resin. Stop! Take the mold apart and look at the grains. Each grain will have a coating of clay or resin. The resin will appear transparent to translucent and will have a yellow to amber to brown color. Compare the unused molding sand grains in The Particle Atlas. Now put the mold back together again, and pour your molten metal. Wait for it to cool, break the mold, remove your cast metal part, and now look at the quartz grains again microscopically. They will now have bits of metal adhering to them, and the coating, if resinous, will be charred. Look at the used molding sand in The Particle Atlas.

Molding aluminum requires a different sand grain size than molding iron, so particle size and morphology are important when identifying sands intended for molding. Some quartz sand deposits already have clays on their surfaces (remember I told you to watch for those surface deposits!), and so not all sand deposits are created equal when it comes to molding purposes, and there is much rivalry in this industry for sands from specific areas. This is a specialty in itself.

Sand is not necessarily quartz, and rarely is it one, single substance. Some of the world’s sands are rutile, as in parts of Florida and, especially, Australia; some sands are gypsum, as in White Sands, New Mexico; then there are the carbonate sands around Salt Lake City; some sands are not mineral at all, as the radiolaria that form the beaches in the Barbados; some sands contain beautiful pieces of pink coral, as from Bermuda; the “green” and “black” sands of Hawaii are quite distinctive, and neither is quartz. Of the sands that do contain quartz, there are always differences in the characteristics of the quartz or the other minerals present, so that it is possible to geo-source a particular sand. For this, there is no substitute for experience and a reference collection. Fortunately, there are two international sand collectors’ organizations whose members collect, sell, and exchange sands from around the world. Again, there is no substitute for actually looking at prepared slide mounts of sands from these various localities.

There is a researcher in France who claims to be able to look at a quartz sand grain, and tell where in the world it came from. I believe that this can be done, but that it is experience dependent. I am reminded here of Siever’s book on Sand which should be read. And by all means see William C. Mahaney’s Atlas of Sand Grain Surface Textures and Applications (Oxford, 2002).

One could go on for a very long time on these two minerals alone, but I think this is sufficient to make the point that the microscopist needs to go far beyond the initial identification of a mineral grain to gain the additional information necessary for geo-sourcing.

Editor’s Note: Part 2 of this series on Combustion Products will appear in the next quarter.