June/July 2001

For the June meeting (Saturday, June 16, 11:00 a.m.), please bring in material collected during spring field trips for shared viewing (or other material you have been working on).

At the last meeting we discussed several topics for the next Conference. The two front runners appear to be Arizona Minerals and Southern California Minerals.

Please give some thought as to potential speakers for either of these themes (or feel free to suggest others). If you cannot attend the June meeting, Email me the names of potential speakers. Paul.M.Adams@aero.org

For this meeting bring your lunch and scopes, any of the above mentioned minerals, or even new acquisitions from interesting sites you have visited. Doors open at 11:00 AM. Just a reminder - the building in which we meet is not heated (or air conditioned?).

Directions: From the 60 Freeway (east), exit on Pyrite Avenue, go north under the freeway to the first signal, turn right, and continue east on Granite Hill to the Center.

In the last issue, we said we would let you know about changes for the Roster. Since that time, we have had the following addition:

Please keep us posted if you change your address or phone so that we can keep our members informed.

We have had no response to our request for e-mail addresses that may not have been included on the new 2001 Roster. Thus, please consider the list complete. If you have an addition, please advise your editor at bcmoreau@4dnet.com.

Following is the web site for SCMM:

http://www.mineralsocal.org

Then click on Southern California Micromineral-ogists in the upper right hand corner.

We will be posting the MicroBits on this site as well as the MSSC Bulletin, and perhaps the So. California California Friends of Mineralogy bulletin.

Once again, we are reporting that Juanita Curtis is still in need of our prayers and support. She is awaiting further surgery for the removal of a skin cancer, but the date has not yet been set. She will see a new surgeon just about the time this issue goes to press. Drop her a card or note to cheer her up. (See your membership list in in the April-May issue for her address.)

Kay Hansen has made some progress with healing following her illness. She has returned to work, but has had to curtail some of her activities along the way. She, too, could use a card or note.

Your communication works wonders when these folks realize we’re out there rooting for their return to good health.

Let your Editor know if there are others who need a bit of Sunshine headed their way.

Member Jack Nelson complained to Jennie Smith a few months ago that he had been scratching the lenses of his glasses on the eyepieces of his microscope. Here was her “tip”:

Take a medical rubber glove and cut two one-inch long tips off the fingers. Then cut the tiniest bit off the tipof each of them. Pull them, big end first, over the eyepieces. The tiny hole you cut in the tip will expand enough to expose the lens, and leave you with two rubber-covered eyepieces that will not scratch your glasses!

In last month’s issue Jack had a short note about the accused spy Robert Hanssen, and his using our monthly meeting place, code named LEWIS, as a drop for espionage materials and their rewards, which included cash and DIAMONDS. I only note that it was about this same time frame that Jack was distributing DIAMONDS as bonuses for Patron or Sponsor memberships. Jack, did you find those diamonds behind the trash can at the Long Branch Park?

Be sure to say “Thanks” to Dad

ELECTRON BACKSCATTER DIFFRACTION IN THE SCANNING ELECTRON MICROSCOPE: A NEW TOOL IN MINERALOGY?

By Paul M. Adams

Nickel (1995) gives the definition of a mineral as: “An element or chemical compound that is normally crystalline and which has formed as a result of a geological process”. The words “crystalline” and “chemical compound” have been bolded in order to highlight the fact that the crystal structure and chemical composition of a new mineral must both be accurately determined (along with certain physical properties) before it can accepted as such. A common response whenever an unknown is encountered is “We need to get that X-rayed in order to identify what it is!”. I think there is a little confusion over what actually happens when a sample “gets X-rayed” because several different techniques involving X rays can be used to determine either the chemical composition or the crystal structure.

X-ray diffraction (XRD) techniques can be used to determine the crystal structure of a mineral (including the locations of atoms in the lattice) and/or “identify” the mineral if the crystal structure is already known. Single crystals are usually required for the former, and powders are generally used for the later (powder XRD). Recently sophisticated indexing and modeling software (ab initio) have been applied to deducing the single crystal structure from powder XRD patterns, and there are special XRD cameras that can generate powder XRD patterns from single crystals without grinding them in to powder. Only crystalline materials diffract X rays, which results from the scattering of the X rays at discrete angles because the interatomic spacing between lattice planes in the crystal is similar to the wavelength of the X rays. The problem with using XRD alone to identify minerals is that some substances with different chemical compositions have similar or nearly identical crystal structures, and therefore, nearly identical XRD patterns. For example, XRD can easily distinguish an amphibole from a pyroxene, feldspar or mica, but being able to distinguish certain pyroxene species from each other, based on the XRD pattern, can often be difficult – to perilous – to impossible. Therefore, one should be suspicious of identifications based on XRD alone if the mineral belongs to a group of isostructural species or can form a solid solution (e.g., olivines: fayalite-forsterite).

In theory (Vegard’s Law), and sometimes in practice, one can determine the composition of a substance which forms a binary solid solution if you use XRD to measure the lattice parameters, of the pure end members and the “unknown” intermediate, with sufficient accuracy and precision (i.e. lattice parameter refinement). This is because the lattice parameters are usually a linear function of composition. The problem with applying this to mineral identification is that (1) nobody goes to the trouble of performing lattice parameter refinement as part of routine identifications, (2) the lattice parameters of minerals in the published XRD data base have usually not been refined and they are not from pure end members, and (3) many minerals are more complicated than simple binary systems (i.e., there can be more than two elements substituting in the structure and changing the lattice parameters each in different ways).

X-ray fluorescence (XRF) techniques are often used to determine the qualitative (approximate) and/or quantitative (exact) chemical composition of minerals. When an inner shell electron in an atom is ejected from the atom, an outer shell electron can drop down to take its place and X rays of a characteristic wavelength and energy, for that element and electron shell transition, are produced in the process. X-rays or high-energy (kilovolt) electrons can be used to eject electrons from atoms, thereby producing the characteristic X rays. Wavelength dispersive (discriminating) and energy dispersive XRF techniques have been developed for chemical analysis. Wavelength dispersive XRF tends to be more time consuming but usually is the method of choice when good quantitative analyses are required. The electron microprobe is a wavelength dispersive XRF technique. Energy dispersive X-ray spectroscopy (EDXS), when coupled with a scanning electron microscope (SEM), is a quick way of obtaining qualitative chemical analyses from relative small volumes (1 micron) and, unlike wavelength dispersive techniques, it analyzes for all detectable elements simultaneously, rather than one at a time. EDXS in the SEM is less quantitative than the electron microprobe, especially for oxides - which include most minerals. It also has limitations in not being able to detect hydrogen, lithium or beryllium, and many EDXS systems can not detect boron, carbon, nitrogen, oxygen or fluorine. As a result, the EDX spectrum of a carbonate, nitrate, borate or hydrated mineral will look nearly identical with that of a simple anhydrous oxide, and since true quantitation is not performed there is no easy way to tell the difference. Therefore, one should be a little bit suspicious about minerals that have been "identified" by EDXS alone, especially if hydrated or carbonate (etc.) species containing the same identification elements exist. Ideally, one should perform both XRD and EDXS analyses on a sample to obtain a true identification, but often both pieces of equipment are not available. There are also limitations to the size of samples that can be analyzed by XRD. EDXS can easily analyze areas as small as one micron in situ, but the practical size limit for XRD is about 50 microns, and this actually requires removing and mounting the 50 micron particle to a glass fiber.

In the last five to ten years there have been great advances in the field of electron backscatter diffraction (EBSD) which is performed in the SEM. Selected area electron diffraction (SAED) performed in the transmission electron microscope (TEM) has been commonplace for many years, but sample preparation for TEM analysis is very time consuming and tedious (samples must be less than 3 mm in diameter in order to fit in the TEM and be electron transparent; i.e., less that 0.1 micron thick). Furthermore, the identification of unknown materials from their SAED patterns alone is very difficult to impossible if the crystallite size is relatively "large" (greater than 0.5 micron!!). EBSD in the SEM has the advantage of being potentially able to identify materials with little to no sample preparation. The main requirements for successful EBSD analysis are that the sample be conductive and that the surface be extremely clean and free from damage. This is because the EBSD pattern is formed from the outer 0.05 micron of the surface of a single crystal. Nonconductive materials, such as minerals, can be made conductive by coating with an extremely thin layer of carbon or a metal such as chromium or platinum. The area that can be analyzed by EBSD can range from one micron, for a conventional SEM, with a tungsten filament, to approximately 0.1 micron with a field emission SEM. The sample must be a single crystal on that scale, or larger, in order to form an EBSD pattern. Figure 1 shows schematically how the EBSD pattern is formed. The sample, or face to be analyzed, must be inclined steeply (20^o) to the electron beam for optimal results. Electrons that penetrate just beneath the surface are scattered back out of the crystal along crystal lattice planes and produce bands (Kikuchi bands) that correspond to each family of planes. The widths of the bands are inversely proportional to the spacing of the lattice planes (d-spacings) and they can be recorded on film or imaged on a phosphor screen. The symmetry elements/operations that are present in the crystal lattice are represented in the EBSD pattern (Fig. 2) and attempts have been made to determine the crystal space group from EBSD patterns (Dingley and Baba-Kishi, 1986). Most of the recent advances in EBSD analysis have been related to the development of very sensitive low light video, or slow speed charge coupled device (CCD) cameras that can be used to view and record the image of the EBSD pattern on the phosphor screen. With the camera coupled directly to a computer the EBSD pattern can be readily analyzed. At present true identifications of unknowns based just on the EBSD pattern is not practical since the accuracy and precision of lattice spacing measurements are not very good (compared with XRD). However, when EBSD is combined with EDXS analysis, "identification" of unknowns becomes possible. The chemical information obtained from EDXS is used to restrict the number of possible compounds to less than about 10-20. Reference crystallographic data from those compounds are then compared with the information in the EBSD pattern. This generally takes the form of creating look-up- tables (LUT) of the angles between possible lattice planes in a known crystal structure, which is then compared with the sets of angles measured between the bands in the EBSD pattern of the unknown, and a figure of merit is generated. For a "good" identification one hopes that the figure of merit for one candidate compound will be very high while those of the others are much lower. A simulated EBSD pattern for the identified compound is then superimposed on the observed pattern, which is then scrutinized for discrepancies between the angles formed by intersecting bands and their widths. An example of an EBSD pattern obtained from a small plattnerite crystal along with a simulated pattern overlay is shown in Figure 3. Another example of patterns obtained from precipitates in steel is shown in Figure 4. There are some limitations to phase identification by EBSD because of its relatively poor sensitivity to differences in lattice parameters (± 5%). As a result, EBSD cannot tell the difference between a crystal structure that is slightly tetragonal vs. one that is cubic, or one that is slightly orthorhombic vs. one that is tetragonal, etc.

For those who take delight in pursuing the trivial and esoteric, this technique potentially has the ability to identify/index all the faces on a complex crystal. The main draw back in actually doing this is that each crystal face must be oriented to the electron beam (20^o) so that an EBSD pattern can be recorded. The orientation of the normal to that face, with respect to the beam and phosphor screen, must also be determined exactly so that the zone axis identified in the EBSD pattern can be correctly correlated with the crystal face. Stereogrammetric methods have been developed for doing this but they are relatively time-consuming (Randle, 1998).

Since EDXS chemical data is combined with structural information from EBSD patterns, the resulting identification is more robust than could be obtained by EDXS or XRD alone. Both EDXS and EBSD can be performed on very small areas (1 micron) in the SEM, and as a result, they potentially could greatly benefit the identification of very small mineral crystals. The main limitation at this time is the cost of EBSD systems and software ($80-100K) since EBSD systems can easily be added onto older existing SEMs.

Figure 2. EBSD pattern of silicon (Si) with symmetry elements identified [2-,3-, 4-fold
rotation axes and mirror-planes (m)] (right). Figure reproduced from Michael (2000).

Figure 3. (left) Observed EBSD pattern and (right) same pattern indexed with structure of plattnerite
(lead oxide -PbO₂). Figures reproduced from Michael (2000).

Figure 4. (a) SEM photograph of a polished metal surface, (b-c) EBSD pattern of globular particles and pattern indexed as hexagonal iron niobium (Fe₂Nb), (d-e) EBSD pattern from needle-like particles and pattern indexed as hexagonal nickel titanium (Ni₃Ti). Figures reproduced from Michael (2000).

1. D. Dingley and K. Baba-Kishi, “Use of electron backscatter diffraction patterns for determination of crystal symmetry elements”, Scanning Electron Microscopy II, 383-391, (1986).

2. V. Randle, “Crystallographic analysis of facets using electron backscatter diffraction”, Journal of Microscopy, 195, 226-232, (1998).

3. J. Michael, “Phase identification using electron backscatter diffraction in the scanning electron microscope”, In Electron Backscatter Diffraction in Materials Science, A. Schwartz, M. Kumar and B. Adams (ed.), Kluwer Academic, New York, 75-89, (2000).

Safety - Sun Exposure

Chuck McKie, CFMS Safety Chair 2001

Q - Under what types of weather conditions do people need to worry about sun exposure?
A: Any time the sun's ultraviolet (UV) rays are able to reach the earth, there exists the risk for excessive sun exposure. Such ultraviolet rays, of course, are present on bright and sunny days. UV rays also actually can penetrate through cloud and haze cover, however, making cloudy and overcast days dangerous as well. Moreover, UV rays reflect off water, cement, sand, and snow. As a result, UV rays can even cause damage in the winter when there is snow on the ground. Relatively speaking, the hours between 10 AM and 4 PM daylight savings time (9 AM to 3 PM standard time) are the most hazardous for UV exposure in the continental United States. UV radiation also is the greatest during the late spring and early summer in North America.

Protection from UV rays, nonetheless, is important all year round, not just during the summer or at the beach.

Q - What are "ultraviolet rays"?
A - The ultraviolet (UV) portion of sunlight is an invisible form of radiation that can penetrate and change the structure of skin cells. Exposure to UV rays has been associated with the development of serious diseases. In fact, UV exposure appears to be the most important environmental factor in the development of skin cancer. UV rays also have been found to be associated with various forms of eye damage, such as cataracts.

More specifically, there are three types of UV rays: ultraviolet A (UVA), ultraviolet B (UVB), and ultraviolet C (UVC). UVA is the most abundant source of solar radiation at the earth's surface, and penetrates beyond the top layer of human skin. Scientists now believe that UVA radiation can cause damage to connective tissue and increase a person's risk of developing skin cancer. UVB is less abundant at the earth's surface than UVA because a significant portion of UVB is absorbed by the ozone layer. UVB does not penetrate as deep into the skin as UVA does, but, nonetheless, can also be damaging and has been associated with the development of skin cancer. UVC radiation is extremely hazardous to skin, but it is completely absorbed by the stratospheric ozone layer and does not reach the surface of the earth.

Q - How can people protect themselves from the sun's UV rays?
A - There are a number of ways you can protect yourself: When possible, avoid outdoor activities during the hours between 10 AM and 4 PM, when the sun's rays are the strongest. As appropriate, wear protective clothing, such as a broad-brimmed hat, long-sleeved shirt, and long pants. Wear sunglasses that provide 100% UV ray protection. Always wear a broad-spectrum (protection against both UVA and UVB) sunscreen with a Sun Protection Factor (SPF) of 15 or higher. Remember to reapply as indicated by the manufacturer's directions.

Q -: What can excessive exposure to these UV rays do to one's skin?
A - While there are a few positive benefits of getting some sun exposure, excessive exposure to the sun can result in premature aging and undesirable changes in skin texture. Such exposure also has been associated with various types of skin cancer, including one of the most serious and deadly forms of such cancer (a form known as melanoma).

Q - What does a suntan indicate? Why does the skin tan when exposed to the sun?
A -: While some people believe otherwise, a suntan by itself is not an indicator of good health. In fact, scientists say that the sun exposure one gets while tanning actually can cause damage to the skin. Some physicians consider tanning a response to injury because it appears after the sun's UV rays have killed some cells on contact and damaged others. It is the penetration of those UV rays to skin's inner layer, which results in the production of more melanin. That melanin eventually moves toward the outer layers of the skin and becomes visible as a tan.

Q - Not everyone burns or tans in the same manner. Are there ways for classifying different skin types?
A - Whether individuals burn or tan depends on a number of factors, including their skin type, the time of year, and the amount of sun exposure they have received recently. The skin's susceptibility to burning can be classified on a five-point scale as outlined in the following table:

Skin Type Sunburn and tanning history
I - Always burns, never tans, sensitive to exposure
II – Burns easily, tans minimally
III - Burns moderately, tans gradually to light brown
IV - Burns minimally, always tans well to moderately brown
V - Rarely burns, tans profusely to dark
VI - Never burns, deeply pigmented, least sensitive

Though everyone is at risk for damage as a result of excessive sun exposure, people with skin types I and II are at the highest risk.

President’s Message

Be sure to say “Thanks” to Dad

By Paul M. Adams

Safety - Sun Exposure

Chuck McKie, CFMS Safety Chair 2001