APPLIED TO ALKALI SYENITE AND CARBONATITE

By: Henry L. Barwood

ABSTRACT

Luminescence excited by electron bombardment, or cathodoluminescence (CL), is a very useful tool in petrographic examination of rocks, minerals and ores. Differences in luminescence colors and intensities allow detection and delineation of mineral species and composition zones difficult to separate visually by normal polarized light microscopy. The cold cathode CL chamber at Indiana University is used with a binocular microscope that has interchangeable attachments for a normal film camera, a digital camera and a fiber optic spectrometer. This allows imaging of the specimens as well as limited spectrographic analysis of the emitted luminescence. The chamber on this CL unit is large enough to accommodate several thin sections, polished mounts, grain mounts or rock slices at one time.

Luminescence petrographic techniques were used to study alkali syenites and carbonatites from the Arkansas. Alkali feldspars in the syenites reveal a history of growth, fenitization and replacement through color and intensity differences. Primary orthoclase luminesces bright blue and is often zoned with a duller blue overgrowth of microcline. Fenitization of the feldspars is demonstrated by dull to bright red luminescence and replacement veins of albite often have a bright yellow green luminescence. Late microcline and perthites from the syenite pegmatites have a dull yellow to orange luminescence. Easily visible on this background of feldspars are grains of apatite (yellow), fluorite (violet), zircon (greenish), sodalite (orange) and mosandrite (green). Complex pseudoleucite syenites from Magnet Cove, Arkansas show similar minerals but also include xenolithic quartz (violet), manganwollastonite (bright orange), benitoite (intense blue) and lorenzenite (green).

Carbonatites from Arkansas contain a wide variety of calcite textures and intensities of luminescence. Grains of carbonatite in well cuttings are easily distinguished from sedimentary calcites by their complex Aflame@ luminescence and the frequent presence of euhedral apatite grains (yellow). Apatite crystals separated from the Arkansas River Valley carbonatites by acid digestion show complex zoning. The cores of these apatites are yellow luminescing due to CL activation by Mn enrichment while the rims are violet luminescing due to REE enrichment. Complex zonation between the cores and rims is easily visible using CL.

DIGITAL IMAGING TECHNIQUES

With the advent of low cost digital cameras with low light capabilities, non-imaging of CL became a reality. For this study a Vicam digital camera with 640 x 480 pixel resolution was adapted to a binocular microscope using nothing more than a simple wood mount and a spare eyepiece (fig 1). It was necessary to replace the standard lens on the camera with a wide angle lens to cut down on vignetting and to replace the infrared filter in the camera with a filter on the microscope. The infrared filters that come with many inexpensive cameras are cheap plastic devices prone to scratches and dirt and are better replaced with a glass filter. Descriptions of other camera mounting techniques may be found on the WWW. Total cost for the camera and mounting can range from under $100 to several thousand dollars, depending on resolution and other factors.

An older Luminoscope Model ELM 2B was upgraded with new window and new anode and cathode supports. This model can generate up to 18KV and when upgraded is adequate for almost all CL work. To cut down on stray light a simple black paper tube was attached to the end of the binocular microscope and simply slides up and down as the scope is focused (fig 2).

After the vacuum has come to operating pressure and a beam established, the image is brought up on the computer screen and all other manipulations are done remotely. Care must be taken when imaging to obtain a Atrue color@ image of the luminescence. Most cameras have an Aauto white balance@ feature that must be disabled to properly image CL. The best method is to take a brightly luminescent sample and observe it visually while adjusting the color balance, gamma and contrast on the computer screen. Once a favorable image has been obtained, these settings should be used for all the specimens. While not an ideal solution, it is highly preferable to the color distortion brought on by the automatic camera settings (fig. 3,4)

Figure 1   Figure 2    Figure 3       Figure 4

CATHODOLUMINESCENCE SPECTROMETRY

In order to measure the spectra of the luminescent minerals, an Ocean Optics fiber optic spectrometer model S-2000 was attached to an eyepiece of the microscope (fig. 5). This simple mount was constructed using a rubber stopper and a piece of PVC pipe. The fiber optic collimator lens should be placed at the focal spot of the eyepiece to assure maximum light gathering capacity. A feature of the software that comes with the S-2000 is that a dark spectrum can be subtracted from the acquired spectrum and this facilitates compensation for stray light and for Ahot@ pixels in the detector. Examples of CL spectra of blue luminescing orthoclase feldspar and red luminescing fenitized orthoclase feldspar are illustrated (fig. 6,7)

Figure 5   Figure 6     Figure 7

CATHODOLUMINESCENCE OF ALKALI SYENITES

Alkali syenites in Arkansas include the Granite Mountain syenite, the Magnet Cove igneous complex, the Potash Sulfur Springs intrusive and the AV@ intrusive. Crushed stone and roofing granules are mined from the Granite Mountain syenite. Limited production of titanium and vanadium has come from Magnet Cove and Potash Sulfur Springs intrusives. Exploration for a number of mineral commodities including rare earth mineralization has taken place at all the syenite intrusives, but no significant mineralization has been located. The syenites and other associated intrusives have been linked to localized metallic mineralization including lead and silver veins.

Luminescence of the individual minerals is discussed in detail below. Application of CL to petrographic problems includes examination of the syenites for fenitization and zeolitization of the feldspars. Production of roofing granules is dependent on the syenite crushing to a high yield of granule sizes. Fenitization and accompanying zeolitization of the feldspars cause yields of granules to fall and increase costs. Fenitization and feldspar alteration is easily observed by CL microscopy.

Exploration for potential rare-earth orebodies is easily carried out using CL. Several rare-earth enriched species luminesce and the luminescent color of apatite is an indicator of rare-earth enrichment. Mobilization of metallic elements, especially V and Ti accompanies fenitization and the degree of alkali fenitization of feldspars is easily assessed using CL.

Feldspar

The earliest crystallized orthoclase feldspars are present in the olivine syenites and they luminesce a deep blue. Later porphyritic alkali syenites have orthoclase that has a lighter blue luminescing rim growing on the earlier feldspars. Partial resorption of the earlier feldspars was followed by deposition of lower Ti content orthoclase that has a lighter CL color (fig. 8). There is a gradual increase in the amount of red luminescing fenitized feldspar, with the earliest examples being auto-fenitization of fine-grained feldspar between the coarse porphyroblasts. Eventually, the ground mass becomes fenitized to the point where there are only relict blue luminescing feldspars in the red CL (fig. 9). Specimens from the border complex and from later nepheline syenite dikes are almost completely fenitized. Spherulitic material from the wall rock chilled zone contains blue luminescing spherulites in a red CL matrix (fig. 10). The coarse nepheline syenite dikes and pegmatites contain orthoclase that luminesces red and microcline that luminesces a dull yellowish-orange. Xenoliths often contain orthoclase that luminesces red. Some late veins in quartz syenites contain albite that luminesces a yellow or greenish-yellow color, but these veins are relatively rare (fig. 11).

Figure 8     Figure 9    Figure 10     Figure 11

Apatite

Grains of apatite are very common in all the syenites and are typically dull to bright yellow luminescing (fig. 12). Zoning is common in the apatites and consists of bright and dull luminescing zones. Where the apatite is from zones of rare earth enrichment at contacts and in pegmatites, it luminesces a blue to violet color. Rarely grains of apatite exhibit cores of yellow luminescence surrounded by rims of blue to violet luminescence from rare earth enrichment.

Figure 12

Fluorite

Colorless grains of fluorite are common in the syenites and are easily located by their bright bluish-violet luminescence.

Zircon

Zircon is rare in the syenites except in the pegmatites and quartz syenites. Grains usually luminesce a bright green.

Mosandrite

Mosandrite is rare in the syenites, but concentrated in the pegmatites. Mosandrite typically luminesces a distinct green. Mosandrite is a good indicator of rare earth enrichment.

Sodalite

Sodalite is a common alteration product of nepheline and is particularly abundant in fenitized zones and pegmatites. It is a good indicator of zeolitic alteration of feldspars. Sodalite typically luminesces a distinct orange to orange-red (fig. 13). Rare lazurite xenoliths have orange luminescing lazurite grains instead of sodalite.

Figure 13

Quartz

Quartz is rare in the syenites and limited to xenoliths and the border quartz syenites, which are often fenites. Quartz from this environment luminesces a dull violet (the so-called Abrown@ luminescing quartz described in the CL literature) (fig.14).

Figure 14

Manganwollastonite

Pseudoleucite syenites often contain gas cavities filled with bright yellow luminescing manganwollastonite.

Benitoite

Rare grains of brilliant blue luminescing benitoite are infrequently encountered in filled gas cavities in the pseudoleucite syenite (fig. 15).

Figure 15

Lorenzenite

Grains of bright green luminescing lorenzenite are frequently encountered in some border fenites of the Magnet Cove igneous complex.

CATHODOLUMINESCENCE OF CARBONATITES

Carbonatites are represented by the Magnet Cove igneous complex, the Potash Sulfur Springs intrusive, the AV@ intrusive and by a series of dikes and sills from the area around Morrillton, Arkansas, known as the Arkansas River Valley carbonatites. The Sinclair Oil and Gas well drilled near Morrillton in the 1950's penetrated some 50 carbonatite bodies and the cuttings were saved and presented to the author for examination. These cuttings are easily identified as carbonatites by the interfingering light and dark orange luminescing calcite grains that give them a Aflame@ texture in CL. Most of the cuttings also contain grains of apatite that luminesce yellow to violet.

Surface exposures of the Arkansas River Valley carbonatites show that most of them are full of xenoliths. CL examination of the xenoliths show varying degrees of alteration from mantle xenoliths with blue luminescing feldspars to partially assimilated shale that has a rim of yellow luminescing albite (?) feldspar. The Arkansas River Valley carbonatites also have xenocrysts of calcite that luminesce a uniform orange color. The larger carbonatite bodies, such as Magnet Cove, have uniform orange luminescing calcite that shows minor calcite xenocrysts. Most apatite grains from the Magnet Cove carbonatite luminesce a pale violet color from rare earth enrichment. Rare veins of bastnaesite and synchisite from the Magnet Cove carbonatite also luminesce orange, but are easily distinguished from the calcite.

Calcite

The characteristic mineral of carbonatites is calcite. Massive carbonatites such as those at the Magnet Cove igneous complex have large crystals of uniform orange luminescing calcite. Earlier formed calcite xenocrysts can be distinguished by a slightly different luminescence intensity (fig. 16). Carbonatite dikes and sills such as those comprising the Arkansas River Valley carbonatites have a different texture entirely. The calcite ground mass consists of interlocking low and high iron calcites that have luminescence ranging from none to bright orange. This gives them a distinct Aflame@ texture in CL (fig. 17,18). These carbonatites also have distinct calcite xenocrysts that have a uniform luminescence ranging from light to dark orange.

Figure 16   Figure 17      Figure 18

Dolomite

Rarely dolomite or ankerite forms around xenoliths and exhibits zoned dull red luminescence

Apatite

Crystals of apatite are common in the carbonatites and range from yellow luminescing grains in the Arkansas River carbonatites (fig. 19,20) to pale violet grains in the Magnet Cove carbonatite (fig. 21). The Arkansas River Valley carbonatite apatites are frequently zoned and often have a rim of violet luminescence from rare earth enrichment.

Figure 19    Figure 20       Figure 21

Quartz

Quartz only occurs in the Arkansas River Valley carbonatites as xenolithic grains and it luminesces a red-violet.

Feldspar

Blue, red and yellow luminescing feldspars are found in the xenoliths. A range of fresh blue luminescing to fenitized red luminescing feldspars exists. Shale xenoliths often have a rim of yellow luminescing feldspar where they have been partially assimilated by the carbonatite.

Bastnaesite/synchisite

Veins of mixed bastnaesite and synchisite cut the carbonatite at Magnet Cove and luminesce orange. The color and intensity are just different enough from calcite to allow their discrimination using CL (fig. 22)

Figure 22 

CONCLUSIONS

Digital imaging of distinctive and mineral specific colors emitted by Cathodoluminescence is a very cost effective method to gather data on petrographic samples. Mineral assemblages in syenites and carbonatites from Arkansas were clearly and diagnostically imaged using the CL technique. Digital imaging provides an almost Ainstant@ imaging format and the low light gathering power of digital cameras allows imaging of features not easily captured by film based techniques. Digital cameras can also Asee@ features that are often too dim to visually observe and that would require long exposure times to capture on film. This technique also allows the investigator to work with a video screen image rather than straining to see the image in a microscope. While there is some loss of resolution (at least with lower cost digital cameras) this is more than offset by the advantages of rapid digital processing of the image and the ability to save and import the image into many other programs.