CLAY MINERALS

SWIR reflectance spectroscopy is a very sensitive analytical method for the clay mineral groups for a number of reasons. The OH and H2O bonds are easily detected in the SWIR range. SWIR does not identify minerals per se, but rather, in the case of clays, the octahedral layer configurations of minerals, as these are so well defined by the OH to cation bonds. The SWIR method is particular good for monitoring and tracking the subtle changes in those layer configurations. This is especially useful when evaluating water rock interactions with clays in different environments.

This method allows the determination of such processes as hydrothermal fluid dissemination through a mineral deposit, ground water channels, paleoweathering influences, and fluid migration along structural features. As SWIR detects the sub-lattice changes in a mineral species, it is a more subtle analytical technique then X-ray diffraction for many applications.

Another major application is the evaluation of industrial clay deposits. SWIR and PIMA™ can also be used in the petroleum industry to log drill core.

This section will present an overview of the different common clay mineral groups.

KAOLINITE GROUP

The Kaolinite Group of minerals includes kaolinite, halloysite, dickite, nacrite, "endellite", allophane, and additional commercial clay types such as ball clay and flint clay.

Figure 1, above, shows the kaolinite group members.

These are [A] nacrite from the Nayarit hydrothermal deposit in Mexico, [B] dickite from a porphyry copper breccia pipe in Tombstone, Arizona, [C] the best known crystalline kaolinite from the Warsaw geode in Indiana, [D] crystalline kaolinite from a hydrothermal location in France. [E] poorly-crystalline kaolinite from a paleoweathering profile in Provins, France, and [F] halloysite from the industrial clay deposit in New Zealand.

Kaolinite is a 1:1 phyllosilicate and an acid pH mineral. It is low temperature from ambient to ~250ƒC, with dickite and nacrite closer to 285ƒC, and is stable from about pH 3-8. It is common as a hydrothermal alteration product in many types of deposits. It can be an acid ground water alteration product of illite.

Spectrally, kaolinite has two very distinctive doublets at 1400 and 2200 nm. The minima are directly linked to the orientation of different octahedral layer structural components, and change in intensity and profile as the octahedral layer bonds and composition vary.

Figure 2 - Georgia and French Kaolinites arranged by decreasing structural order

Changes in crystal structural ordering were observed by clay mineralogists working in the famous Georgia Kaolinite industrial clay deposits (Lyons and Murray, 1963) with X-ray diffraction techniques. Their Crystallinity Index has been re-cast with SWIR spectroscopy to show a gradational change in spectral profile and water content (Figure 2), which is a manifestation of changes in ordering.

The increasing water content is seen in the 1900nm feature and indicates weakening of interlayer bonds as a function of disorder and the development of interlayer spaces that accommodate water and hypothesized development of smectitic layers.

Note in this expanded scale version of Figure 2, below, how the kaolinite doublet in the 2200nm region softens and broadens from a sharp, defined minimum to a shoulder as ordering decreases. This change in profile can be mathematically defined and utilized as an index. It is a function of random and sporadic filling of metal sites in the octahedral layer of the mineral. Empty and random sites decrease order and therefore the sharpness of the profile.

Figure 2A - Expanded view of Figure 2.

The change in slope, flagged with the red arrows, can be measured and used as a crystallinity index.

Mixed Layer Kaolinite/Smectite Clays

The interstratification of kaolinite and smectite is found in many paleoweathering profiles. The best documented occurrence is in the Paris Basin of France.

The following plot shows a progression from kaolinite-dominated (KSPB95) to smectite-dominated (KSPB00). The numbers in the file names refer to approximate percentages of kaolinite. These are estimated from X-Ray Diffraction work on the samples. Note the changes in the features at 1400nm and 2200nm as the progression moves from kaolinite with doublets to smectite with single features.

Figure 3 - This figure compiles a progression from high (95%) kaolinite layers [A] through [B] 90% kaolinite, [C] 80% kaolinite, [D] 75% kaolinite, [E] 55% kaolinite [F] 45% kaolinite, [G] 20% kaolinite, [H] 2% kaolinite plus calcite, to [I] all smectite layers.

ILLITE --> KAOLINITE

A commonly observed phenomenon in mineral deposits with illitic, sericite or phyllic alteration is the kaolinitization of the illite/mica. Figure 4 shows spectra from a porphyry copper deposit. The illite, which was probably originally phyllic muscovite, is now being altered by acid ground water to kaolinite. What is most likely happening is that layers of 1:1 phyllosilicate configurations are developing within the 2:1 mica-->illite structure. Therefore, although this is treated here as a discrete mineral phase, it probably is not discrete. This hypothesis has never been physically tested, as it is very difficult to separate the secondary clay minerals from each other because they are very fine grained. X-ray diffraction is usually not sensitive to these changes.

Figure 4 shows illite [A], illite with minor kaolinite (note the profile changes of the 2200nm feature)[B], poorly crystalline kaolinite with minor illite [C] and [D] kaolinite. The changes in [B] are particularly subtle, but diagnostic of acid ground water flow.

ILLITES

Illites are environmental monitors and show a crystallinity change with temperature. This can be used to trace illites through an environment and vector towards geologic and mineralization changes.

Figure 5, below, compiles illites from [A] Morenci, AZ, porphyry copper phyllic zone, [B] Leadville, CO, base metal veins, [C] Carlin, NV, sediment hosted epithermal gold, [D] Hog ranch, NV, volcanic hosted disseminated gold, [E] Fithian illite from sediments in Illinois, and [F] ammonia-bearing illite from black shales above the Mercur Mine, UT.

Figure 5 - Illites by environment.

Illite will also have different cations substituting into various sites with diagnostic changes in the spectral profile. In Figure 6, below, spectrum [A] is an interstratification of illite and smectite with higher aluminum contents in two octahedral layers, shown by a shift to lower wavelengths (2188nm) of the 2200nm feature. Contrast this with [B], an illite from a low temperature gold system, which shows higher aluminum then the average illite (see next section) but less than in [A]. In spectrum [C], NH4 is substituting for potassium and creating an ammonium-illite. This manifestation is shown in the features flagged with red arrows. When iron substitutes into the octahedral layer, the features shift to a higher wavelength (2310nm). This is glauconite, the iron-illite [D].

Figure 6 - Illites by chemistry.

Chemical Variability - Illites

The aluminum content of illites can be estimated from the 2.2m absorption feature, which shifts relative to the percent aluminum present. There appears to be a deposit-specific correlation. When illite/"sericite"/muscovite alteration is present, higher amounts of aluminum are apparently associated with the ore zones. This has also been documented by Post and Noble (1993) and their data is plotted below against spectral wavelength values collected from their published samples.

Feature positions for a cross section of muscovites and illites from SPECMIN™ range from 2198nm to 2212nm, with the majority falling within 2200-2204nm.

The illites plotted in Figure 7 are from different environments, and from top to bottom are: [A] Hog Ranch, Nevada, epithermal gold deposit; [B] Chuquicamata, Chile, porphyry copper; [C] Leadville, Colorado, gold vein system; [D] Cananea, Mexico, porphyry copper deposit; [E] Round Mountain, Nevada, disseminated gold deposit; [F, G, H] sedimentary illites from Illinois shales.

The red line is positioned at the minimum for the 2200nm absorption feature. The hydrothermal samples all line up near 2200nm, with is high alumina - about 35% Al2O3. The sedimentary illites, however, all show shifts to the higher wavelengths, indicating less alumina than observed in the hydrothermal varieties.

Figure 7 - Cross section of illites showing a spectral shift from 2198 (top) to 2222nm (bottom).

Smectite --> Mica Plot

Figure 8 plots a series from smectite ([A] Arizona) through mixed-layer smectite/illite [B] and illite/smectite [C, D], to epithermal gold illite [E] Hog Ranch, NV, [F] Round Mountain, NV, to vein illite [G] El Indio, Chile, to phyllic illite in porphyry copper systems from [H] Silver Bell, Az and Cananea, Mx [I].

Figure 8 Smectite --> Muscovite Spectral Sequence Plot

This is a classic sequence of changing crystallinity and proportional chemistry. The structures are very similar as are the compositions with like-octahedral layer configurations. Interlayer positions in smectite are occupied with water and common cations such as Ca, K, Mg and Na. As order increases and potassium replaces the water and fills the interlayer sites, illite and then muscovite evolve.

Although it is easy to differentiate the end-members, it is not so easy to distinguish between near neighbors. This is one of the more common series of minerals encountered in infrared spectroscopy. It gives the most difficulty in identification.

Note in the plot below how the wavelength position of the 2200nm feature shifts. This is a function of the aluminum composition of the octahedral layers. As stated, the structure of the individual members within this series of mineral phases is very similar. The major differences are ordering and chemistry. The mixed layer constituents contain the most aluminum [C, D], then the illites from porphyry copper systems [G, H, I], then the illites from epithermal gold systems [E, F], with smectite [A] and the smectite dominated Illite/Smectite carrying lesser amounts.

Figure 8A - Expanded view of the 2200 nm region from Figure 8.

The chemical compositions are one way of differentiating the species, but since there is overlap, it must be done within restricted environments and usually is difficult, except in generalities, to cross environments of paragenesis.

This underlines the need to understand environments better from a spectral perspective and also to utilize the concept of site-specific databases.

SMECTITES

Smectites are sensitive to their environment and form at low temperatures (less then 150ƒC), usually in alkaline, neutral conditions. They will reflect the type, amount, and transit of cation-bearing ground waters through the rock or soil.

Figure 9 - Smectite Group minerals; [A] beidellite-Al, [B] montmorillonite-Al/Mg, [C] hectorite-Li, [D] nontronite-Fe, [E] iron saponite-Mg/Fe, [F] saponite-Mg, and [G] sauconite-Zn.

Smectites are a common constituent of soils and occur as a weathering product of other clays. They are excellent low temperature indicators. In hydrothermal ore deposits, they occur in the zones of argillic alteration, away from the mineralization. The exception to this is beidellite, the aluminum species. Beidellite is found near the ore zones in some low temperature gold and silver deposits.

Saponites are indicators for clay alteration of the serpentines found within kimberlite pipes.

Nontronite is associated with enriched porphyry copper deposits and found as a layer under the supergene-enriched ore blanket.

Hectorite is a lithium source and found in playas and evaporitic lakes.

Sauconite is the Zn-smectite. It is found as ore in zinc deposits such as the Skorpion zinc mine in Namibia

The most notable thing about the SWIR spectra of smectites is the ability to differentiate the Fe, Mg, Al, Zn and Li cations using the wavelength positions of the hydroxyl absorption features. By X-Ray diffraction, the conventional method of identification, this is a time consuming process. SWIR accomplishes it in 30 seconds.

Figure 9 summarizes the smectite group members. These include; [A] beidellite-Al, [B] montmorillonite-Al/Mg, [C] hectorite-Li, [D] nontronite-Fe, [E] iron saponite-Mg/Fe, [F] saponite-Mg, and [G] sauconite-Zn

The red line flags aluminum species in the 2200nm region; green the iron species; cyan the magnesium species, and gold the zinc phase.

CHLORITE GROUP

The chlorite group is large and very complicated. Chlorites are ubiquitous and can be found in low temperature regimes where they are considered more as clays and in higher temperature environments associated with porphyry copper, with VMS, with kimberlites and base metal systems.

Because of the solid solution substitution series that exists within the various chlorite species, especially with iron, magnesium and aluminum, chlorites respond well to SWIR analyses and provide valuable information about their environments and alteration processes.

Figure 10 shows the two chlorite "end members", which are iron-bearing [A] and magnesium-bearing [B]. Note the different slopes and spectral profiles.

Figure 10 - Iron [A] and Magnesium [B] bearing chlorites.

The changes in the spectra and absorption feature positions are dramatic and, for the most part, when chlorites are present in perhaps at least 15% amounts, their species can be determined. The identification between an iron and non-iron chlorite is helped by the positive slope to the continuum seen in the above plot between ~1400 - 1850nm. There are also diagnostic shifts in the wavelength positions. The shift in the 2300nm feature indicates iron and magnesium substitution in the octahedral layers.

Chemical Variability - Chlorite

The following plot in Figure 11 shows a series from magnesium-dominated chlorite [A] through various chemical combinations of magnesium, iron and aluminum, to the iron end member [F].

Figure 11 - Mg --> Fe Chlorites

These spectra are from samples archived at the Royal Ontario Museum in Toronto, and have chemical analyses providing percentages of Mg, Fe, and Al. This data is available through Spectral International in the SPECMIN™ reference database. By overlaying reference spectra from this database, it is usually possible to obtain a reasonable idea of the composition of the unknown chlorite.