An evaluation of the relationship between physical/mechanical properties and mineralogy of landscape rocks as determined by hyperspectral reflectance

Abstract

We investigated the relationships between mineral content and the physical and mechanical properties of landscape rock using a non-destructive remote sensing method applied in the laboratory. Using this technique, the spectral properties of the landscape rock could be collected at different wavelengths without harming the samples. Differences in spectral reflectance were compared with the physical and mechanical properties of the stone. Significant correlations were observed between reflectance values and the rocks’ mechanical and physical properties, with correlation coefficients of 95 to 99 %. However, establishing a correlation between two variables is not a sufficient condition to establish a causal relationship. Mineral densities and mineral content are characteristics used for the classification of landscape rock. We have concluded that although spectral signatures from landscape rock can be used for predicting which stones might have similar features when comparing two batches of stone, the high correlations we discovered cannot confirm a cause and effect relationship that would allow for the prediction of a rock’s physical and mechanical properties. Although this conclusion is disappointing, the mineral content and the significant correlations discovered by hyperspectral reflectance scanning can be used as supplementary information when comparing two samples of landscape rock.

Annex-A

Al–OH

Many Al–OH containing minerals have subtle compositional variations that are indicative of the chemistry of fluids present at the time of the alteration event. Since these compositional variations result in wavelength shifts of the Al–OH absorption feature near 2200 nm, the wavelength position of this feature is an indicator of geochemical conditions at the time of the alteration event.

Kx

The phyllosilicate kaolinite is produced by both weathering and alteration events. The crystallinity of the resulting kaolinite is strongly influenced by the temperature of the fluids at the time of formation. Since kaolinite crystallinity influences the shape of the Al–OH absorption feature centered near 2200 nm, a measurement of this feature shape is indicative of the temperature of formation. Kx scalar values greater than 1 are indicative of a weathering-produced kaolinite, while values less than 1 are generally associated with those produced by higher temperature alteration events; the lower the Kx value, the higher the temperature of formation.

Illite spectral maturity (ISM)

The phyllosilicates in the illite-muscovite group are produced over a broad range of geochemical conditions. With increasing metamorphic grade, the reflectance spectrum of these minerals indicate a loss of molecular water (as inferred from the depth of the H₂O absorption feature near 1950 nm) along with an increase in crystallinity (as inferred from the depth of the Al–OH absorption feature near 2200 nm). ISM scalar values greater than 1 are indicative of a low metamorphic grade illite, while values less than 1 are generally associated with those produced by higher temperature alteration events; the lower the ISM value, the higher the alteration temperature can be observed.

Chlorite spectral maturity (CSM)

The chlorite group minerals are produced over a broad range of geochemical conditions. With increasing metamorphic grade, the reflectance spectrum of these minerals indicate a loss of molecular water (as inferred from the depth of the H₂O absorption feature near 1950 nm) along with an increase in crystallinity (as inferred from the depths of the Mg–OH, near 2350 nm, and Fe–OH, near 2260 nm, absorption features).

Mg–OH

Many Mg–OH-containing minerals have subtle compositional variations that are indicative of the chemistry of fluids present at the time of the alteration event. Since these compositional variations result in wavelength shifts of the Mg–OH absorption feature near 2350 nm, the wavelength position of this feature is an indicator of geochemical conditions at the time of the alteration event.

Fe–OH

Many Fe–OH-containing minerals have subtle compositional variations that are indicative of the chemistry of fluids present at the time of the alteration event. Since these compositional variations result in wavelength shifts of the Fe–OH absorption feature near 2260 nm, the wavelength position of this feature is an indicator of geochemical conditions at the time of the alteration event.

Fe3t

Iron oxides and oxyhydroxides are produced under a wide range of geologic conditions. All the Fe³⁺ minerals, the most common of which are hematite and goethite, have a similar feature in the 750–1000-nm region. The position of this feature shifts depending on the identity of the Fe³⁺ mineral.

Fe3i

Iron oxides and oxyhydroxides are produced under a wide range of geologic conditions. All the Fe³⁺ minerals, the most common of which are hematite and goethite, have strong absorption short of about 550 nm. The intensity of this feature depends strongly on the abundance of Fe³⁺ minerals. The Fe3i scalar tracks the intensity of this feature; the higher the Fe3i value, the more intense the Fe³⁺ absorption.

Al–Fe–Mg

The relative depths of the Al–OH, Fe–OH, and Mg–OH features centered in the 2160- to 2370-nm wavelength range and seen in the reflectance spectrum of an alteration produced mineral assemblages can provide an indication of geochemical conditions. The Al–Fe–Mg scalar reports the wavelength of the deepest absorption feature of these three.