Roughness length and drag coefficient at two MONTBLEX-90 tower stations

Using MONTBLEX-90 mean velocity data, roughness lengths and drag coefficients are estimated at Jodhpur and Kharagpur. At Jodhpur, since the surface is not uniform the roughness length is estimated separately in three different subsectors within the range of prevailing wind directions and averages to 1.23 cm in the sector between 200° and 230° which is relatively flat with no obstacles on the ground. At Kharagpur, where the terrain is more nearly homogeneous, the average value (for all prevailing wind directions) is 1.94 cm. The drag coefficient C_D at Jodhpur shows variation both with the roughness subsector and with wind speed, the average over all directions increasing rapidly as themean wind speed Ū₁₀ at 10m height drops according to the power lawC _D = 0.05 Ū ₁₀ ^t-1.09 in trie range 0.5 < Ū₁₀ < 7 m s⁻¹. At Kharagpur, the drag coefficient is smaller than at Jodhpur by nearly 50% for the same range of wind speeds (> 3 ms⁻¹).     

An analysis of MONTBLEX data on heat and momentum flux at Jodhpur

Parameterization of sensible heat and momentum fluxes as inferred from an analysis of tower observations archived during MONTBLEX-90 at Jodhpur is proposed, both in terms of standard exchange coefficientsC _H andC _D respectively and also according to free convection scaling. Both coefficients increase rapidly at low winds (the latter more strongly) and with increasing instability. All the sensible heat flux data at Jodhpur (wind speed at 10 m Ū₁₀ < 8 ms⁻¹) also obey free convection scaling, with the flux proportional to the ‘4/3’ power of an appropriate temperature difference such as that between 1 and 30 m. Furthermore, for Ū₁₀ < 4 ms⁻¹ the momentum flux displays a linear dependence on wind speed.     

Folds in multilayered rocks of Proterozoic age,Rajasthan, India

First phase folds F₁ developed in polydeformed Ajabgarh Group rocks of Proterozoic age are studied using various geometrical methods of analysis for compatibility of homogeneous strain in both class 1–3 pairs by correlatingt′ _ga/α plots with existing curves for competent layers and matchingt _ga/α plots with the flattening curves for the incompetent layers. F¹ folds were initiated by the process of buckling but underwent [(λ₂/λ₁) = 0.2 to 0.7] for competent layers andR- values of 1.1 to 5 for incompetent layers. The varying flattening is also revealed by the geometry of folds. The apparent buckle shortening of folds which ranges between 49 and 67 per cent with a majority of the folds having shortening values between 50% and 55% (exclusive of layer parallel strain) and inverse thickness method strain up to 50%. Besides flattening, the fold geometry was also modified by the pressure solution. This is borne by the presence of dark seams rich in phyllosilicates and disseminated carbonaceous material offsetting limbs of buckled quartz veins in slates     

Identification of a surface layer structure and analysis of humidity data in two weather situations at Jodhpur (26°18′N, 73°04′E), India,during MONTBLEX 1990

The Monsoon Trough Boundary Layer Experiment held in 1990 was a multi-institutional effort to probe the atmospheric boundary layer over the monsoon trough over northern India. For this experiment, four micrometeorological towers were set up at four different locations along the normal position of the trough. One such tower of 30m height was located at Jodhpur (26‡18′N, 73‡04′E), Rajasthan. The fast and slow response data available during the experiment have been used in the present study to determine a suitable layer-structure of the surface layer for evaluation of sensible heat flux using the multilayer hypothesis of Kramm (1989).     

Dielectric constants of chrysoberyl,spinel, phenacite,and forsterite and the oxide additivity rule

The dielectric constants and dielectric loss values of BeAl₂O₄ (chrysoberyl), MgAl₂O₄ (spinel), Be₂SiO₄ (phenacite), and Mg₂SiO₄ (forsterite) were measured at 1 MHz using a two-terminal method and empirically determined edge corrections. The results are: chrysoberyl, κ′ _a =9.436, κ′ _b =9.071, κ′ _c =8.269; spinel, κ′ _a =8.18; phenacite, κ′ _a =6.28, κ′ _c =6.06; and forsterite, κ′ _a =6.867, κ′ _b =7.392, κ′ _c =6.739. The agreement between measured dielectric polarizabilities as determined from the Clausius-Mosotti equation and those calculated from the sum of oxide polarizabilities according to α_D(M₂M′X₄) = 2α_D(MX)+α_D(M′X₂) is ~ 1.0%.     

Thermodynamic structure of the marine atmosphere over the region 80–87°E along 13°N during August (phase II) BOBMEX-99

Thermodynamic structure of the marine atmosphere in the region between 80 and 87‡E along 13‡N over the Bay of Bengal was studied using 13 high resolution radiosonde profiles from surface-400 hPa collected onboard ORV Sagar Kanya during the period 27th–30th August, during BOBMEX-99. Saturation point concept, mixing line analysis and conserved variable diagrams have been used to identify boundary layer characteristics such as air mass movement and stability of the atmosphere. The results showed relatively dry air near the ocean surface between 1000 and 950 hPa. This feature is confirmed by the conserved teta_v structure in this layer. Further, teta_v seldom showed any inversions in this region. The teta_e and teta_es profiles showed persistent low cloud layers between 900 and 700 hPa. The conserved variable diagrams (teta_e-q) showed the existence of double mixing line structures approximately at 950 and 700 hPa levels.     

The Direct Shear Strength and Dilatancy of Sand–gravel Mixtures

A total of 87 direct shear tests in a large direct shear-box apparatus have been used to investigate the strength and dilatancy of sand–gravel mixtures. This paper focuses on the differences in behaviour between a silica sand (yellow Leighton Buzzard sand) and sand–gravel mixtures obtained by adding fractions of two kinds of gravel to the sand. The purpose is to find a relation between the grain-size characteristics of the materials and the shearing resistance. Experimental results are analysed in terms of the frictional and dilatant contributions to the strength of mixtures as a function of their relative density, and are compared with dilatancy theories and empirical equations. The addition of gravel to the mixtures, even at low fractions (less than 0.1 by volume), causes an increase in peak friction angle (Φ ′_peak) which results both from higher dilatancy at failure (ψ_max) and higher constant volume friction angle (Φ ′_cv). Use of the minimum voids ratio (e_min) of the materials allows the data for the two families of mixtures to be normalized and interpreted in terms of Φ ′_cv and the ratio (Φ ′_peak − Φ ′_cv)/ψ_max. The relationships between relative density (D_r), ψ_max and Φ ′_peak − Φ ′_cv are only partly explained on a physical basis, so we develop empirical equations to predict the peak shear resistance of sand–gravel mixtures (up to gravel contents of 0.5) on the basis of easily measurable quantities. Such equations constitute a practical tool to overcome the problems arising from the impracticality of testing coarse material in the standard shear-box apparatus.     

Solubility of water in the α, β and γ phases of (Mg,Fe)2SiO4

 The solubility of hydroxyl in the α, β and γ phases of (Mg,Fe)₂SiO₄ was investigated by hydrothermally annealing single crystals of San Carlos olivine. Experiments were performed at a temperature of 1000° or 1100 °C under a confining pressure of 2.5 to 19.5 GPa in a multianvil apparatus with the oxygen fugacity buffered by the Ni:NiO solid-state reaction. Hydroxyl solubilities were determined from infrared spectra obtained of polished thin sections in crack-free regions ≤100 μm in diameter. In the α-stability field, hydroxyl solubility increases systematically with increasing confining pressure, reaching a value of ∼20,000 H/10⁶Si (1200 wt ppm H₂O) at the α-β phase boundary near 13 GPa and 1100 °C. In the β field, the hydroxyl content is ∼400,000 H/10⁶Si (24,000 wt ppm H₂O) at 14–15 GPa and 1100 °C. In the γ field, the solubility is ∼450,000 H/10⁶Si (27,000 wt ppm H₂O) at 19.5 GPa and 1100 °C. The observed dependence of hydroxyl solubility with increasing confining pressure in the α phase reflects an increase in water fugacity with increasing pressure moderated by a molar volume term associated with the incorporation of hydroxyl ions into the olivine structure. Combined with published results on the dependence of hydroxyl solubility on water fugacity, the present results for the α phase can be summarized by the relation C _OH = A(T)f nH₂Oexp(−PΔV/RT), where A(T) = 1.1 H/10⁶Si/MPa at 1100 °C, n = 1, and ΔV = 10.6×10^–6 m³/mol. These data demonstrate that the entire present-day water content of the upper mantle could be incorporated in the mineral olivine alone; therefore, a free hydrous fluid phase cannot be stable in those regions of the upper mantle with a normal concentration of hydrogen. Free hydrous fluids are restricted to special tectonic environments, such as the mantle wedge above a subduction zone. Received: 10 February 1995 / Accepted: 23 October 1995     

Do carbonate liquids become denser than silicate liquids at pressure? Constraints from the fusion curve of K<Subscript>2</Subscript>CO<Subscript>3</Subscript> to 3.2 GPa

Brackets on the melting temperature of K₂CO₃ were experimentally determined at 1.86 ± 0.02 GPa (1,163–1,167°C), 2.79 ± 0.03 GPa (1,187–1,195°C), and 3.16 ± 0.04 GPa (1,183–1,189°C) in a piston-cylinder apparatus. These new data, in combination with published experiments at low pressure (<0.5 GPa), establish the K₂CO₃ fusion curve to 3.2 GPa. On the basis of these experiments and published thermodynamic data for crystalline and liquid K₂CO₃, the high-pressure density and compressibility of K₂CO₃ liquid were derived from the fusion curve. The pressure dependence of the liquid compressibility (K′₀ = dK ₀/dP, where K ₀ = 1/β₀) is between 16.2 and 11.6, with a best estimate of 13.7, in a third-order Birch–Murnaghan equation of state (EOS). This liquid K′₀ leads to a density of 2,175 ± 36 kg/m³ at 4 GPa and 1,500°C, which is ∼30% lower than that reported in the literature on the basis of the falling-sphere method at the same conditions. The uncertainty in the liquid K′₀ leads to an error in melt density of ± 2% at 4 GPa; the error decreases with decreasing pressure. With a K′₀ of 13.7, the compressibility of K₂CO₃ at 1,500°C and 1 bar (K ₀ = 3.8 GPa) drops rapidly with increasing pressure ( ), which prevents a density crossover with silicate melts, such as CaAlSi₂O₈ and CaMgSi₂O₆, at upper mantle depths.     

Crystal structure and compression of an iron-bearing Phase A to 33 GPa

Fe-bearing dense hydrous magnesium silicate Phase A, Mg_6.85Fe_0.14Si_2.00O₈(OH)₆ has been studied by single-crystal X-ray diffraction at ambient conditions and by high-pressure powder diffraction using synchrotron radiation to 33 GPa. Unit cell parameters at room temperature and pressure from single crystal diffraction are a=7.8678 (4) Å, c=9.5771 (5) Å, and V=513.43 (4) ų. Fitting of the P–V data to a third-order Birch-Murnaghan isothermal equation of state yields V ₀=512.3 (3) ų, K _T,0=102.9 (28) GPa and K′=6.4 (3). Compression is strongly anisotropic with the a-axes, which lie in the plane of the distorted close-packed layers, approximately 26% more compressible than the c-axis, which is normal to the plane. Structure refinement from single-crystal X-ray intensity data reveals expansion of the structure with Fe substitution, mainly by expansion of M-site octahedra. The short Si2–O6 distance becomes nearly 1% shorter with ~2% Fe substitution for Mg, possibly providing additional rigidity in the c-direction over the Mg end member. K _T obtained for the Fe-bearing sample is ~5.5% greater than reported previously for Fe-free Phase A, despite the larger unit cell volume. This study represents a direct comparison of structure and K _T–ρ relations between two compositions of a F-free dense hydrous magnesium silicate (DHMS) phase, and may help to characterize the effect of Fe substitution on the properties of other DHMS phases from studies of the Fe-free end-members.     

Seismic moment release along selected major transforms on the Central Indian Ridge,Western Indian Ocean,and its tectonic implications

A detailed seismicity map of the Central Indian Ridge for the period 1912–1993 is presented, and the earthquakes pertaining to four major transforms offsetting the ridge are utilized to study the moment release pattern. The scalar moment release for the period 1912–1993, and the summed moment rate tensors for both short period (1977–1993) and long period (1912–1993) bring out a unified picture of moment release pattern. The fraction of seismic slip calculated based on depths of 100°C and 400°C limiting temperatures suggests that the Marie-Celeste transform requires a slip almost to a depth of 400°C isotherm to account for the observed moment, and the Argo transform requires depth of faulting much above the 400°C isotherm. A very small fraction of slip is accounted seismically for Vema (53%) and 12° 12′S (23%) even to depths of 100°C isotherm, suggesting a very low order of moment release along these transforms. The horizontal plate velocities and the corresponding strain rates obtained from moment tensor summation of long period data (82 years) give rise to (V _y ^y ; V _y ^x mm. yr⁻¹) of 6.0 and 6.1 along Marie-Celeste, 1.3 and 0.50 along Argo, 0.06 and 0.06 along 12° 12′S, 1.6 and 0.25 along Vema transforms. The corresponding strain rates (ε _y ^y ;ε _y ^x × 10⁻¹⁵ S⁻¹) are 12.7 and 6.8 along MarieCeleste, 6.9 and 1.4 along Argo, 0.27 and 0.14 along 12° 12′S, 7.3 and 0.58 along Vema transforms. These results suggest that the strain rates were highest and almost all predicted motion is taken up seismically along the Marie-Celeste transform. The strain rates are lower along Argo transform and the observed moment release require shallower depth of faulting in order to slip to be accounted seismically. The Vema and 12° 12′S transforms are characterized by low strain rates and less than 15 per cent of motion is accommodated seismically within the seismogenic layer. It is proposed that the deficiency of moment release along the Vema and 12° 12′S multiple transform system may be due to most of the plate motion occurring aseismically.     

Dehydration kinetics of muscovite by in situ infrared microspectroscopy

Dehydration behavior of muscovite flake was investigated at 760–860°C by using in situ high-temperature IR microspectroscopy for the OH absorption band around 3,620 cm⁻¹. Isothermal kinetic heating experiments at each temperature gave detailed decrease curves of the OH band area with time. These curves have been simulated by the first and second order reactions or mono- and two-dimensional diffusion processes. The mono-dimensional diffusion was found to give the best fit to the experimental data and apparent diffusion coefficients D were determined at 760–860°C with the activation energy of 290 ± 20 kJ/mol. The apparent diffusion coefficients D varied with the sample thickness L. This variation can be explained by an m layers model with a unit length of L′ with a constant diffusion coefficient D′. Therefore, the dehydration process might be rate-limited by mono-dimensional diffusion through tetrahedral silicate sheet perpendicular to (001) planes of muscovite with a unit length of L′.     

The initiation and linkage of surface fractures above a buried strike-slip fault: An experimental approach

Surface fractures in the overburdened sedimentary rocks, formed above a deep-seated basement fault, often provide important information about the kinematics of the underlying master fault. It has already been established that these surface fractures dynamically evolve and link one another with progressive displacement on the master fault below. In the present study, two different series of riedel-type experiments were carried out with clay analogue models under different boundary conditions viz., (i) heterogeneous simple shear of the cover rocks above a buried strike slip fault (wrench system) and (ii) heterogeneous simple shear with a component of shear-normal compression of the overburden package above a basement fault (transpressional system), to observe the initiation and linkage of surface fractures with varying T′ (where T′ = thickness of the overburden normalized with respect to the width of the master fault). In the wrench system, Riedel (R) shears were linked by principal displacement (Y) shears at early stages (shear strain of 0.8 to 1) in thin (2 < T′ < 18) models and finally (at a minimum shear strain of 1.4) gave rise to a through-going fault parallel to the basement fault without development of any other fracture. Conjugate Riedel (R′) shears develop only within the thicker (T′ > 18) clay models at a minimum shear strain of 0.7. With increasing deformation (at a minimum shear strain of 1.2) two R′ shears were joined by an R shear and finally opened up to make a sigmoidal vein with an asymmetry antithetic to the major fault-movement sense. Under transpression, the results were similar to that of heterogeneous simple shear for layers 2 < T′ < 15. In layers of intermediate thickness (15 < T′ < 25) early formed high angle R shears were cross cut by low angle R shears (at a minimum shear strain of 0.5 and shortening of 0.028) and “Riedel-within-Riedel” shears were formed within thick (T′ > 25) models (at minimum shear strain of 0.7 and shortening of 0.1), with marked angularity of secondary fault zone with the master fault at depth.     

Viscoelastic behaviour of mica-based glass-ceramic aggregate

The present study deals with the small strain torsion deformation of MACOR glass-ceramic samples at high temperatures (450–850 °C) and over a range of low frequencies (20 Hz–5 mHz). The samples of MACOR ceramic consist of 55 vol% randomly oriented, sheet-like fluorophlogopite mica crystals (∼100–20 μm in planar size, 1–2 μm in thickness) and 45 vol% of isotropic alumino-borosilicate glass matrix. Measurements of the complex shear modulus show that the sample does not possess the relaxed shear viscosity even at temperatures above the glass transition temperature of the glass matrix. The maximum of the imaginary component G ^′′() of the shear modulus is ∼0.15 of the unrelaxed value G _∞, the relaxation strength Δ≈0.9. The activation energy of the peak of G ^′′() is ∼245 kJ mol⁻¹. Using this value of E _a , the data obtained at various frequencies and temperatures have been reduced to a master curve using the dimensionless variable ωτ, where ∼₀ exp(−E _a /RT). The internal friction Q⁻¹(ωτ) is ∝1/()^0.35−0.4 in the low-temperature high-frequency range (1); passes through a maximum at ∼1 and trends asymptotically to a value Q⁻¹∼0.25–0.30 at ≪1. The behaviour of Q ⁻¹(ωτ) differs from that of a Caputo body by the presence of the resolved peak which may be attributed to the slow mechanical relaxation of mica crystals due to rotation as well as flexing and bending modes of crystal deformation. Received: 26 June 1998 / Revised, accepted: 13 January 1999     

Variability in the surface wind direction at a coastal site of complex terrain

Variability in the standard deviation of surface wind direction (σ_θ), under different Pasquill stability regimes on diurnal, seasonal and interannual scales has been investigated making use of a 10-year data set collected at Visakhapatnam (17°42′ N., 82° 18′ E) during January, April, August and October for winter, pre-monsoon, monsoon and post-monsoon seasons respectively. The diurnal scale variability in σ_θ is more pronounced during day time than in night. The seasonal variability in σ_θ is only moderate around noon while relatively large fluctuations are noticed on inter-annual scale only during day time in January and August. The seasonal dispersion in σ_θ decreased from most unstable regime to most stable regime.     

Atmospheric boundary layer studies at Jodhpur during MONTBLEX using sodar and tower

A monostatic sodar was set up at Jodhpur, near the western end of the monsoon trough, to investigate the atmospheric boundary layer dynamics. A 30 m instrumented tower was also located close to the sodar antenna. Data were collected from June to August during the monsoon period of 1990, as also from July 1992 to September 1993. Thermal plumes, surface-based stable layers (both flat or short spiky top and tall spiky top), elevated/multi-layers with or without undulations and dot echo structures were seen; however, erosion of the morning inversion layer in the form of a rising layer with growing thermal plumes under it was rarely seen, and that too only during the winter period. The observed structure of the stable layer with tall spikes and its depth have been found to be correlated with the intensity of the monsoon spell; the dot echoes have been found to be correlated with the approach of a monsoon depression near Jodhpur; and the elevated/multilayers have been attributed to the formation of a subsidence (shear instability).     

Room-temperature equation of state of Li<Subscript>2</Subscript>VOSiO<Subscript>4</Subscript> up to 8.5 GPa

High-pressure single-crystal X-ray diffraction measurements of lattice parameters of the compound Li₂VOSiO₄, which crystallises with a natisite-type structure, has been carried out to a pressure of 8.54(5) GPa at room temperature. Unit-cell volume data were fitted with a second-order Birch-Murnaghan EoS (BM-EoS), simultaneously refining V ₀ and K ₀ using the data weighted by the uncertainties in V. The bulk modulus is K ₀ = 99(1) GPa, with K′ fixed to 4. Refinements of third order equations-of-state yielded values of K′ that did not differ significantly from 4. The compressibility of the unit-cell is strongly anisotropic with the c axis (K ₀(c) = 49.7 ± 0.5 GPa) approximately four times more compressible than the a axis (K ₀(a) = 195 ± 3 GPa).     

A New Approach to the Modeling of the Pressure Dependency of the Strength of Rocks

Based on the hypothesis that shearing stresses and normal tensile stresses both play a decisive role in fracturing and brittle failure of rock material, a novel strength criterion was developed in one of the earlier works of the author. In the criterion, a certain parameter ν′ occurs which depends on the structure of the material. Originally, the parameter was treated as a constant, which resulted in a linear form of the strength function ^F σ ₁ = f(σ ₃), where σ ₃ = σ ₂. Although the linear strength criterion is sometimes found to be applicable to various particular rock materials, it is not, in general, of a universal character. Analysis of the triaxial test results for 75 different sandstones revealed that parameter ν′ usually increases in an exponential or linear manner as confining pressure increases, and only in isolated cases does it seem to be independent of the confining pressure. For these three types of function ν′ = f(p) appropriate strength criteria ^F σ ₁ = f(σ ₃) are given in the present paper. These criteria were used to fit all of the collected empirical data sets. In general, a very good fit to the data was obtained.     

Elastic behaviour and structural evolution of topaz at high pressure

The elastic behaviour and the high-pressure structural evolution of a natural topaz, Al_2.00Si_1.05O_4.00(OH_0.26F_1.75), have been investigated by means of in situ single-crystal X-ray diffraction up to 10.55(5) GPa. No phase transition has been observed within the pressure range investigated. Unit-cell volume data were fitted with a third-order Birch-Murnaghan Equation of State (III-BM-EoS). The III-BM-EoS parameters, simultaneously refined using the data weighted by the uncertainties in P and V, are: V ₀=345.57(7) ų, K _T0=164(2) GPa and K′=2.9(4). The axial-EoS parameters are: a ₀=4.6634(3) Å, K _T0(a)=152(2) GPa, K′(a)=2.8(4) for the a-axis; b ₀=8.8349(5) Å, K _T0(b)=224(3) GPa, K′(b)=2.6(6) for the b-axis; c ₀=8.3875(7) Å, K _T0(c)=137(2) GPa, K′(c)=2.9(4) for the c-axis. The magnitude and the orientation of the principal Lagrangian unit-strain ellipsoid were determined. At P−P ₀=10.55 GPa, the ratios ε₁:ε₂:ε₃ are 1.00:1.42:1.56 (with ε₁||b, ε₂||a, ε₃||c and |ε₃| > |ε₂| > |ε₁|). Four structural refinements, performed at 0.0001, 3.14(5), 5.79(5) and 8.39(5) GPa describe the structural evolution in terms of polyhedral distortions.     

A note on radioactive riverine placers around Midtul,Bastar craton,Central India

Reconnaissance survey in the Midtul area (toposheet no. 65F/1; 18°52′30″N: 81°09′55″E) of the Bastar district, Chhattisgarh, revealed well-developed, 1 to 4 m thick riverine placers, derived from granitic terrain in the environs of the Mari River. The sediments comprise largely unconsolidated, cross-stratified gravels, sands, and silts, with low flat-topped surfaces which, at places, correspond to the morphology of terraces that are preserved on convex banks of the present day meanders, indicating their deposition by fluvial processes as channel bars. Available field data suggest deep-seated secular weathering of granites of the area, extensive reworking of such weathered products by fluvial processes, and their deposition as riverine placers in the vicinity of the streams draining the area. The granitic rocks of the area record radioactivity of 0.01–0.02 milli Roentgen per hour (mR/hr), with spots of higher radioactivity of 0.04–0.08 mR/hr, whereas riverine placers derived from them show radioactivity up to 0.25 mR/hr. Hand-panned polymineral concentrates from such placer deposits, analyzed by wavelength-dispersive x-ray fluorescence (WDXRF) spectrometry, showed 0.006–0.67 (n = 47; av. 0.092) % Y₂O₃ and 0.019–4.729 (n = 46; av. 0.834) % Ce₂O₃, with low values of Nb (41–467; n = 15; av. 197 ppm), Ta (115–598, n = 13; av. 230 ppm), and Sn (17–468; n = 14; av. 104 ppm). From the observed heavy mineral assemblage of monazite, zircon, ilmenite, epidote, hematite, and magnetite, as revealed by x-ray diffraction (XRD) studies, it is clear that the radioactivity is contributed by cerium, besides monazite (a phosphate of La, Ce, Th), and yttrium, possibly by zircon. Small amounts of niobium, tantalum, and tin are considered to be associated with ilmenite, epidote, and biotite. An occasional higher value of cerium (∼5% Ce₂O₃), together with restricted higher radioactivity, indicates only localized and uneconomic concentration of light REE-bearing mineral, monazite in isolated patches in such riverine placers.