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Neoproterozoic island arc assemblage of the Arabian–Nubian Shield (ANS) in the Eastern Desert (ED) of Egypt comprises juvenile suites of metavolcanics (MV), large amounts of meta-sedimentary rocks (MS), and voluminous metagabbros-diorites (MGD) and syn-tectonic intrusions of older granitoids (OG). We report here the updates of these four rock units in terms of classification, distribution, chemical characteristics, geodynamic evolution, metamorphism, and ages. In addition, we discuss these integrated data to elucidate a reasonable and reliable model for crustal evolution in the ANS. The main features of these rock units indicate their relation to each other and the geodynamic environment dominated by early immature oceanic island arcs to primitive continental arcs. Integrated information of the island arc metavolcanic and plutonic rocks (gabbros, diorites, tonalites, and granodiorites) furnish evidence of the genetic relationships. These include proximity and a coeval nature in the field; all protolith magmas are subalkaline in nature following calc-alkaline series with minor tholeiitic affinities; common geochemical signature of the arc rocks and subduction-related magmatism; their similar enrichment in LREEs; and similar major element compositions with mafic melts derived from metasomatized mantle wedge. The volcano-sedimentary and the OG rocks underwent multiphase deformation events whereas the MGD complexes deformed slightly. Based on the magmatic, sedimentological, and metamorphic evolutions constrained by geochronological data as well as the progressive evolutionary trend from extensional to compressional regimes, a possible gradual decrease in the subducted slab dip angle is the most influential in any geodynamic model for arc assemblage in the ED of Egypt. [ABSTRACT FROM AUTHOR]

In the south Eastern Desert of Egypt, two contrasting types of magmatism (mafic and felsic) are recorded in the Wadi Kalalat area, and form the Gabal El Motaghiarat and Gabal Batuga intrusions, respectively. The two intrusions post-dates ophiolitic and arc associations represented by serpentinite and metagabbro-diorite, respectively. The mafic intrusion has a basal ultramafic member represented by fresh peridotite, which is followed upward by olivine gabbro and anorthositic or leucogabbro. This mafic intrusion pertains to the Alaskan-type mafic–ultramafic intrusions in the Arabian–Nubian Shield (ANS) being of tholeiitic nature and emplaced in a typical arc setting. On the other hand, the Gabal Batuga intrusion comprises three varieties of fresh A-type granites of high K-calc alkaline nature, which is peraluminous and garnet-bearing in parts. A narrow thermal aureole in the olivine gabbro of the mafic intrusion was developed due to the intrusion of the Batuga granites. This results in the development of a hornfelsic melagabbro variety in which the composition changed from tholeiitic to a calc-alkaline composition due to the addition of SiO2, Al2O3, alkalis, lithosphile elements (LILEs) such as Rb (70 ppm) and Y (28 ppm) from the felsic intrusion. Outside the thermal aureole, Rb amounts 2–8 ppm and Y lies in the range < 2–6 ppm. It is believed that the Gabal Batuga felsic intrusion started to emplace during the waning stage of an arc system, with transition from the pre-collisional (i.e., arc setting) to post-collisional and within plate settings. Magma from which the Gabal Batuga granites were fractionated is high-K calc-alkaline giving rise to a typical post-collisional A-type granite (A2-subtype) indicating an origin from an underplating crustal source. Accordingly, it is stressed here that the younger granites in the ANS are not exclusively post-collisional and within-plate but most likely they started to develop before closure of the arc system. The possible source(s) of mafic magmas that resulted in the formation of the two intrusions are discussed. Mineralogical and geochemical data of the post-intrusion dykes (mafic and felsic) suggest typical active continental rift/within-plate settings. [ABSTRACT FROM AUTHOR]

El Bakriyah Ring Complex (BRC) is a prominent Neoproterozoic post-collisional granite suite in the southern part of the Central Eastern Desert of Egypt. The BRC bears critical materials (F, B, Nb, and Ta) in appreciable amounts either in the form of rare-metals dissemination or in the form of fluorite and barite vein mineralization. The complex consists of inner syenogranite and outer alkali feldspar granite that have been emplaced in a Pan-African assemblage made up of granitic country rocks (granodiorite and monzogranite), in addition to post-collisional fresh gabbro as a part of the Arabian-Nubian Shield (ANS) in northeast Africa. Granites of the BRC are characterized by enrichment in silica, alkalis, Rb, Y, Ga, Nb, Ta, Th, and U and depletion in Sr, Ba, and Ti. Geochemical characterization of the BRC indicates that the magma is a crustal melt, which originated from the partial melting of metasedimentary sources. Concentrations of rare-earth elements (REEs) differ in magnitude from the ring complex and its granitic country rocks but they have similar patterns, which are sub-parallel and show LREEs enrichment compared to HREEs. The presence of a negative Eu anomaly in these rocks is related to plagioclase fractionation. The abundance of fluorine (F) in the different granite varieties plays an important role in the existence of a tetrad influence on the behavior of REEs (TE1, 3 = up to 1.15). Geochemical parameters suggest the crystallization of the BRC granite varieties by fractional crystallization and limited assimilation. Mn-columbite and Mn-tantalite are the most abundant rare-metals dissemination in the BRC granite varieties. We present combined field, mineralogical and geochemical data that are in favor of magma originating from a metasedimentary source for the BRC with typical characteristics of A-type granites. Our geodynamic model suggests that the Gebel El Bakriyah area witnessed the Neoproterozoic post-collisional stage of the ANS during its late phase of formation. This stage was characterized by the emplacement of fresh gabbros followed by the syenogranite and alkali-feldspar granite of the BRC into an arc-related assemblage (granodiorite and monzogranite). It is believed that the mantle-derived magma was interplated and then moved upward in the extensional environment to a shallower level in the crust owing to events of lithospheric delamination. This presumably accelerated the processes of partial melting and differentiation of the metasedimentary dominated source (Tonian-Cryogenian) to produce the A-type granites building up the BRC (Ediacaran). [ABSTRACT FROM AUTHOR]

Since Antiquity, sustainable resources of gold and copper have been mined at two prominent prospects in the north Eastern Desert of Egypt, namely the south Gabel Um Monqul (SGUM) and Gabal Al Kharaza (GKZ). Mineralization is hosted by Neoproterozoic shield rocks represented by dacite and monzo- to syenogranite at the SGUM prospect whereas they are diorite, granodiorite, and quartz feldspar porphyry at the GKZ prospect. These hosts have been emplaced in an island arc environment from calc-alkaline magmas with a peraluminous to metaluminous signature. They are hydrothermally altered including albitization, sericitization, silicification, epidotization, and chloritization. The Au and Cu mineralizations are confined to shear zones that lately filled with auriferous quartz veins adjacent to mineralized alteration zones. In the GKZ prospect, the old trenches trend mainly in a NW–SE direction whereas it is NE–SW and NW–SE in the SGUM prospect. Evidence of shearing ranges from megascopic conjugate fractures and shear planes in the outcrops to microscopic sheared and crumbled Au-Cu ore assemblages dominated by Fe-Cu sulfides, specularite, and barite. Microscopic investigation suggests that the formation of specularite is due to the shearing of early existing magnetite. The ore textures and paragenetic sequence indicate that pyrite in the alteration zones is oxidized, leading to the liberation of gold up to 3.3 g/t. The formulae of the analyzed electrum lie in the range Au74.5-76.8 Ag22.2-24.5. Integration of the field, geochemistry, and mineral chemistry data, combined with the gold fire assay data prove the presence of sustainable amounts of disseminated Au and Cu, not only in the mineralized quartz veins, but also in the alteration zones. Data materialized in our paper show similarities in the style of mineralization at the SGUM and the GKZ prospects with iron oxide-copper-gold (IOCG) deposits elsewhere in the Arabian-Nubian Shield (ANS) and other world examples. [ABSTRACT FROM AUTHOR]

In this paper, characterization and discrimination of some silicate gems (gemstones and low-gem quality varieties) from ancient Egyptian mines have been performed for the first time adopting molecular and elemental spectroscopic techniques. The selected gem groups are peridot, beryl/emerald, amazonite, and amethyst. In this sense, characterization of the genuine Egyptian gems and the importance of mineralogical and chemical signatures in a historical context as well as the scope of geoheritage can be achieved for the first time. Laser induced breakdown spectroscopy (LIBS) analysis has been found to be an effective method for the discrimination among different gems groups where a characterizing element for each group in a specific spectral window has been found. Raman and Fourier transform infrared (FTIR) spectroscopy spectra have proven to be fingerprints with the ability to distinguish future alteration of the gemstone depending on the molecular vibrational spectroscopy. FTIR provides the functional group that can absorb the infrared radiation and be responsible for the coloring of the gemstones. [ABSTRACT FROM AUTHOR]

In this work, we present new field observations, geochemical data, mineral chemistry, and interpretations from the late Neoproterozoic El-Bakriya mafic intrusion in the central Eastern Desert of Egypt to enhance our knowledge and understanding of the formation of the juvenile continental crust in the Arabian-Nubian Shield (ANS). The field relationships indicate that the gabbroic intrusion is younger than the syntectonic granodiorite but older than the postcollisional El-Bakriya granites. The El-Bakriya mafic intrusion is neither deformed nor metamorphosed and preserves typical primary mineralogical features and texture. The intrusion is composed of three main varieties of gabbro with prominent gradational contacts, namely, troctolite, olivine gabbro, and hornblende gabbro. Both megascopic and imperceptible layering is recognized in the intrusion. El-Bakriya gabbros are characterized by an enrichment in large-ion lithophile elements relative to high field strength elements and a noticeable alkaline affinity owing to fractional crystallization. Different varieties of gabbro show continuous linear composition trends in all major- and trace-element variation diagrams, indicating a cogenetic origin source. The initial magma was an alkaline mafic melt that derived from the mantle with minor crustal assimilation before its emplacement in the upper crust in a stable postcollisional cratonic setting. Mineralogical and geochemical data are combined evidence of an asthenosphere upwelling and lithosphere delamination scenario for the evolution of the El-Bakriya mafic intrusion. On the basis of geothermobarometric calculations, the gabbroic rocks crystallized at a pressure of 5–6.8 kbar and a temperature of 750°C–900°C. The petrological, geochemical, and mineralogical characteristics of the El-Bakriya mafic intrusion are akin to those of the Egyptian postcollisional younger gabbros. These gabbros are totally different from the metamorphosed oceanic and arc-related gabbros, which constitute the earlier juvenile crustal components that formed during the development of the ANS in the late Neoproterozoic. [ABSTRACT FROM AUTHOR]

Tourmaline occurrences are found in the Eastern Desert of Egypt, particularly at the Wadi El-Gemal area, which is the locality of famous beryl mines dated back to the Roman times including the so-called "Cleopatra's Emerald Mines". The collected tourmaline occurs in beryl-rich pegmatites and quartz veins. Infrared data of tourmaline indicate that dravite from three localities (Wadi Sikait, Wadi Um Sleimat and Wadi Um Addebbaa) can be distinguished based on the nature of the OH group and its vibrational bands. Also, reflectance spectra of fresh and altered dravite are distinguishable. Mineral inclusions (mostly hafnon zircon and REE-bearing apatite and monazite) in dravite affects the FTIR (Fourier transform infrared) spectra. Such spectra show influence of cationic substitutions in dravite, particularly M2+at the X-site and M3+ at the Y-site. Spectrometric measurements suggest that radioelements in tourmaline are not considerably high, whereas high radioactivity is restricted to the pegmatite bodies away from the metasomatic front with the serpentinite country rocks. [ABSTRACT FROM AUTHOR]

Mineralogical and geochemical characteristics of stream sediments collected from 30 stations in the Dahab-Wadi Kid area (southeastern Sinai, Egypt) are presented. The studied sediments contain considerable amounts of heavy minerals, including abundant Fe–Ti oxides, with up to 25.94% index figure that measures the relative abundance of heavy minerals to light minerals. Immaturity of the sediments and nature of Fe–Ti oxide intergrowths, in addition to some non-opaque heavy minerals, indicate multiple provenances with contributions from a variety of Neoproterozoic mafic to felsic intrusions. Appreciable amounts of heavy minerals are derived from schists that belong to the so-called Wadi Kid volcano-sedimentary group. Owing to the considerable contribution of the felsic intrusions represented by within-plate granites, the sediments are enriched in REE-bearing minerals (e.g., monazite, cerianite, La-cerianite and allanite), V-bearing thorite, zircon, and a ZrO2 phase. The majority of Fe–Ti oxides are derived from the mafic intrusions, and they show evidence of pre-, syn- and post-depositional phases of alteration and formation of intergrowths. As to the geochemical signature of the bulk silt fraction, there is remarkable enrichment of LREEs with distinct negative Eu anomaly, and this supports dominance of a provenance dominated by felsic intrusions (i.e., the A-type within-plate granite and associated pegmatites). Also, the felsic provenance is responsible for slight abnormality of ∑U + Th content (up to ~ 37 ppm) and few anomalous samples bear uraniferous zircon and Ce-type monazite. The present paper sheds light on the fertility of the Dahab stream sediments in some economic minerals for possible future exploitation, not for gold only, but for potential REE-bearing minerals that are needed for a wide spectrum of modern technological industries. [ABSTRACT FROM AUTHOR]

The paper presents new insights to gold mineralization at the Jabal Umm Watirah (JUW) area in western Saudi Arabia using surface and sub-surface information from field, mineralogical, and geochemical studies. Host rocks are Neoproterozoic serpentinites and arcmeta-andesite. Gold in serpentinites is enriched in listwaenite that results from silica and CO2 metasomatism. In the meta-andesite, prominent NW-SE alteration zones follow the Najd fault system (NFS) along the Ruwah fault zone (RFZ). These alterations are sulfide-rich and show sericitization, chloritization, silicification, and hematitization. The auriferous pyrite-rich listwaenite contains relict of Cr-spinel of the ophiolitic serpentinite precursor. Anomalous gold up to 10 g/t in listwaenite is encountered in one borehole whereas the rest assay up to 0.27 g/t with the exception of silicified serpentinite and quartz–carbonate rocks with up to 0.59 and 0.57 g/t gold, respectively. Hydrothermally altered arc meta-andesite contains up to ~ 25% sulfides (mostly pyrite, arsenopyrite, and chalcopyrite) with up to 1.62 wt% As2O5 and up to ~ 21 g/t gold. The results indicate that arsenic is a pathfinder for the studied gold mineralization. On the other hand, fresh meta-andesite is very poor in gold, up to 0.05 g/t only. Remobilized visible gold ore can be seen in some quartz veins and stringers with 150–170 μm wide gold along fractures. Gold enrichment is confirmed in the meta-andesite from boreholes up to ~ 250 m deep that are dug beneath ancient workings and dumps at the two sectors in the JUW prospect. Gold mineralization in the JUW prospect is structurally controlled orogenic gold mineralization because of the following reasons: (1) proximity to the major first order structure, (2) host rocks are metamorphosed in the greenschist facies conditions, and (3) listwaenitization by carbonic and silicic metasomatism of the ophiolitic serpentinite. [ABSTRACT FROM AUTHOR]

Beryl (mostly emerald) was mined in Egypt since Antiquity, particularly the so-called "Cleopatra's Mines" in the Eastern Desert were a major source of emerald in the world during the Roman era in northeast Africa. Emerald from the Nugrus belt is located at Sikait, Um Sleimat, Um Addebbaa and Um Kabu areas where the ancient mines are confined structurally and genetically to "schist-type" deposits as a part of the Neoproterozoic ophiolitic mélange. At Homret Mikpid and Homr Akarem occurrences, non-gem beryl occurs in greisinized granites, pegmatites and quartz veins. Fourier transform infrared (FTIR) spectroscopy is able to distinguish Egyptian emeralds from non-gem beryls. Different types of H2O molecules and OH-groups can be identified in all especially at ~ 3435–3480 cm−1 as an indication of X-site occupancy of Na and vacancy (□) and to the Y-site M2+ and M3+ cation distribution. High wavenumber bands > 3600 cm−1, e.g. at ~ 3865–3988 cm−1 are characteristics of non-gem beryl only. Structural CO2 molecules are seen in both emerald and beryl at ~ 2360 cm−1. About 97–99% (at a wavelength range of 335–611 nm.) infrared reflectance of non-gem beryl is much higher (97–99%) whereas it never exceeds ~ 88% (at ~ 360 nm) in emerald. [ABSTRACT FROM AUTHOR]

The paper concerns with potential utilization of Landsat-8 data combined with TerraSpec spectro-radiometer and field studies for mapping and discriminating the different granitic varieties of Batuga pluton in the South Eastern Desert of Egypt. Integrated data of band combination, principal component analysis (PCA), band rationing, fusion technique, and TerraSpec measurements substantially improve visual interpretation to differentiate between granite varieties as well as basalt dikes traversing the pluton. Compilation of field, petrographic, and spectral data enables to distinguish three varieties of granite, namely alkali feldspar granite, garnet-bearing leucogranite, and two-mica granite (monzogranite). False color composite (FCC) image PC1, PC3, and PC4 in R, G, and B, respectively, is successful to discriminate the western part of Batuga pluton as mica granite (monzogranite) but it failed to discriminate the other two granite types. On the other hand, the band ratio combination 6/7, 5/6, and 2/4 successfully used to discriminate the three varieties of granite. Also, the image fusion technique of false color composite (7, 5, & 2) and band ratio (6/7, 5/6, & 2/4) combinations with the high spatial resolution band 2 of Sentinel-2 is applied using the Gram-Schmidt method. Regarding with TerraSpec measurements, the alkali feldspar granite displays two absorption features around 1.4 and 2.25 μm. Moreover, the garnet-bearing leucogranite shows four little absorption features at 1.4, 2.21, 2.25, and 2.33 μm, while the mica granite shows low reflectance values throughout the VNIR and SWIR regions compared to the other two varieties, and low four absorption features at 1.4, 2.21, 2.25, and 2.33 μm wavelengths. The basalt dikes show a low, nearly flat spectral profile in the VNIR and SWIR wavelength regions and small four absorption features also observed around the 0.65, 1.0, 1.2, and 2.32 μm wavelengths. Data materialized in the paper suggests the usefulness of Landsat-8 and TerraSpec spectroradiometer data as a powerful combination to discriminate the granitic rocks and the traversing basalt dikes as well. [ABSTRACT FROM AUTHOR]

Metagabbros and gabbros in the Ablah-Shuwas belt (western Saudi Arabia) represent part of significant mafic magmatism in the Neoproterozoic Arabian Shield. The metagabbros are Cryogenian, occasionally stratified and bear calcic amphiboles (hornblende, magnesio-hornblende and actinolite) typical of calc-alkaline complexes. These amphiboles suggest low pressure (~1-3 kbar), high $$f_{{{\text{O}}_{2} }}$$ and crystallization temperature up to 727 °C, whereas it is 247-275 °C in the case of retrograde chlorite. Rutile and titanite in metagabbros are Fe-rich and replace Mn-bearing ilmenite precursors at high $$f_{{{\text{O}}_{2} }}$$ . On the other hand, younger gabbros are fresh, layered and comprised of olivine gabbro and olivine-hornblende gabbro with an uppermost layer of anorthositic gabbro. The fresh gabbros are biotite-bearing. They are characterized by secondary magnetite-orthopyroxene symplectitic intergrowth at the outer peripheries of olivine. The symplectite forms by deuteric alteration from residual pore fluids moving along olivine grain boundaries in the sub-solidus state. In fresh gabbros, ortho- and clinopyroxenes indicate crystallization at 1300-900 and 800-600 °C, respectively. Geochemically, the Cryogenian metagabbros (~850-780 Ma) are tholeiitic to calc-alkaline in composition and interpreted as arc-related. Younger, fresh gabbros are calc-alkaline and post-collisional (~620-590 Ma, i.e., Ediacaran), forming during the late stages of arc amalgamation in the southern Arabian Shield. The calc-alkaline metagabbros are related to a lithospheric mantle source previously modified by subduction. Younger, fresh gabbros were probably produced by partial melting of an enriched mantle source (e.g., garnet lherzolite). [ABSTRACT FROM AUTHOR]

The present work deals with some Pan-African serpentinites of Neoproterozoic age from five localities in the Eastern Desert of Egypt namely, Abu Fannani, Fawakhir, Barramiya, Ras Shait and Wadi Ghadir that derivedmostly from lherzolite to harzburgite protoliths. The M-value of antigorite is anindicator of the metamorphic grade which is lowest at Fawakhir (greenschist facies) and highest at Abu Fannani (lower amphibolite facies). Antigoritization during progressive regional metamorphism at Fawakhir is limited and its M-value is much higher than 8.52 indicating crystallization temperatures of 220-250 C whereas it is ≥300 C in the rest. Antigorite recrystallized at T ≈ 400-450 C in the contact metamorphic aureoles due to the emplacement of post-orogenic leucogranites at Fawakhir and Gebel Ghadir. Towards the contact with the granites, M-value of antigorite is low (6.48) compared to 8.52 in the least recrystallized antigorite due to the thermal effect. Antigorite that forms in the thermal aureoles is characterized by two types of substitution; Tschermak substitution (Al and Cr for Si in the tetrahedral sites and Mg in the octahedral sites) and non-Tschermak substitutionin the octahedral sites (2R = 3R). Generally, both ortho- and clinochrysotiles are common with MgO contents of 41-41.13 wt% and 39.38-40.93 wt%, respectively. In addition to chlorite, high- and low-Al lizardites are present with almost constant total iron (~0.24 cations) as Fe(vi) and variable Mg. This suggests that significant R in the octahedral sites are mostly occupied by Al and Cr and not iron. In the lizardite structure, there is Tschermak substitution of Si by some trivalent cations in the tetrahedral sites. Using the TEM images, antigorite is distinguishable from chrysotile and they suggest the presence of polyhedral or polygonal serpentine (spherical or circular with alternating sectors of lizardite). Crack-seal microstructures are displayed by chrysotile and brucite veins developed by extensional shearing that post-dates antigoritization. [ABSTRACT FROM AUTHOR]

The Wadi Hamama area is a volcanogenic Zn-Cu-Au-Ag prospect. It is hosted by a Neoproterozoic bimodal-mafic sequence, which comprises basalt, dacite and rhyolite along with volcaniclastic rocks. The rocks have a low-K tholeiitic affinity and are enriched in large ion lithophile elements over high field strength elements, which indicated their formation in an intra-oceanic island arc tectonic setting. The area was intruded by a tonalite-trondhjemite body, which has an intra-oceanic island arc affinity and later by diorite, which has a cordilleran-margin geochemical affinity. These rock units were intruded by post-tectonic granite dykes, which have a within-plate geochemical signature. There is a quartz-carbonate horizon extending along the contact between the basalt and the volcaniclastic rocks, mainly banded and lapilli tuffs. This horizon is of exhalative origin and is underlain by a mushroom-shaped alteration zone extending from the horizon down to the massive basalt. The footwall alteration is characterized by a silica-rich core surrounded by a thick chlorite sheath. Both the quartz-carbonate horizon and the footwall-altered rocks enclose historical trenches and pits. Sulfide-rich core samples are enriched in Zn, relative to Cu, and in Ag, which indicates the low-temperature nature of the hydrothermal system. The prospect was affected by supergene processes, which led to the widespread occurrence of secondary copper minerals and gold enrichment relative to the leached base metals, especially Zn. The prospect formed through a limited rifting of an intra-oceanic island arc which resulted in the formation of a small-scale volcanogenic Zn-Cu-Ag-Au prospect. [ABSTRACT FROM AUTHOR]

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Fe-rich "oxydravite" and dravite from the Late Proterozoic ophiolitic mélange of the Arabo-Nubian Shield, located in Egypt and Saudi Arabia, were structurally and chemically characterized by using crystal structure refinement based on single-crystal X-ray diffraction data, electron microprobe analysis, and Mössbauer spectroscopy. Structural formulae obtained by optimization procedures indicate disordering of Al, Mg, and Fe2+ over the Y and Z sites, and an ordering of Fe3+ at Y. The disordering can be explained by the substitution mechanisms 2YMg+ZAl+WOH = 2YAl+ZMg+WO²- and 2YFe2++ZFe3++WOH = 2YFe3++ZFe2++WO²-, which are consistent with reducing the mismatch in dimensions between YO6 and ZO6 octahedra. To explain the Mg-Al disordering process, as well as the occurrence of B at the T site in tourmaline, analogies have been drawn between the crystal structure of tourmaline and that of lizardite. A critical constraint in both structures is the geometrical fit of the six-membered tetrahedral ring with the attached group of three YO6 octahedra. In tourmaline, the disordering of Mg and Al over Y and Z relieves the strain due to the misfit in dimensions of the larger triads of edge-sharing MgO6 octahedra and the smaller Si6O18 tetrahedral rings. In Al-rich tourmaline, where the octahedral cluster is smaller, the strain can be relieved by incorporating B in the tetrahedra. An opposite effect is observed by substitution of Al for Si at the tetrahedral site in Mg-rich tourmaline. Because the Al radius is intermediate between those of Mg and Si, Al plays an important structural role in accommodating the potential misfit between YO6, ZO6, and TO4 polyhedra. The amount of Al and its distribution in the structure strongly affects the values of the unit-cell parameters of tourmaline and yields volume variations according to a quadratic model. This results from the effect of ZAl combined with the occurrence of B at T in Al-rich tourmaline. ZAl has a greater effect than YAl as long as Al does not fully occupy the Z site. [ABSTRACT FROM AUTHOR]