滇西小龙潭矿区富碱斑岩地球化学特征与成因

岩石地球化学分析表明小龙潭斑岩高钾(K_2O/Na_2O=0.85~8.39)、富碱(K_2O+Na_2O=9.05%~12.45%),总体属于钾玄岩系列的岩石。岩石富集Rb、Ba、Th、LREE等大离子亲石元素而亏损Ti、Nb、Ta、HREE等高场强元素,具有较高的ΣLREE/ΣHREE比值和(La/Yb)N比值,具中等程度Eu负异常。从岩石地球化学特征分析,小龙潭富碱斑岩起源于金沙江洋壳俯冲交代作用形成的富集地幔,富碱斑岩可能与Cu、Mu、Ag成矿作用密切相关。     

Copper(II) sorption onto goethite, hematite and lepidocrocite: a surface complexation model based on ab initio molecular geometries and EXAFS spectroscopy

We measured the adsorption of Cu(II) onto goethite (α-FeOOH), hematite (α-Fe₂O₃) and lepidocrocite (γ-FeOOH) from pH 2-7. EXAFS spectra show that Cu(II) adsorbs as (CuO₄H_n)ⁿ⁻⁶ and binuclear (Cu₂O₆H_n)ⁿ⁻⁸ complexes. These form inner-sphere complexes with the iron (hydr)oxide surfaces by corner-sharing with two or three edge-sharing Fe(O,OH)₆ polyhedra. Our interpretation of the EXAFS data is supported by ab initio (density functional theory) geometries of analogue Fe₂(OH)₂(H₂O)₈Cu(OH)₄and Fe₃(OH)₄(H₂O)₁₀Cu₂(OH)₆ clusters. We find no evidence for surface complexes resulting from either monodentate corner-sharing or bidentate edge-sharing between (CuO₄H_n)ⁿ⁻⁶ and Fe(O,OH)₆ polyhedra. Sorption isotherms and EXAFS spectra show that surface precipitates have not formed even though we are supersaturated with respect to CuO and Cu(OH)₂. Having identified the bidentate (FeOH)₂Cu(OH)₂⁰ and tridentate (Fe₃O(OH)₂)Cu₂(OH)₃⁰ surface complexes, we are able to fit the experimental copper(II) adsorption data to the reactions

     

祁雨沟金矿田S、O、C、Pb同位素组成及成矿物质来源

祁雨沟金矿田是由以隐爆角砾岩型为主、石英脉型、构造破碎带蚀变岩型和斑岩－角砾岩型等金矿床组成的一个成矿亚系列．该矿田硫同位素组成变化范围较窄,δ３４ＳΣＳ＝－０．１７‰,硫来源于深沉岩浆;氧同位素变化范围为－４．３１％０～４．６０‰,主成矿期与幔源岩浆水氧同位素值相近,晚期出现负值,表明有大气降水参与成矿;成矿晚期联同位素的δ１３ＣΣＣ＝－３．８３‰,为岩浆碳与地层碳酸盐不同比例混合的结果;铅同位素研究表明．矿床铅为正常铅混合型,μ值较低,成矿铅主要来源于岩浆,有少量地层铅混入．经同位素、构造、地层、岩浆岩、矿物微量元素和包裹体等综合研究,表明本区金矿成矿物质主要来自壳幔层同熔的燕山期花岗质钙碱性岩浆,部分来自上地壳太华群变质岩．     

Hydrogen bonding in basic copper salts: a spectroscopic study of malachite,Cu2(OH)2CO3, and brochantite,Cu4(OH)6SO4

Infrared and Raman spectra of the basic copper salts malachite, Cu₂(OH)₂CO₃, and brochantite, Cu₄(OH)₆SO₄, as well as of deuterated and ¹³C substituted samples are presented and discussed in terms of group theory and the hydrogen bonds present. The main results are that (i) the hydrogen donor strengths of the OH^? ions are strongly increased due to the very great synergetic effect of the copper ions, (ii) the acceptor strengths of the H-bond acceptor groups (SO₄ ^2-, CO₃ ^2-, and OH^? ions) are significantly modified by the linkage and coordination of the acceptor atoms — this complicates true assignment of the OH bands observed to the two and six different OH^? ions present in malachite and brochantite, respectively -, and (iii) the Cu — O stretching modes at 430–590 cm^?1 and 420–520 cm^?1 for malachite and brochantite, respectively, exhibit strong, partially covalent Cu — O bonding.     

Aqualite,a new mineral species of the eudialyte group from the Inagli alkaline pluton,Sakha-Yakutia,Russia, and the problem of oxonium in hydrated eudialytes

Aqualite, a new eudialyte-group mineral from hydrothermally altered peralkaline pegmatites of the Inagli alkaline pluton (Sakha-Yakutia, Russia) is described in this paper. Natrolite, microcline, eckermanite, aegirine, batisite, innelite, lorezenite, thorite, and galena are associated minerals. Aqualite occurs as isometric crystals up to 3-cm across. The color is pale pink, with a white streak and vitreous luster. The mineral is transparent. The fracture is conchoidal. The mineral is brittle; no cleavage or parting is observed. The Mohs’ hardness is 4 to 5. The density is 2.58(2) g/cm³ (measured by the volumetric method) and 2.66 g/cm³ (calculated). Aqualite is optically uniaxial (+), α = 1.569(1) and β = 1.571(1). The mineral is pleochroic from colorless to pale pink on X and pink on Y, α < β. Aqualite is weakly fluorescent with a dull yellow color under ultraviolet light. The mineral is stable in 50% HCl and HNO₃ at room temperature. Weight loss after ignition at 500°C is 9.8%. Aqualite is monoclinic, and the space group is R3. The unit-cell dimensions are a = 14.078(3) Å, c = 31.24(1) Å, V = 5362 ų, and Z = 3. The strongest reflections in the X-ray powder pattern [d, Å (I)(hkl)] are: 4.39(100)(2005), 2.987(100)(315), 2.850(79)(404), 10.50(44)(003), 6.63(43)(104), 7.06(42)(110), 3.624(41)(027), and 11.43(39)(101). The chemical composition (electron microprobe, H₂O determined with the Penfield method) is as follows (wt %): 2.91 Na₂O, 1.93 K₂O, 11.14 CaO, 1.75 SrO, 2.41 BaO, 0.56 FeO, 0.30 MnO, 0.17 La₂O₃, 0.54 Ce₂O₃, 0.36 Nd₂O₃, 0.34 Al₂O₃, 52.70 SiO₂, 12.33 ZrO₂, O.78 TiO₂, 0.15 Nb₂O₅; 1.50 Cl, 9.93 H₂O,-O=Cl₂ 0.34; where the total is 99.46. The empirical formula calculated on the basis of Si + Zr + Ti + Al + Nb = 29 apfu is as follows: [(H₃O)_7.94Na_2.74K_1.20Sr_0.49Ba_0.46Fe_0.23Mn_0.12]_Σ13.18(Ca_5.79REE_0.19)_Σ5.98 (Zr_2.92Ti_0.08)_Σ3.0(Si_25.57Ti_0.21Al_0.19Nb_0.03)S_26.0[O_66.46(OH)_5.54]_Σ72.0 [(OH)_2.77Cl_1.23]_Σ4.0. The simplified formula is (H₃O)₈(Na,K,Sr)₅Ca₆Zr₃Si₂₆O₆₆(OH)₉Cl. Aqualite differs from typical eudialyte by the extremely low contents of Na and Fe, with more than 50% Na being replaced with the (H₃O)⁺ group. The presence of oxonium ions is confirmed by IR spectroscopic and X-ray single-crystal diffraction analysis. The mineral is compared with five structurally studied high-oxonium analogues from alkaline plutons of other regions. All of these minerals were formed at a relatively low temperature through the ion-exchange transformation of “protoeudialytes”; the successor minerals inherited the principal structural and compositional features of the precursor minerals. The name aqualite is derived from the Latin aqua in reference to its specific chemical composition. The type material of aqualite is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.     

Batagayite,CaZn2(Zn,Cu)6(PO4)4(PO3OH)3·12H2O,a new phosphate mineral from Këster tin deposit (Yakutia,Russia): occurrence and crystal structure

Mineralogy and Petrology - Batagayite, CaZn2(Zn,Cu)6(PO4)4(PO3OH)3·12H2O, is a new secondary phosphate mineral from the Këster deposit, Arga-Ynnykh-Khai massif, NE Yakutia, Russia. It is...     

Microanalysis of the iron oxidation state in (Mg,Fe)O and application to the study of microscale processes

We report application of the flank method using the electron microprobe to a suite of twelve (Mg,Fe)O samples with composition 2–47 wt% Fe and Fe³⁺/ΣFe = 1 to 11%, where Fe³⁺/ΣFe was determined independently using Mössbauer spectroscopy on the same grains used for the flank method measurements. A calibration curve of the form Fe²⁺ = A + B × (ΣFe)² + C × (Lβ/Lα) was fit to the data and gave excellent agreement between Fe³⁺/ΣFe calculated from the flank method and Fe³⁺/ΣFe determined using Mössbauer spectroscopy. We found the method to be sufficiently sensitive to determine meaningful variations in Fe³⁺/ΣFe for geophysically relevant compositions of (Mg,Fe)O (<25 wt% Fe), and calibration parameters remained constant within experimental uncertainty over the course of the entire study (20 months). Flank method measurements on an inhomogeneous sample of synthetic (Mg,Fe)O showed evidence of diffusion processes resulting from rupture of the capsule during the high-pressure experiment and the possibility to measure Lβ/Lα variations with a spatial resolution of a few microns. We detected the presence of exsolved magnesioferrite in a suite of (Mg,Fe)O single crystals using transmission electron microscopy and Mössbauer spectroscopy. Flank method measurements on the same suite of single crystals showed enhanced Fe³⁺/ΣFe values, consistent with the presence of magnesioferrite even though the grains were too small to be resolved by conventional electron microprobe measurements.     

Fe-rich Li-bearing magnesionigerite-6N6S from Xianghualing tin-polymetallic orefield, Hunan Province, P.R. China

The Fe-rich Li-bearing magnesionigerite-6N6S occurs in the Xianghualing tin-polymetallic ore field, Linwu County, Hunan Province, Peoples Republic of China. It was found near the outer contact zone of the Laizhiling granite body and in the Middle-Upper Devonian carbonate rocks of Qiziqiao Formation. The mineral formed during the skarn stage. Its empirical formula is Sn_1.81Li_0.67(Fe_1.43Zn_1.19 Mn_0.41)_Σ3.03(Al_14.89Mg_1.46 Ti_0.11Si_0.01)_Σ16.47O₃₀(OH)₂. The structure for magnesionigerite-6N6S was solved and refined in space group R-3?m, with a?=?5.7144(8), c?=?55.446(11) Å, V?=?1568.0(4) ų, to R1?=?0.0528. Based on the structural refinement of single crystal diffraction data the formula of magnesionigerite-6N6S is Sn_1.80Li_0.97(Fe_1.89Zn_0.91) _Σ2.80 (Al_14.60Mg_1.63 Ti_0.20)_Σ16.43O₃₀(OH)₂ with Z?=?3. Fe-rich Li-bearing magnesionigerite-6N6S contains 0.74 wt.% Li₂O. The idealized charge-balanced composition of magnesionigerite-6N6S may be expressed by bivalent and trivalent cations: (Mg²⁺)₄(Al³⁺)₁₈O₃₀(OH)₂. The simplified general formula for the 6N6S polysomes in the nigerite and högbomite groups can be given as A _x B_18-x O₃₀(OH)₂, x?=?~4, where A?=?Mg²⁺, Fe²⁺, Zn²⁺; B?=?Al³⁺, Sn⁴⁺, Ti⁴⁺, Li⁺, □.     

Crystal Structure of Yushkinite [(Mg0.60Al0.30V0.10)Σ1.0(OH)2][V0.875S2]: An Example of a Commensurate Combination of Brucite and Sulfide Layers

Doklady Earth Sciences - A structural model of the rare mineral yushkinite [(Mg0.60Al0.30V0.10)Σ1.0(OH)2] [V0.875S2] is studied using the data of electron microdiffraction and the X-ray powder...     

Corrosion of brass in a marine environment: mineral products and their relationship to variable oxidation and reduction conditions

Minerals coating brass ammunition shells that rested at the bottom of Halifax Harbour, Nova Scotia, for 52 a have been identified by X-ray diffraction and analytical scanning electron microscopy. The admiralty brass shells, partially buried in anoxic muds, straddle a strong Eh gradient ranging from 0 mV to values characteristic of oxygenated seawater. Whereas the brass surface in contact with the sediment has been preserved, parts of the shells exposed to seawater have corroded throughout their thickness. The corrosion products identified include metallic Cu, djurleite (Cu_1.96S), cuprite (Cu₂O), atacamite (Cu₂Cl(OH)₃), spertiniite (Cu(OH)₂) and hydrozincite (Zn₅(CO₃)₂(OH)₆). These products are those predicted thermodynamically on the basis of ambient Eh and pH. However, this study also revealed the presence of a mineral not previously known to exist and tentatively identified as Cu₁₄Zn₁₄Cl₅(SO₄)₅(OH)₄₁.H₂O. This “new” mineral seems to have a stability field in Eh–pH diagrams similar to that of connellite (Cu₁₉Cl₄SO₄(OH)₃₂.2H₂O).     

Die Kristallstruktur des Johannits,Cu(UO2)2(OH)2(SO4)2·8H2O

Zusammenfassung Die Kristallstruktur des Johannits wurde anhand eines verzwillingten Kristalls von Joachimsthal, Böhmen, mit dreidimensionalen Röntgendaten bestimmt und für 2005 unabhängige Reflexe aufR=0,039 verfeinert. Johannit kristallisiert triklin, RaumgruppeP1, mita=8,903 (2),b=9,499 (2),c=6,812 (2) Å, =109,87 (1) =112,01 (1), =100,40 (1)° undV=469,9 ų. Chemische Formel und Zellinhalt lauten Cu(UO₂)₂(OH)₂(SO₄)₂·8H₂O, das ist um zwei H₂O-Moleküle mehr als bisher angenommen. In der Struktur sind pentagonal dipyramidale (UO₂)(OH)₂O₃-Polyeder paarweise über eine von zwei OH-Gruppen gebildete Kante zu Doppelpolyedern und diese wiederum durch SO₄-Gruppen zu (UO₂)₂(OH)₂(SO₄)₂-Schichten parallel (100) verknüpft. Die Schichten sind parallel über gestreckte Cu(H₂O)₄O₂-Oktaeder und Wassermoleküle miteinander verbunden. Folgende Bindungslängen wurden gefunden: U–O=1,78 Å (2x) und 2,34–2,39 Å (5x); Cu–O=1,97 Å (4x) und 2,40 Å (2x); =1,47 Å; O–O in Wasserstoffbrücken 2,71–2,91 Å (8x) und 3,30 Å.

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The crystal structure of johannite, Cu(UO₂)₂(OH)₂(SO₄)₂·8H₂O

Summary The crystal structure of johannite has been determined from threedimensional X-ray data measured on a twinned crystal from Joachimsthal, Böhmen, and has been refined toR=0.039 for 2005 independent reflections. Johannite crystallizes triclinic, space groupP1, witha=8.903 (2),b=9.499 (2),c=6.812 (2) Å, =109.87(1), =112.01(1), =100.40 (1)° andV=469.9 ų. Chemical formula and cell content are Cu(UO₂)₂(OH)₂(SO₄)₂·8H₂O, by two H₂O molecules more than previously assumed. Pairs of pentagonal dipyramidal (UO₂) (OH)₂O₃ polyhedra form double polyhedra by edgesharing via two OH groups. The double polyhedra are linked by the SO₄ tetrahedra to form layers (UO₂)₂(OH)₂(SO₄)₂ parallel zu (100). These layers are interconnected parallel toa by elongated Cu(H₂O)₄O₂ octahedra and water molecules. Following bond lengths have been observed: U–O=1.78 Å (2x) and 2.34–2.39 Å (5x); Cu–O=1.97 Å (4x) and 2.40 Å (2x); =1.47 Å; O–O for hydrogen bonds 2.71–2.91 Å (8x) and 3.30 Å.

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Mit 2 Abbildungen     

巴颜喀拉盆地长石头山二叠纪—三叠纪碳酸盐岩丘的沉积岩石学和地球化学特征

青海巴颜喀拉盆地长石头山二叠纪—三叠纪碳酸盐岩丘是由罕见的块状纯灰岩组成,灰岩具团块结构,由泥微晶方解石团块和栉状生长的泥微晶方解石胶结物组成,部分泥微晶方解石发生了重结晶作用。灰岩的矿物组成主要为方解石,平均含量达97%,化学成分除CaO外,其余均低于5‰。灰岩的5%HNO3可溶相（碳酸盐矿物）稀土元素页岩配分模式具...     

The crystal structure of arsentsumebite, Pb2Cu[(As, S)O4]2(OH)

Summary The crystal structure of arsentsumebite, ideally, Pb₂Cu[(As, S)O₄]₂(OH), monoclinic, space group P2₁/m, a = 7.804(8), b = 5.890(6), c = 8.964(8) ?, β = 112.29(6)°, V = 381.2 ?³, Z = 2, d_calc. = 6.481 has been refined to R = 0.053 for 898 unique reflections with I> 2σ(I). Arsentsumebite belongs to the brackebuschite group of lead minerals with the general formula Pb₂ Me(XO₄)₂(Z) where Me = Cu²⁺, Mn²⁺, Zn²⁺, Fe²⁺, Fe³⁺; X = S, Cr, V, As, P; Z = OH, H₂O. Members of this group include tsumebite, Pb₂Cu(SO₄)(PO₄)(OH), vauquelinite, Pb₂Cu(CrO₄)(PO₄)(OH), brackebuschite, Pb₂ (Mn, Fe)(VO₄)₂(OH), arsenbracke buschite, Pb₂(Fe, Zn)(AsO₄)₂(OH, H₂O), fornacite, Pb₂Cu(AsO₄)(CrO₄)(OH), and feinglosite, Pb₂(Zn, Fe)[(As, S)O₄]₂(H₂O). Arsentsumebite and all other group members contain M = M–T chains where M = M means edge-sharing between MO₆ octahedra and M–T represents corner sharing between octahedra and XO₄ tetrahedra. A structural relationship exists to tsumcorite, Pb(Zn, Fe)₂(AsO₄)₂ (OH, H₂O)₂ and tsumcorite-group minerals Me(1)Me(2)₂(XO₄)₂(OH, H₂O)₂. Received June 24, 2000; revised version accepted February 8, 2001     

Surface complexation model for multisite adsorption of copper(II) onto kaolinite

We measured the adsorption of Cu(II) onto kaolinite from pH 3-7 at constant ionic strength. EXAFS spectra show that Cu(II) adsorbs as (CuO₄H_n)ⁿ⁻⁶ and binuclear (Cu₂O₆H_n)ⁿ⁻⁸ inner-sphere complexes on variable-charge ≡AlOH sites and as Cu²⁺ on ion exchangeable ≡X--H⁺ sites. Sorption isotherms and EXAFS spectra show that surface precipitates have not formed at least up to pH 6.5. Inner-sphere complexes are bound to the kaolinite surface by corner-sharing with two or three edge-sharing Al(O,OH)₆ polyhedra. Our interpretation of the EXAFS data are supported by ab initio (density functional theory) geometries of analog clusters simulating Cu complexes on the {110} and {010} crystal edges and at the ditrigonal cavity sites on the {001}. Having identified the bidentate (≡AlOH)₂Cu(OH)₂⁰, tridentate (≡Al₃O(OH)₂)Cu₂(OH)₃⁰ and ≡X--Cu²⁺ surface complexes, the experimental copper(II) adsorption data can be fit to the reactions

     

九连墩楚墓青铜器锈蚀产物的拉曼光谱分析

利用拉曼光谱对几件九连墩楚墓出土青铜器的腐蚀产物进行了测试。结果表明,九连墩楚墓出土的青铜器上主要的锈蚀产物为孔雀石[CuCO_3·Cu(OH)_2],存在部分蓝铜矿[2CuCO_3·Cu(OH)_2]和少许副氯铜矿[Cu_2(OH)_3Cl];此批青铜器锈蚀情况比较复杂,锈蚀种类比较丰富。在此基础上,探讨了科学保护此批青铜器的方法。     

硅酸盐体系的化学平衡:(2)反应热力学

通过具体的应用实例,系统介绍了在矿物材料学研究中硅酸盐体系的多相平衡反应热力学的基本原理。对硅酸盐体系的典型多相平衡反应进行了热力学计算,包括:(1)微晶玻璃制备过程中的硅酸盐熔融反应;(2)霞石正长岩和高铝粉煤灰利用技术中的硅酸盐烧结反应;(3)S iO2-CaO-H2O体系和KA lS i3O8-CaO-H2O体系雪硅钙石、硬硅钙石的水热晶化反应;(4)高铝粉煤灰和霞石正长岩烧结产物的溶解反应;(5)Na[A l(OH)4]-A l(OH)3-H2O体系和Na2SO4-Ca(OH)2-H2O体系中α-A l(OH)3和CaSO4.2H2O的析晶反应。研究成果可望对矿物材料制备实验方案设计、工业生产过程优化及改进产品性能提供理论指导,同时为同类材料学研究提供借鉴。     

安徽狸桥地区晚石炭世硅质岩的发现及其地质意义

安徽狸桥地区出露一套硅质建造,其赋存于黄龙组灰岩与五通组石英砂岩之间,呈层状产出,显微镜下主要由隐晶质石英组成,具角砾结构,未见硅质生物,局部有硅化、褐铁矿化现象。其地球化学特征显示,本区硅质岩Si O2含量为93.57%~98.06%(平均为97.22%),∑REE含量低,为4.89×10~(-6)~18.40×10~(-6)(平均为11.29×10~(-6)),δCe为0.50~0.68(平均为0.58),呈负异常,δEu为1.13~2.98(平均为1.49),呈明显的正异常。Al/(Al+Fe+Mn)值为0.11~0.64(平均为0.45),Al_2O_3,TiO_2与Si O2相关性较差,而Fe2O3和Si O2呈明显的正相关。结合双变量SiO_2-Al_2O_3、SiO_2-MgO,三变量Al-Fe-Mn,Fe-Mn-(Ni+Co+Cu)×10图解,指示其为热水沉积成因;其δEu、Mn O/TiO_2、Fe2O3/TiO_2和Al_2O_3/(Al_2O_3+Fe_2O_3)值分析,其沉积环境为远离陆源物质供给的大陆边缘,受到强烈的热水活动影响。综上表明:晚石炭世时期,狸桥地区处于受基底深断裂及同沉积断裂控制的二级断陷盆地内,可能由于地壳的持续拉张、减薄,导致火山热液或喷气活动沿基底深断裂在海底火山喷发间歇式或旋回期发生热水喷流作用,形成了具有热水喷流沉积成因的角砾状硅质岩。这一发现对于长江中下游地区晚古生代热水沉积硅质岩的研究提供一定的借鉴,也为长江中下游地区至少在石炭纪时存在一定的热水活动提供了有利的证据。     

Peralkalinity, Al{rightleftharpoons}Si Substitution, and Solubility Mechanisms of H2O in Aluminosilicate Melts

The solubility mechanisms of H₂O in peralkaline sodium aluminosilicatequenched melts (anhydrous NBO/T = 0.5) have been studied withRaman spectroscopy as a function of Al/(Al + Si) (0–0–3)and H²O content (0–7.5 wt.%). The coexisting structuralunits in the anhydrous quenched melts are TO2 (Q4), T2O5(Q3),and TO3 (Q2). In Al-free Na₂Si₄O₉ (NS4) melt, H₂O forms complexes with Na⁺(Na–OH bonds) and with Si⁴⁺ (Si–OH bonds). MolecularH₂O is also detected. TO3 structural units are not detectedin this composition. In the H₂O concentration range between0 and 4 wt.%, there is an approximately 20% increase in NBO/Tresulting from the increased abundance ratio, T₂O₅/TO2. Withfurther increments in water activity, the NBO/T of hydrous NS4melt is reduced. The depolymerization results from hydroxylationof the silica tetrahedra, whereas polymerization is due to formationof complexes with Na–OH bonding. In Al-bearing compositions on the Na₂Si₄O₉–Na₂(NaAl)₄O₉–join, there is evidence for Al–OH bonding in additionto Na–OH and Si–OH bonds. Among these complexes,the relative abundance of those with Si–OH bonds diminisheswith increasing Al/(A1 + Si), whereas complexes with Al–OHand Na–OH bonds become more important. Complexes withNa–OH bonds dominate for H₂O4 wt.%, whereas complexeswith Al–OH dominate at higher water content. The threestructural units, TO₃, T₂O₅, and TO², were observed in bothanhydrous and hydrous peralkaline sodium aluminosilicate melts.Their abundance varies, however, with the H₂O concentrationin the melts. The NBO/T decreases to a minimum (a 30–50%lowering of NBO/T relative to anhydrous materials) for low H₂Ocontents (3–4 wt.% H₂O), and increases as the H₂O contentis increased further.