A-type granites and related rocks: Evolution of a concept, problems and prospects

International Tables for X-Ray Crystallography (Published for the International Union of Crystallography.) Vol. 1: Symmetry Groups. Edited by Norman F. M. Henry and Kathleen Lonsdale. Pp. xi + 558. (Birmingham: Kynoch Press, 1952.) 105s.

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International Tables for X-Ray Crystallography

The circum-Mediterranean anorogenic Cenozoic igneous province

A nomenclature for tourmaline-supergroup minerals is based on
chemical systematics using the generalized tourmaline
structural formula: XY3 Z6(T6O18)(BO3)3V3W where the common
ions at each site are X = Na1+, Ca2+, K1+ and vacancy; Y =
Fe2+, Mg2+, Al3+, Li1+, Fe3+ and Cr3+; Z = Al3+, Fe3+, Mg2+ and
Cr3+; T = Si4+, Al3+ and B3+; B = B3+; V = OH1- and O2-; and W
= OH1-, F1- and O2-. Most compositional variability occurs at
the X, Y, Z, W and V sites.

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Nomenclature of the tourmaline-supergroup minerals

The chemical speciation of arsenic in sediments and porewaters of aquifers is the critical factor that determines whether dissolved arsenic accumulates to potentially toxic levels. Sequestration of arsenic in solid phases, which may occur by adsorption or precipitation processes, controls dissolved concentrations. We present synchrotron x-ray absorption spectra of arsenic in shallow aquifer sediments that indicate the local structure of realgar (AsS) as the primary arsenic-bearing phase in sulfate-reducing conditions at concentrations of 1-3 mmol.kg(-1), which has not previously been verified in sediments at low temperature. Spectroscopic evidence shows that arsenic does not substitute for iron or sulfur in iron sulfide minerals at the molecular scale. A general geochemical model derived from our field and spectroscopic observations show that the ratio of reactive iron to sulfur in the system controls the distribution of solid phases capable of removing arsenic from solution when conditions change from oxidized to reduced, the rate of which is influenced by microbial processes. Because of the difference in solubility of iron versus arsenic sulfides, precipitation of iron sulfide may remove sulfide from solution but not arsenic if precipitation rates are fast. The lack of incorporation of arsenic into iron sulfides may result in the accumulation of dissolved As(III) if adsorption is weak or inhibited. Aquifers particularly at risk for such geochemical conditions are those in which oxidized and reduced waters mix, and where the amount of sulfate available for microbial reduction is limited.

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The influence of sulfur and iron on dissolved arsenic concentrations in the shallow subsurface under changing redox conditions

Epidote-group minerals are monoclinic in symmetry and have topology consistent with space group P21/m and the general formula A2M3(T2O7)(TO4)(O,F)(OH,O). Zoisite is an orthorhombic polymorph of clinozoisite Ca2Al3(Si2O7)(SiO4)O(OH) and is thus not considered a member of the epidote-group. Epidote-group minerals are divided into three subgroups. (1) Members of the clinozoisite subgroup are derived from the mineral clinozoisite Ca2Al3(Si2O7)(SiO4)O(OH) by homovalent substitutions only. The key cation- and anion-sites are A1 = M2+, A2 = M2+, M1 = M3+, M2 = M3+, M3 = M3+, O4 = O2-, O10 = (OH)-. In other words, the dominant valence as listed above must be maintained. (2) Members of the allanite subgroup are REE-rich minerals typified by the eponymous mineral "allanite". This subgroup may be derived from clinozoisite by homovalent substitutions and one coupled heterovalent substitution of the type A2(REE)3+ + M3M2+ → A2Ca2+ + M3M3+. Thus the valences on the key sites are: A1 = M2+, A2 = M3+, M1 = M3+, M2 = M3+, M3 = M2+, O4 = O2-, O10 = (OH)-. (3) Members of the dollaseite subgroup are REE-rich minerals typified by the eponymous mineral "dollaseite". This subgroup may be derived from clinozoisite by homovalent substitutions and two coupled heterovalent substitutions of the type A2(REE)3+ + M3M2+ → A2Ca2+ + M3M3+ and M1M2+ + O4F - → M1M3+ + O4O-2. Thus the valences on the key sites are: A1 = M2+, A2 = M3+, M1 = M2+, M2 = M3+, M3 = M2+, O4 = F-, O10 = (OH)-. The key cation-sites M3 and A1 (and, in principle, M2) determine the root name. In both clinozoisite and allanite subgroups no prefix is added to the root name if M1 = Al. The prefixes ferri, mangani, chromo, and vanado indicate dominant Fe3+, Mn3+, Cr3+, and V3+ on M1, respectively. In the dollaseite subgroup no prefix is added to the root name if M1 = Mg. Otherwise a proper prefix must be attached; the prefixes ferro and mangano indicate dominant Fe2+ and Mn2+ at M1, respectively. The dominant cation on A2 (other than Ca) is treated according to the Extended Levinson suffix designation. This simple nomenclature requires renaming of the following approved species: Niigataite (old) = clinozoisite-(Sr) (new), hancockite (old) = epidote-(Pb) (new), tweddillite (old) = manganipiemontite-(Sr) (new). Minor modifications are necessary for the following species: Strontiopiemontite (old) = piemon- tite-(Sr) (new), androsite-(La) (old) = manganiandrosite-(La) (new). Before a mineral name can be assigned, the proper subgroup has to be determined. The determination of a proper subgroup is made by the dominating valence at M3, M1, and A2 expressed as M2+ and or M3+, not by a single, dominant ion (i.e., Fe2+, or Mg, or Al). In addition, the dominant valence on O4: X - or X 2- must be ascertained. (M2+)A2 > 0.50, (M3+)M3 > 0.50 → clinozoisite subgroup, (M3++ M4+)A2 > 0.50, (M2+)M3 > 0.50 → allanite subgroup, {(M2+)M3+M1 - (M3++ M4+)A2 } > 0.50 and (X - )O4 > 0.5 → dollaseite subgroup. Coupled heterovalent substitutions in epidote-group minerals require a special application of the so-called 50 % rule in solid-solution series. (1) Clinozoisite subgroup: The dominant trivalent cation on M3 determines the name, whereas the A2 cation appearing in the suffix has to be selected from among the diva- lent cations. (2) Allanite and dollaseite subgroups: For the sites involved in the charge compensation of a heterovalent substitution in A2 and O4 (i.e. M3 in the allanite subgroup; M3 and M1 in the dollaseite subgroup), identification of the relevant end-member formula must take into account the dominant divalent charge-compensating octahedral cation (M2+) and not the dominant cation

Recommended nomenclature of epidote-group minerals

Afin d'eclaicir le probleme du changement de symetrie dans la solution solide entre stannite et kesterite [Cu 2 FeSnS 4 -Cu 2 ZnSnS 4 ], nous avons entrepris une etude structurale de monocristaux synthetiques de Cu 2 Fe 1 - x Zn x SnS 4 , x = 0, 0.2, 0.5, 0.7, 0.8 and 1, respectivement. Nous avons determine la distribution des atomes metalliques parmi les cavites tetraedriques par affinement de differents modeles dans les groupes spatiaux 14 et 142m. La meilleure solution est celle que nous obtenons dans le groupe spatial I42m, meme pour les membres riches en zinc de la serie. Toutefois, deux mecanismes differents d'incorporation sont en operation le long de la serie stannite-kesterite. Dans le cas de la stannite pure et de la stannite zincifere (x = 0, 0.2, 0.5), la position 2a (0,0,0) serait surtout remplie par (Fe,Zn), tandis que le Cu predomine a la position 4d (0,½,1/4). Dans le cas de la kesterite ferreuse et de la kesterite pure (x = 0.7, 0.8, 1), c'est la position 2a qui est remplie de Cu, tandis que (Zn,Fe) et le reste du Cu sont desordonnes a la position 4d. A la lumiere des resultats structuraux, nous proposons des longueurs de liaison Me-S ideales pour Fe, Cu, et Zn dans les sites 2a et 4d, et nous obtenons ainsi la distribution des atomes metalliques sur les sites tetraedriques. Pour les compositions ayant x ≥ 0.7, la distance Me-S pour l'atome situe au site 2a correspond a celle pour l'atome situe au site 4d, ce qui mene a une trame plus reguliere. Par consequent, les indices de distorsion λ et σ 2 du tetraedre S(Me 3 Sn) diminuent a mesure qu'augmente la proportion de Zn, C'est en fait la raison de la symetrie pseudocubique du reseau observee sur l'intervalle de compositions riches en zinc (2a se rapprochant du parametre c). Le volume de la maille augmente de facon lineaire a mesure qu'augmente la proportion de zinc, ce qui confirme le caractere surtout covalent des liaisons dans ces composes. L'inversion dans la pente des parametres a x = 0.7 qui avait ete etablie anterieurement correspond au point dans la serie ou le Cu devient predominant au site 2a. Notre modele rend compte des variations structurales et geometriques le long de la serie stannite-kesterite, sans meme avoir a supposer un changement du groupe spatial.

A MODEL FOR THE MECHANISM OF INCORPORATION OF Cu, Fe AND Zn IN THE STANNITE – KËSTERITE SERIES, Cu2FeSnS4 – Cu2ZnSnS4

Crystallisation and genesis of peralkaline magmas from Pantelleria Volcano, Italy: an integrated petrological and crystal-chemical study

Abstract Transformations induced by light in realgar and β-As4S4 have been studied by means of X-ray diffraction (powder and single-crystal) and Raman laser spectroscopy. Both polychromatic and monochromatic light alter realgar and the β phase into pararealgar. By lowering the reaction rate (long-wavelength pass filter, low laser power) an intermediate product, corresponding to the so-called “χ phase,” can be observed during the alteration process from β-As4S4 to pararealgar (polychromatic filtered light and laser light) and that from realgar to pararealgar (only with polychromatic filtered light). The powder pattern of the χ phase is fairly similar to that of β-As4S4 but differs from realgar, and it can be indexed on the basis of an expanded β-phase unit cell. Lattice parameters are as follows: a = 9.758(5), b = 9.522(5), c = 9.074(5) Å, 0 = 100.84(5)°, and a = 9.757(4), b = 9.512(7), c = 9.089(4) Å, 0 = 100.97(3)° for the χ phase obtained from alteration of realgar and β-As4S4, respectively. The formation of the χ phase is preceded by a strong anisotropic increase of the unit-cell volume of realgar and the β phase. The effects of light exposure on lattice parameters were studied using single-crystal X-ray diffraction: In realgar a and c sin β increase linearly with increasing exposure times, whereas b remains substantially unchanged. In the β-As4S4 crystals, b and c sin β increase while a decreases so that, after 1560 min, the unit-cell dimensions match those of the χ phase. Raman spectroscopy, which allows monitoring of the alteration process in real time, indicates the presence of the same As4S4 cage molecule in the χ phase as in realgar and β -As4S4 and confirms the similarity between the structure of χ and that of β. However, a less-ordered crystal structure and a lower molecular symmetry is suggested from analysis of the spectroscopic results.

Silvio Menchetti

Papers

Recommended nomenclature of epidote-group minerals

A MODEL FOR THE MECHANISM OF INCORPORATION OF Cu, Fe AND Zn IN THE STANNITE – KËSTERITE SERIES, Cu2FeSnS4 – Cu2ZnSnS4

Crystallisation and genesis of peralkaline magmas from Pantelleria Volcano, Italy: an integrated petrological and crystal-chemical study

Light-induced variations in realgar and beta -As 4 S 4 ; X-ray diffraction and Raman studies

Spectroscopic investigation and normal mode analysis of As4S4 polymorphs