The origin of anomalous birefringence of grossular-andradite (grandite) garnets from skarns in Mali and Russia was considered. The crystals had complex optical patterns which can be induced by superposition of two phenomena: mismatch compositional strain (stress birefringence) and growth ordering of atoms (growth dissymmetrization). Study of the crystals using several experimental techniques (optical microscopy, microprobe analysis, X-ray diffraction topography and X-ray single crystal diffraction) as well as calculations of anomalous birefringence has confirmed this hypothesis. Depending on the crystal composition and growth conditions, the relative magnitude of each phenomenon controls the various optical effects. As a result one can see two groups of crystals which are found to have fundamentally different anomalous optical properties: crystals with low (<0.001) and high (0.001 0.015) values of birefringence. The spatial distribution of birefringence within each group is different and this fact is related to different mechanisms causing optical anomalies: stress birefringence and growth dissymmetrization for these two groups, respectively.

The available data on compositional zoning in grossular-andradite (grandite) garnets occurring at different scales, mainly due to the variations of their Fe3+/Al ratios, and the hypotheses on the origin of this zoning are reviewed. Four zoned crystals of grandites showing different optical properties have been studied by means of X-ray diffraction. Optical and structural studies suggest three superimposed scales of Fe3+/Al zonality along [110]. The drastic change of the structural characteristics from sample to sample correlates with the change of their optical patterns. The superfine oscillatory zoning has been described in terms of irregular interstratified structures with a high degree of segregation. The self-affinity of zoning at different scales testify to the possible origin of the zonality due to the non-linear dynamics at the growth front. The wide variation of the layer compositions revealed by the modelling of the X-ray diffraction profiles seems to contradict the hypothesis of unmixing in grandite garnets.

The paper presents the detailed mineralogical and petrographic study of spinel-bearing marbles at the Luc Yen deposit, North Vietnam. The LA-ICP-MS analysis of 74 spinel grains, combined with mineralogical data, allows us to discriminate them into fve types according to mineral assemblages and trace element geochemistry. Forty seven minerals are identifed as inclusions in spinel grains and 38 minerals are described at Luc Yen deposit for the frst time including leonardsenite, tintisite, manasseite, chalcoalumite, cobaltite and spherocobaltite. The mineral assemblages, trace element pattern, and specifc mineral inclusions indicate the involvement of hydrothermal fuids related to magma intrusions in the formation of, at least, two of fve spinel types at the deposit.

We report a new occurrence of preiswerkite, the rare sodium analog of eastonite, the trioctahedral mica, from marble-hosted noble spinel deposits of the Luc Yen district, northern Vietnam. It is found in marble for the first time. The preiswerkite is anhedral and associated with phlogopite, aspidolite, sadanagaite, pargasite, spinel, corundum, dolomite and calcite. The average compositions of preiswerkite is (Na0.88Ca0.08K0.01)Σ0.97(Mg2.29Al0.72Fe0.04)Σ3.05)[(Al1.95Si2.05)Σ4.00O10](OH)2. The compositions of preiswerkite have a narrow range of Mg# values (0.96–0.99) and define a preiswerkite-aspidolite solid-solution series. Compared with other occurrences, the Luc Yen preiswerkite has a low iron content, which attains 0.09 atoms per formula unit (1.53 wt.% FeO). The formation of preiswerkite is favored by the proportion of Mg, Al and Si in the precursor rocks and the increased activity of sodium and H2O in the fluid phase.

In this paper, we report the very first occurrence of titanian hydroxylclinohumite in the marble-hosted gem spinel deposits of the Luc Yen district, northern Vietnam. Hydroxylclinohumite is anhedral and associated with forsterite, tremolite, pargasite, diopside, spinel, dolomite and calcite. Hydroxylclinohumite from the Luc Yen deposit was characterized via electron microprobe analysis, single-crystal X-ray diffraction study, and Raman spectrometry. The average composition is (Mg0.69Ti0.29Fe0.02)Σ1.00Mg7.91(SiO4)4.08[(OH)1.10F0.53O0.37]Σ2.00. (ideally (Mg0.7Ti0.3)Σ1Mg8.0(SiO4)4[(OH)1.2F0.5O0.3]2). The compositions of the analyzed hydroxylclinohumites have a narrow range of Mg/(Mg+Fe+Ti) values (0.96–0.97) and a defined hydroxylclinohumite solid-solution series. Compared with other occurrences, the Luc Yen hydroxylclinohumite has an average titanium content, which attains 0.31 atoms per formula unit (3.93 wt.% TiO2) and a low iron content of 0.04 atoms per formula unit (0.42 wt.% FeO). The formation of hydroxylclinohumite is favored by the proportion of Mg, and Si in the precursor rocks and the increased activity of H2O in the fluid phase.

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A new method of heavy mineral (HM) separation and assessment of gold grade was compared with the results of conventional AAS analysis. Sixteen gold micronuggets and a number of particles of native metal and metal alloys (brass, tin, bismuth, lead) were extracted from 100 g of till fines (< 50 µm). From the size, number, and composition of micronuggets, the total gold grade (58 ppb) of till fines was evaluated. The assessments agree well with the results of AAS analysis (57 ppb). A slightly lower value (44 ppb) was obtained by Flame Atomic Absorption Analysis with Fire Assay (FAAS FA) method of the extracted HM. Mineralogical investigations allow identification of two types of gold micronuggets thus revealing a complex origin for the geochemical anomaly. The association of brass-pyroxene (Mg# = 80-82) with complex gold-brass-lead-tin intergrowths indicates that some gold in till is derived from ultramafic rocks.

Microprobe, optical, and x-ray diffraction analyses of lithium micas from pegmatites in five regions of the USSR show that the structural heterogeneity, as indicated by the polytype complexity and abnormal optical properties peaks at intermediate composi tions (2-4 wt. % Li 2 O) in the muscovite-polylithionite series. The different polytypes of the intergrowths have identical chemical compositions, i.e., there is no miscibility gap between the di-and trioctahedral micas. The structural heterogeneity appears to result from stresses in the growing crystal associated with the misfit of zones and sec tors of different chemical compositions.Polytypic compounds characteristically form "syntactic intergrowths" [1], i.e., parallel inter growths of polytypes with different layering, which morphologically form a "single crystal." If the com pound has a simple composition (for example, SiC or ZnS), such intergrowths are caused by the regular occurrence of stacking faults and are usually accom panied by twinning on the stacking planes and by layering disorder. The stacking faults responsible for polytype intergrowths may originate by transforma tion [24], growth [16] or autodeformation [7]. In any case, the polytype in an intergrowth have the same composition. This also occurs for compounds with more complicated structures but constant composi tions such as potassium ferrocyanide [9] or fluorophlogopite [10].However, with minerals of variable composition, in particular natural micas, there is in principle another possibility: intergrowth in a "single crystal" of various polytypes differing in composition. With the micas, there is fairly strict crystallochemical con trol of polytypism [4], so that possibility is quite realistic. Some investigators have interpreted the structures of intermediate members in the isomorphous lithium mica series on such a basis [18,5]. The end-members in the series are dioctahedral muscovite, dioctahedral lithian muscovite, and two trioctahedral lepidolites: trilithionite and polylithionite [2]. Analyses of natural lithium micas indicate that the predominant substitution is Al VI + Al IV = 2Li + Si . It is assumed that there is only limited isomorphous miscibility between the end-members muscovite and polylithionite. Intermediate lepi dolites represent anomalous mixed crystals, i.e., they are composed of alternating layers with mus covite and polylithionite compositions. The crys tallochemical control means that the muscovite layers are the 2M 1 polytype and the polylithionite layers 1M.There is, however, an opposing view [20, 23]: The layers of the various polytypes in a lepidolite crystal have the same composition, which in essence means that there is no miscibility gap in the isomorphous series. The main evidence for lack of complete mis cibility in the lithium mica series is the polytype complexity of intermediate-composition micas. Con sequently, the miscibility gap is as problematic as differences in composition between the polytype layers. The uncertainty is further increased by the fact th...