Oxygen diffusion in amphiboles

The kinetics of oxygen isotope self-diffusion in natural samples of hornblende, tremolite, and richterite have been measured. Samples were run under hydrothermal conditions using ¹⁸O enriched water. Profiles of ${}^{18}\text{O}\text{(}{}^{16}\text{O +}{}^{18}\text{O)}$vs depth into the crystal were obtained using an ion microprobe; the depths of sputtered holes were measured using an optical interferometer. At 1000 bars (100 MPa) water pressure, the activation energies (Q) and pre-exponential factors (D₀) for diffusion parallel to c are: D₀(cm²/sec) Q (kcal/gm-atom) T (°C) Hornblende 1⁺²⁰_?1 × 10^?741 ± 6 650–800 Tremolite 2⁺³⁰_?2× 10^?8 39 ± 5 650–800 Richterite 3⁺⁵_?2 × 10^?4 57 ± 2 650–800The diffusion coefficient (D) for hornblende at 800°C and 1000 bars water pressure measured parallel to the c crystallographic direction is at least ten times greater than that parallel to the a or b directions. An increase in water pressure from 200 to 2000 bars increases D by a factor of 2.7 for diffusion parallel to c at 800°C. The D value for hornblende at 800°C is about 0.01 that for quartz and 0.001 that for anorthite. As a result, closure temperatures for oxygen exchange in natural primary amphiboles are significantly higher than for quartz or feldspars. It is unlikely that amphiboles will exchange oxygen isotopes by diffusion under most crustal conditions.     

Statistical chemistry of calcic amphiboles

Principal components analysis is used to study the chemistry of 639 calcic amphiboles. Eigenvectors representing multiple partial correlation coefficients give various sets of substitutional relationships. The relative significance of each set can be noted by the percent variation of the data it represents. The highest percent variation (36%) is associated with the substitutions $$Si + Mg \rightleftharpoons Al^{IV} + Al^{VI} + Ti + Fe^{3 + } + Fe^{2 + } + Na + K$$ . Other expected substitutions among the ions such as Al^IV + Na ? Si, the positive correlation between Al^IV and Al^VI etc. are shown statistically. The substitution of Al in T ₁ and T ₂ positions imposes an ordering in the M ₁, M ₂ and M ₃ sites. Variability of OH in the amphiboles is found to be significant. There is no definite correlation between OH and Fe³⁺ but OH and Ti are positively correlated. Under certain conditions and provided the concentration of Al^IV does not change significantly, Fe and Mg may be assumed to mix ideally in the amphibole solid solution.     

Thermodynamic study of calcic amphiboles

The paper reports original thermochemical data on six natural amphibole samples of different composition. The data were obtained by high-temperature melt solution calorimetry in a Tian–Calvet microcalorometer and include the enthalpies of formation from elements for actinolite Ca_1.95(Mg_4.4Fe _0.5 ²⁺ Al₀₁)[Si_8.0O₂₂](OH)₂(–12024 ± 13 kJ/mol) and Ca_2.0(Mg_2.9Fe _1.9 ²⁺ Fe _0.2 ³⁺ )[Si_7.8Al_0.2O₂₂](OH)₂, (–11462 ± 18 kJ/mol), and Na_0.1Ca_2.0(Mg_3.2Fe _1.6 ²⁺ Fe _0.2 ³⁺ )[Si_7.7Al_0.3O₂₂](OH)₂ (–11588 ± 14 kJ/mol); for pargasite Na_0.5K_0.5Ca_2.0-(Mg_3.4Fe _1.8 ²⁺ Al_0.8)[Si_6.2Al_1.8O₂₂](OH)₂ (–12316 ± 10 kJ/mol) and Na_0.8K_0.2Ca_2.0(Mg_2.8Fe _1.3 ³⁺ Al_0.9) [Si_6.1Al_1.9O₂₂](OH)₂ (–12 223 ± 9 kJ/mol); and for hastingsite Na_0.3K_0.2Ca_2.0(Mg_0.4Fe _1.3 ²⁺ Fe _0.9 ³⁺ Al_0.2) [Si_6.4Al_1.6O₂₂](OH)₂ (?10909 ± 11 kJ/mol). The standard entropy, enthalpy, and Gibbs free energy of formation are estimated for amphiboles of theoretical composition: end members and intermediate members of the isomorphic series tremolite–ferroactinolite, edenite–ferroedenite, pargasite–ferropargasite, and hastingsite.     

Clustering of cations in amphiboles

The OH^? stretching frequencies of clino-amphiboles are known to depend on the cations to which the OH^? is co-ordinated, and the intensities of the corresponding infra-red (ir) absorption bands have been used to obtain evidence as to the occupancy of the M1 and M3 sites. However, the possible effects on the method arising from clustering together of like cations, or of the mutual avoidance of like cations (anti-clustering), have not hitherto been explicitly analysed. A model is set up which permits these to be systematically explored. The results are exemplified for occupation of M1 and M3 by (Fe, Mg). Clustering of Fe increases the frequency of both Fe₃ and Mg₃ triads, though not equally, and the effect of anti-clustering is in the opposite sense. The effect on the frequencies of MgFe₂ and Mg₂Fe triads is much more complicated and the sense of the changes depends on both the overall Fe content and on the degree of clustering or anti-clustering.     

Chemical etching of amphiboles and pyroxenes

Chemical etching of defect structures in pyroxenes and amphiboles has been investigated. Many features, such as very thin (～1 micrometer) exsolution lamellae not observed by transmission optical microscopy of thin sections, have been observed in reflected light after chemical etching of cleaved or polished crystal surfaces. A new feature in the microstructure of pyroxenes, not previously reported in the literature, has been revealed by one of the etchants.     

Metabasite amphiboles of the Scottish Dalradian

Blue-green hornblendes are observed in metabasite assemblages throughout the chlorite, biotite and garnet zones of the southwest Scottish Highlands. Actinolites are common in more Mg-rich metabasites in these zones. At low grade, hornblendes are relatively edenite-rich, and may sometimes occur together with a more Mg-rich, Al-poor actinolite. Within the garnet zone, hornblendes are pargasitic, showing extensive tschermakite substitution. Textural and chemical evidence do not indicate the presence of any miscibility gap between hornblende and actinolite within the chlorite to garnet zones in the southwest Highlands. The occurrence of hornblende-actinolite pairs in metabasites of the Scottish Dalradian, and perhaps also in other metamorphic terrains, is considered to reflect the incomplete chemical equilibration of lower grade actinolitic amphibole during prograde metamorphism, rather than a miscibility gap. The paucity of amphibole compositions intermediate between hornblende and actinolite in many metamorphic terrains is thought to reflect the rapid but continuous change of stable amphibole compositions in metabasites over a small range of increased metamorphic grade.     

Characteristics of asbestiform and non-asbestiform calcic amphiboles

In terms of morphology there are four major types of calcic amphibole; massive, prismatic, finely acicular and asbestos. Representatives of each of these types have been examined by optical microscopy, X-ray diffraction, scanning and transmission electron microscopy, and electron probe microanalysis. Massive specimens (nephrite) consist of randomly oriented clusters of fine, roughly lath-shaped, sub-microscopic crystals; within each cluster the lath lengths (z) are approximately aligned but neighbouring laths are rotated with respect to one another. Finely acicular specimens (“byssolites”) have well-formed crystals bounded mainly by {110} (100) and (010) faces and characteristically have striations parallel to their lengths. Asbestiform varieties range from finer (flexible) to coarser (more brittle) specimens and many specimens contain a mixture of fine and coarse fibrils. The fibrils in a bundle are aligned parallel to z but are in a range of azimuthal orientations. It is inferred that they are formed by multiple independent nucleation and growth parallel to z rather than through parting or cleavage on {110} planes. (100) defect or twin planes, or on (010) planar defects.

The {110} cleavage in amphiboles is well reported but (100) features are rarely mentioned in the literature. Our observations reveal the importance of (100) as a cleavage or parting as well as the tendency in nephrites, byssolites and asbestos towards a lath-like (parallel to z) morphology with flattening on (100). In the latter varieties therefore, the y-direction is that of second fastest crystal growth, after z.

When subjected to moderate grinding, the comminution of asbestos fibres proceeds more by separation of fibrils and less by fracturing to shorter lengths as compared with prismatic and byssolite specimens. Prolonged grinding does, however, shorten lengths of even the least brittle asbestos.

Transmission electron microscopy revealed extensive sub-grain boundaries and dislocation networks (suggesting a deformation history) in all prismatic and nephrite specimens. Fine multiple (100) twinning was observed in asbestos but not in other varieties. Although chain-width defects [on (010)], with visibility enhanced by beam damage, were most abundant in nephrites and fibrous tremolites, there appears to be no completely consistent relationship between such features and morphological type.

Electron probe analyses showed that specimens that contain more than a very small amount of aluminium do not have asbestiform habit. Asbestos specimens also have lower contents of Mn, Na and K and have formulae closer to the ideal Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂. Small departures from this in asbestos involve Na in the A site compensated by Na for Ca rather than Al for Si whereas the reverse is true in byssolites. Chemical substitutions in prismatic specimens are much less constrained.

The characteristics of the four morphological sub-groups correlate reasonably well with what is known of their geological environments.     

Multiple-chain and other unusual faults in amphiboles

A combination of high resolution transmission electron microscopy and computer simulation has revealed the existence of multiple-chain and other faults in nephrite jade. Attention is drawn to the subunit cell reorganization involved with many of these faults, which generally appear to be free from severe structural strain.     

Sub-solidus dehydration of amphiboles in an andesitic magma

Several petrologic experiments have demonstrated that in igneous and metamorphic reactions amphibole minerals can break down by a subsolidus dehydration reaction, but evidence for the reaction in natural rocks has been lacking. Evidence for the breakdown of an edenite-pargasite amphibole by a subsolidus dehydration reaction has now been found in an andesite flow from Garner Mountain, southern Cascase Range. The andesite contains one modal percent of crystal clots formed of crystallites of opx, cpx, plag, K-spar, opaque and quartz. The crystal clots retain the original amphibole morphology and intra-clot pyroxenes are aligned with crystallographic c parallel to c in the amphibole precursor; these conditions would not be duplicated by a melting reaction.Microprobe analyses of the bulk clot and the intra-clot minerals suggest the solid-state reaction: 100 amph+10 SiO₂=>55 cpx+33 plag+22 opx+ 1 opq+1 ksparPyroxene thermometry of the andesite groundmass pyroxenes and the intra-clot pyroxenes demonstrates that the amphibole dehydration reaction occurred in the xenocrystic amphiboles as a result of heating by the near-solidus andesite magma.     

Chemical types of amphiboles from the Tyrnyauz deposit

Based on the results of microprobe analyses, six groups of calcium amphiboles were distinguished for the Tyrnyauz deposit. Group I (mainly edenite) includes amphiboles of hornfels and periskarn rocks. Groups II-VI (actinolite-hastingsite series) are related to post-skarn metasomatic rocks of the productive stage after hornfels (II), the tonalite and plagiogranite complex (III), ultrabasic rocks (IV), pyroxene-plagioclase periskarn rocks (V), and skarn (VI). Group IV is represented by actinolite; group VI, by hastingsite; groups II, III, and V, by hornblende varieties.     

Coexisting amphiboles in an eclogite from the Western Alps: new constraints on the miscibility gap between sodic and calcic amphiboles

Abstract Crystal-chemical relationships between coexisting sodic and calcic amphiboles have been studied in eclogitic metagabbros from the Aosta Valley, Western Alps. Textural analysis gives evidence of three successive high-pressure parageneses:
1. Pre-kinematic high-grade blueschist assemblages, preserved as polymineralic inclusions in garnet cores and made of glaucophane and actinolite (stage A).
2. Synkinematic eclogite assemblages, composed of garnet + omphacite + glaucophane ± actinolite ± white mica ° Clinozoisite + quartz + rutile (stage B).
3. Post-kinematic epitactic overgrowths of barroisitic amphibole on glaucophane and actinolite (stage C).
P–T conditions of the eclogitic metamorphism have been estimated at around 500–550°C, 16 kbar.
Glaucophane and actinolite coexist as discrete grains in stage A and B assemblages. This texture and the chemistry of the amphiboles unambiguously denotes the existence of a miscibility gap between sodic and calcic amphiboles (from Na^M4= 0.80 in actinolite to Na^M4= 1.70 in glaucophane at T = 500–550°C). A comparison with published analyses allows a new solvus along the glaucophane–actinolite join to be drawn.
The later barroisitic amphibole (stage C) exhibits strong chemical zonation indicating disequilibrium growth. This amphibole cannot either be used to define a miscibility gap with glaucophane or actinolite or be considered as an intermediate stage between these two end-members.     

Greenschist amphiboles from Haast river,New Zealand

A section across the Haast Schist Group in the Southern Alps of New Zealand shows a sequence of metamorphosed eugeosynclinal sediments. Meta-basic rocks (greenschists) have been studied to determine the nature of the actinolite-hornblende transition and to investigate the change in amphibole composition through the Metamorphic Facies Series.Electron microprobe analyses of 21 representative amphiboles, including 3 amphibole pairs can be shown to support theories of a miscibility break in the calciferous amphibole solid solution series. The existence of a miscibility break is further supported by the widespread appearance, even at low metamorphic grades, of exsolution lamellae in actinolite and hornblende amphiboles.Amphibolite facies amphiboles differ from greenschist facies amphiboles in that (a) there are increased amounts of Ti entering the lattice and (b) that there is an increased occupancy of the A site at higher metamorphic grades.     

A Comparison of amphiboles from medium- and low-pressure metabasites

A comparison of published metabasite amphibole analyses from medium and low-pressure metamorphic terrains reveals that there is no systematic variation in Na, Na^M4, Al or Al^VI as a function of pressure. This may be due to blurring of the differences by variation in oxidation state, or by analytical differences between laboratories. It is not due to variable Mg/Fe in whole rocks. Differences that can be recognised are generally higher Ti/Al ratios in the low-pressure amphiboles, and a very poorly developed compositional gap between actinolite and hornblende compared with a well-developed gap at medium pressures. These features, together with the relatively low-grade appearance of calcic plagioclase at low pressures, provide the best means of distinguishing metabasites from the two facies series.All three features can be explained by the configuration of cation-exchange equilibria at the greenschist/amphibolite facies boundary. Enrichment in Ti at low-pressures is due to the positive slope of reactions partitioning Ti into the amphibole. The composition gap in amphiboles at medium-pressure is due to overstepping of the tschermakite-enriching equilibrium. At low pressures this overstepping still occurs, but the equilibrium tschermakite-content in the amphibole is much lower for a given amount of overstepping. The relatively low-grade appearance of oligoclase at low pressures is due to convergence of the tschermakite and anorthite-enriching equilibria with decreasing pressure.     

Optical spectroscopy study of natural Fe, Ti-bearing calcic amphiboles

Over thirty samples of natural Ti-bearing amphiboles with Ti- and Fe-contents ranging from 0.111 to 0.729 atom per formula unit (a.p.f.u.) and from 0.479 to 2.045 a.p.f.u., respectively, were studied by means of optical absorption spectroscopy and microprobe analysis. Thirteen samples were also studied by Mössbauer spectroscopy. A strong pleochroic absorption edge, causing the dark brown colours of Ti-bearing amphiboles, is attributed to ligand-metal and metal-metal charge transfer transitions involving both iron and titanium ions (O^2?→ Fe³⁺, Fe²⁺, O^2?→ Ti⁴⁺ and Fe²⁺ + Ti⁴⁺→ Fe³⁺ + Ti³⁺). A broad intense Y-polarized band ～22?000?cm^?1 (ν_1/2?≈?3700?cm^?1) in spectra of two low iron amphiboles with a relatively low Fe³⁺/Fe_total ratio, both from eclogite-like rocks in kimberlite xenoliths, was attributed to electronic Fe²⁺(M3) + Ti⁴⁺(M2)→Fe³⁺(M3)+Ti³⁺(M2) IVCT transitions. The IVCT bands of other possible ion pairs, involving Ti⁴⁺ and Fe²⁺ in M2 and M1, M4 sites, respectively, are presumed to be at higher energies, being obscured by the absorption edge.     

Shock-metamorphic features in amphiboles from the Xiuyan crater of China

Amphibole-bearing gneiss fragments are common in the impact breccias of the Xiuyan crater, China. Three kinds of amphibole-bearing gneiss fragments with different shock-metamorphic levels have been identified. Shock-metamorphic features of amphiboles in these gneisses were investigated in situ by optical microscope, electron microprobe, Raman spectroscopy, and X-ray diffraction. Amphiboles in the weakly shocked gneiss (shock pressure less than 10 GPa) basically remain intact. Amphiboles in the moderately shocked gneiss (shock pressure range between 35 and 45 GPa) show strong deformation, reduced optical interference color, and partial loss of OH^?. In the strongly shocked gneiss (shock pressure above 50 GPa), amphiboles are completely melted and dendritic pyroxenes crystallize from the melt. The formation of dendritic pyroxenes shows nearly complete loss of water in the amphibole melt at shock-induced high temperature above 1,500 °C. The occurrence of both diopside and pigeonite dendrites crystallized in the same amphibole melt shows inhomogenous melt composition and rapid cooling of the melt.     

Sodic amphiboles and pyroxenes from fenites in East Africa

During alkali metasomatism of the country-rock associated with ijolite-carbonatite complexes the development of sodic amphibole and/or pyroxene is characteristic. In this paper, some new chemical analyses of these minerals, together with published analyses from fenites of Kenya, Uganda and Tanzania, include those of co-existing pairs of amphibole and pyroxene. The common amphiboles of the fenites are magnesioarfvedsonites with 100 Mg: Mg+Fe+Mn ranging from 67 to 36. They co-exist with aegirines having 0.75 to 0.89 ions Fe⁺³. Most of these minerals are poor in Ca; co-existing pairs tend to show corresponding increases in Ca and in Fe⁺². In the syenitic fenites of Tororo and Budeda, considered to have formed at higher temperatures, the stable mineral is aegirine-augite. New analyses of richterite, magnesioarfvedsonite and aegirine from carbonate-rich rocks are also presented, and the relation between fenites and carbonatites is discussed.     

Stability and composition relations of calcic amphiboles in ultramafic rocks

Calcic amphiboles are observed in ultramafic rocks that have equilibrated under a broad span of geological conditions and might prove to be good indicators of metamorphic grade if their stabilities could be determined as a function of their compositions. Experiments were performed on the stability of tremolite plus forsterite in the system H₂O-CaO-MgO-SiO₂ from 5 to 20 kbar. A univariant curve was fitted to the experimental brackets using volume, water fugacity, and heat capacity data. The results indicate that the maximum stability of tremolite in the presence of forsterite is about 825° C at 5 kbar. Addition of Al₂O₃ to this system increases the stability of tremolitic amphibole by only 20°–40° C and induces solubility of 5–7 wt.% Al₂O₃ in the amphibole, as determined from quantitative SEM analyses of individual amphibole crystals. Thus substitution of the tschermakite component (Ca₂(Mg₃Al₂) (Si₆Al₂) O₂₂(OH)₂) alone cannot lead to the greatly enhanced Al₂O₃ contents or thermal stability of natural calcic amphiboles. Comparison of the results from this study with experimental results from other studies on synthetic calcic amphiboles indicates that the high thermal stability of natural amphiboles is strongly linked with the substitution of alkalies (Na in particular) in the form of the component Na-Ca₂(Mg₄Al) (Si₆Al₂)O₂₂(OH)₂ (pargasite). Accordingly, experimental data from studies on pargasite have been combined with the appropriate univariant curves to obtain a phase diagram for amphibole-bearing ultramafic rocks modelled by the system H₂O-Na₂O-CaO-MgO-Al₂O₃-SiO₂.     

Alkali amphiboles intermediate in composition between actinolite and riebeckite

Alkali amphiboles of an intermediate composition in the magnesioriebeckite-eckrite series have been found in the metamorphic terrane of Leros Island. A complete compositional gradation has been demonstrated by a series of electron microprobe scans and analyses of spots. Electron microscopic examination revealed no exsolution.The present analyses and other data indicate a closing of the Na-Ca amphiboles gap towards the magnesian end members at temperatures higher than that usually associated with the blueschist facies.     

MinIdent: An application to the identification and classification of amphiboles

Summary Amphibole data in the MinIdent database (Smith andLeibovitz, 1986) were initially entered using species names quoted in the original source. The database has been updated by reclassifying these early data using the program AMPHTAB supplied by N. M. S. Rock and by adding supplemental data from the more recent literature, with the species names again checked using AMPHTAB. Associated MinIdent mineral identification software was utilized to determine which minerals in the database most closely resemble a series of unknown specimens chemically, as expressed in the Chemical Matching Index, CM, a relative figure-of-merit. Chemical data fromMogessie and Tessadri (1982) and Hawthorne (1983) were used to check the agreement between MinIdent and AMPHTAB for the classification of 221 unknown amphiboles.With 450 amphibole analyses entered and compiled in MinIdent, the name assigned by AMPHTAB showed the highest value of CM in MinIdent for 127 of the 221 unknown amphiboles (57.5°/x) and the second highest value for another 32 (14.5%). A chemically adjacent amphibole field had the highest value of CM for 59 of the 221 unknowns (26.7%), where chemically adjacent refers to a change in one chemical parameter. The greatest discrepancy between the two programs occurred in the hornblendes, with an agreement of just 20%, although for 58% of the unknowns the species with the highest CM in MinIdent was in a chemical field adjacent to the species name assigned by AMPHTAB. In many cases the disagreement between MinIdent and AMPHTAB could be ascribed to a lack of data in MinIdent.A comparison of the two programs suggests that the assignment of a single name to an unknown amphibole by AMPHTAB with no direct indication of its reliability may be; misleading. Standard analytical errors are frequently sufficient to overlap the arbitrary boundaries between amphibole species fields. In such cases it may be preferable to use a program such as MinIdent which, rather than assigning an arbitrary amphibole name, presents a list of 20 amphiboles with the degree of similarity between them and the unknown amphibole indicated. MinIdent offers the additional benefit of allowing input of other than chemical data and bases the match between unknown and standard data upon all input data. This will become more of an advantage as instruments such as automated refractometers become available for routine use.

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Zusammenfassung Ausgangspunkt war das Amphibol-Datenmaterial (Smith und Leibovitz, 1986) mit den dort verwendeten Artnamen. Diese Basisdaten wurden vervollständigt und erneuert durch Reklassifizierung mittels des AMPHTAB Programms, ergänzt durch N. M. S. Rock, und durch Hinzufügung weiterer Daten aus der neuesten Literatur, deren Speciesnamen wiederum mit AMPHTAB überprüft wurden. Außerdem wurde eine MinIdent Mineralidentifizierungs-Software verwendet, um die Minerale zu bestimmen, die in ihrem Chemismus am ehesten einer Serie von unbekannten Amphibol-Species entsprechen, wie sie im Chemical Matching Index (CM) aufscheinen. Zur Klassifikation von 221 unbekannten Amphibolen wurden chemische Daten von Mogessie und Tessadri (1982) verwendet um die Übereinstimmung zwischen MinIdent und AMPHTAB zu überprüfen.Unter den 450 in MinIdent zusammengestellten und eingegebenen Amphibolanalysen zeigen die bei AMPHTAB angegebenen die höchsten CM Werte, nämlich 127 von 221 unbekannten Amphibolen (57,5%) und weitere 32 (14,5%) die zweithöchsten Werte. Innerhalb eines chemisch benachbarten Amphibolfeldes hatten 59 der 221 unbekannten Amphibole (26,7%) die höchsten CM Werte, wobei unter achemisch benachbart die Änderung eines chemischen Parameters zu verstehen ist. Die größten Unterschiede zwischen den beiden Programmen traten bei den Hornblenden auf. Die Übereinstimmung lag bei nur 20%, obwohl bei 58% der unbekannten Amphibole die Species mit dem höchsten CM Wert in MinIdent in ein chemisches Feld zu liegen kamen, welches zu den bei AMPHTAB angegebenen Speciesnamen eine benachbarte Position einnimmt. Die Unterschiede zwischen MinIdent und AMPHTAB könnten in vielen Fällen auf ein Fehlen von Daten in MinIdent zurükzuführen sein.Ein Vergleich beider Programme deutet an, daß die Angabe eines Einzelnamens für ein unbekanntes Amphibol im AMPHTAB Programm ohne Angaben über die Zuverlässigkeit zu Mißverständnissen führen kann. Normale analytische Fehler können bereits dazu führen, daß die Grenzen zweier willkürlicher Amphibolfelder überlappen. In derartigen Fällen emphiehlt sich die Anwendung des MinIdent Programmes, welches eben nicht einen willkürlichen Amphibolnamen angibt, sondern eine Liste von 20 Amphibolen mit dem Grad ihrer Ähnlichkeit, und einem Hinweis auf den unbekannten Amphibol. MinIdent bietet den zusätzlichen Vorteil, daß man außer chemischen auch andere Daten eingeben kann, und stellt dann sämtliche Daten des unbekannten Amphibols den Standard Daten gegenüber. Dieser Klassifizierungsvorgang wird mit der zunehmenden Routineanwendung von automatischen Refraktometern verstärkte Anwendung finden.