Peralkaline silicate lavas at Oldoinyo Lengai, Tanzania

A detailed study of Oldoinyo Lengai has led to the recognition of two major cone-building stages. An early, predominantly phonolitic stage, Lengai I, forms the southern cone. The recent nephelinitic Lengai II developed following a major sector collapse event over Lengai I. Petrography of Lengai II lavas show that nephelinite is combeite- and wollastonite-bearing. All Oldoinyo Lengai lavas are peralkaline and highly evolved in terms of low Mg#, Ni and Cr values. Within the unique Lengai II combeite–wollastonite–nephelinite (CWN) peralkalinity increases with time to extreme values (Na + K)/Al = 2.36. Mineralogical expression of peralkalinity is the presence of combeite and Na-rich clinopyroxene. In addition, exceptionally high Fe₂O₃ (up to 10.28 wt.%) in nepheline is an indicator for alumina deficiency. Combeite also shows high Fe³⁺. Phonolite and CWN of Lengai I and Lengai II show similarly enriched LILE and LREE values and generally parallel patterns in PM normalized and REE plots.     

Primary magmas at Oldoinyo Lengai: The role of olivine melilitites

The paper describes olivine melilitites at Oldoinyo Lengai, Tanzania, and from tuff cones from the Tanzanian rift valley in the vicinity of Oldoinyo Lengai. Oldoinyo Lengai is the only active carbonatite volcano and is distinguished by its alkali-rich natrocarbonatites. Lengai is also unique for its extreme peralkaline silicate lavas related directly to the natrocarbonatites. Primitive olivine melilitites are, according to their Mg# and Ni, Cr contents, the only candidates in the Lengai area for primary melt compositions. Incompatible trace elements, including REE, constrain the melting process in their sub-lithospheric sources to very low degrees of partial melting in the garnet stability field. The strong peralkaline trend at Oldoinyo Lengai is already recognisable in these primary or near-primary melts. More evolved olivine melilitites, with Mg# < 60 allow the fractionation line in its major and trace element expressions to be followed. Nevertheless, a large compositional gap separates the olivine melilitites and olivine-poorer melilitites from the phonolites and nephelinites that form the bulk of the Lengai cone. These silicate lavas show a high degree of peralkalinity and are highly evolved with very low Mg, Ni and Cr. Prominent examples of the recent evolution are the combeite–wollastonite nephelinites that are unique for Lengai. In their Sr, Nd, and Pb isotope relationships the olivine melilitites define a distinct group with the most depleted Sr and Nd ratios and the most radiogenic Pb isotopes. They are closest to a supposed HIMU end member of the Lengai evolution, which is characterised by an extreme spread in isotopic ratios, explained as a mixing line between HIMU and EM1-like mantle components.     

Activity of the carbonatite volcano Oldoinyo Lengai, 1966

From 1960 to August, 1966, the activity of Oldoinyo Lengai took the form of quiet extrusion of carbonatite lava. In August, 1966, the style of activity changed abruptly and violent ash eruptions took place. The activity varied from minor emissions of ash to major Plinian and Vulcanian type eruptions. A new ash-cone built up within the crater and ash was widely distributed on the slopes of the volcano and over the surrounding countryside.The ash consists of sodium carbonate mixed with crystals of nepheline, pyroxene, wollastonite, apatite, melanite and pyrite. Also blocks of ijolite and melteigite were ejected during the activity.

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Zusammenfassung Von 1960 bis zum August 1966 bestand die Tätigkeit des Vulkans Oldoinyo Lengai/Ostafrika in ruhigen Lava-Extrusionen. Im August 1966 änderte sich plötzlich die Art seiner Tätigkeit, und heftige Aschenbrüche fanden statt. Diese Tätigkeit variierte von kleineren Ascheneruptionen bis zu größeren Ausbrüchen plinianischen und vulkanischen Typs. Ein neuer Aschenkegel entstand in dem aktiven Krater, und Asche wurde weithin über die Abhänge des Vulkans und über die Umgebung verteilt.Die Asche besteht aus Natrium-Karbonatit mit einer Beimischung von Kristallen von Nephelin, Pyroxen, Wollastonit, Apatit, Melanit und Pyrit. Während des Ausbruchs wurden auch Ijolith- und Melteigitblöcke ausgeworfen.

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Résumé De 1960 jusqu'en août, 1966, l'activité du volcan Oldoinyo Lengai consistait en coulées tranquilles de lave carbonatitique. En août, 1966, le genre d'activité changea abruptement et de violentes éruptions de cendres se produisirent. L'activité consistait tantôt en de petites émissions de cendres, tantôt en éruptions majeures du genre Plinien et Vulcanien. Un cône neuf de cendres s'amoncelait dans le cratère actif, et les cendres se dispersaient sur le pays environnant.Les cendres se composaient de carbonatite alcaline avec des cristaux de nephéline, pyroxène, wollastonite, melanite et de pyrite. D'ailleurs des blocs d'ijolite et de melteigite furent projetés hors du cratère pendant l'activité.

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Oldoinyo Lengai ( ). 1960 1966 . , . Na- , , , , .

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Dedicated to Professor Dr. A.Rittmann on the occasion of his 75. birthday     

Unusual Carbonatite Lavas from Active Volcano, Oldoinyo Lengai

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Unusual Carbonatite Lavas from Active Volcano, Oldoinyo Lengai     

Mineralogical and chemical transformation of Oldoinyo Lengai natrocarbonatites, Tanzania

The minerals of Oldoinyo Lengai natrocarbonatite lavas are unstable under atmospheric conditions. Subsolidus mineral assemblages in natrocarbonatites were studied in 105 samples from contemporary eruptions ranging from present day to about 100 years old. The subsolidus minerals in natrocarbonatites were formed (i) along cracks on the lava surface from hot gases escaping during cooling, (ii) as atmospheric alteration by solution of water-soluble minerals, in particular halides and gregoryite, and by hydration of nyerereite under the influence of meteoric water and (iii) by reaction with fumarole gases. After solidification, the lavas were cut by a network of thin cracks, the edges of which are covered by polymineralic encrustations. Samples collected 2–24 h after eruption contain nahcolite, trona, sylvite, and halite with accessory kalicinite and villiaumite. Atmospheric humidity results immediately (≥ 2 h after eruption) in alteration of black lavas that is marked by the appearance of white powdery thermonatrite with nahcolite on the lava surface. Subsequent reaction (weeks, months, years) of natrocarbonatite with meteoric water and the atmosphere results in the formation of pirssonite, gaylussite, shortite, trona, thermonatrite, nahcolite and calcite. Generally, the first important step is the formation of pirssonite and the end-members are calcite carbonate rocks or loose aggregates. Fumarolic activity is common for the active northern crater of the volcano. Reaction of hot (54–141 °C) fumarolic gases with natrocarbonatite leads to the formation of sulphur, gypsum, calcite, anhydrite, monohydrocalcite, barite and celestine. Changes in mineralogy of the natrocarbonatite lead to substantial chemical transformation. The most obvious chemical changes in this process are the loss of Na, K, Cl and S, combined with an increase in H₂O, Ca, Sr, Ba, F and Mn. The oxygen and carbon isotopic composition of altered natrocarbonatites shows a significant shift from the primary “Lengai Box” to high values of δ¹⁸O and δ¹³C. Calcite exhibits δ¹³C values between − 2‰ and − 4‰ PDB and δ¹⁸O values of + 23‰ to + 26‰ SMOW. The observed assemblages of secondary minerals formed by reaction with atmosphere and meteoric water, the changes in chemical composition of the natrocarbonatite and field observations suggest that alteration of natrocarbonatite is an open-system low-temperature process. It takes place at temperatures between 8 and 43 °C with the addition of H₂O to the system and the removal of Na, K, Cl and S from the carbonatites. Low-temperature thermodynamic models developed for alkali carbonate systems can be used for the interpretation of Oldoinyo Lengai subsolidus mineralization.     

Liquid immiscibility during crystallization of forsterite-phlogopite ijolites at Oldoinyo Lengai Volcano,Tanzania: study of melt inclusions

The paper is concerned with study of melt inclusions in minerals of ijolite xenoliths at Oldoinyo Lengai Volcano. Melt inclusions with different phase compositions occur in forsterite macrocrysts and in diopside, nepheline, fluorapatite, Ti-andradite, and Ti-magnetite crystals. Nepheline contains primary melt inclusions (silicate glass + gas-carbonate globule ± submicron globules ± sulfide globule ± daughter/trapped phases, represented by diopside, fluorapatite, Ti-andradite, and alumoakermanite). The gas-carbonate globule consists of a gas bubble surrounded by a fine-grained aggregate of Na-Ca-carbonates (nyerereite and gregoryite). Fluorapatite contains primary carbonate-rich melt inclusions in the core, which consist of nyerereite, gregoryite, thenardite, witherite, fluorite, villiaumite, and other phases. Their mineral composition is similar to natrocarbonatites. Primary melt inclusions (glass + gas bubble ± daughter phases) are rare in diopside and Ti-andradite. Diopside and forsterite have trails of secondary carbonate-rich inclusions. Besides the above minerals, these inclusions contain halite, sylvite, neighborite, Na-Ca-phosphate, alkali sulfates, and other rare phases. In addition, diopside contains sulfide inclusions (pyrrhotite ± chalcopy- rite ± djerfisherite ± galena ± pentlandite). The chemical compositions of silicate glasses in the melt inclusions vary widely. The glasses are characterized by high Na, K, and Fe contents and low Al contents. They have high total alkali contents (16–23 wt.% Na₂O + K₂O) and peralkalinity index [(Na + K)/Al] ranging from 1.1 to 7.6. The carbonate-rich inclusions in the ijolite minerals are enriched in Na, P, S, and Cl. The data obtained indicate that the parental melt in the intermediate chamber was heterogeneous and contained silicate, natrocarbonate, and sulfide components during the ijolite crystallization. According to heating experiments with melt inclusions, silicate-carbonate liquid immiscibility occurred at temperature over 580 °C.     

Chemical composition of nyerereite and gregoryite from natrocarbonatites of Oldoinyo Lengai volcano,Tanzania

Alkali carbonates nyerereite, ideally Na₂Ca(CO₃)₂ and gregoryite, ideally Na₂CO₃, are the major minerals in natrocarbonatite lavas from Oldoinyo Lengai volcano, northern Tanzania. They occur as pheno- and microphenocrysts in groundmass consisting of fluorite and sylvite; nyerereite typically forms prismatic crystals and gregoryite occurs as round, oval crystals. Both minerals are characterized by relatively high contents of various minor elements. Raman spectroscopy data indicate the presence of sulfur and phosphorous as (SO₄)²⁻ and (PO₄)³⁻ groups. Microprobe analyses show variable composition of both nyerereite and gregoryite. Nyerereite contains 6.1–8.7 wt % K₂O, with subordinate amounts of SrO (1.7–3.3 wt %), BaO (0.3–1.6 wt %), SO₃ (0.8–1.5 wt %), P₂O₅ (0.2–0.8 wt %) and Cl (0.1–0.35 wt %). Gregoryite contains 5.0–11.9 wt % CaO, 3.4–5.8 wt % SO₃, 1.3–4.6 wt % P₂O₅, 0.6–1.0 wt % SrO, 0.1–0.6 wt % BaO and 0.3–0.7 wt % Cl. The content of F is below detection limits in nyerereite and gregoryite. Laser ablation ICP-MS analyses show that REE, Mn, Mg, Rb and Li are typical trace elements in these minerals. Nyerereite is enriched in REE (up to 1080 ppm) and Rb (up to 140 ppm), while gregoryite contains more Mg (up to 367 ppm) and Li (up to 241 ppm) as compared with nyerereite.     

Petrology and Mineral Chemistry of Plutonic Igneous Xenoliths from the Carbonatite Volcano, Oldoinyo Lengai, Tanzania

Magmatic plutonic rocks in the Oldoinyo Lengai pyroclasticsare jacupirangite, pyroxenite, ijolite, nepheline syenite andwollastonitite. Mainly cumulates, they are combinations of nepheline,clinopyroxene, Ti-andradite, spinel (sensu lato), apatite, perovskite,titanite, wollastonite, sulphides, mica, glass and K-rich feldspar,most of which are strongly zoned. Low analytical sums for glasses,vesiculation of intergranular glasses, and the generally explosivenature of the volcanicity point to significant concentrationof dissolved volatiles in the parent magma; the absence of hydrousphases suggests that the dominant volatile is CO₂. Cumulatetextures, widely variable modes, veining and variation in specimenconsolidation and metasomatism all indicate derivation froma structurally complicated and multiply injected sub-volcaniccomplex. Complex zoning of phases and mineral disequilibrium is attributedto convective percolation of fluids through permeable cumulates,possibly complicated by magma replenishment during crystallizationof individual magma batches. Olivine, mica and pyroxene megacrystsin some ijolites indicate polybaric crystallization; mixingof potassic and sodic magmas may be the cause of these megacryst-bearingijolites, but the main parent is highly evolved, of carbonatedijolitic (nephelinitic) composition and with Nd and Sr isotopecharacteristics slightly more depleted than Bulk Earth. KEY WORDS: xenoliths; ijolite; jacupirangite; nepheline syenite ^*Corresponding author. Telephone 031 650 4837. Fax: 031 668 3184. e-mail: jbdawson{at}glg.ed.ac.uk     

Skeletal crystallization and residual glass compositions in a cellular alkalic pyroxenite nodule from Oldoinyo Lengai

The alkalic pyroxenite nodule consists of megacrysts of diopside, apatite, perovskite and titanomagnetite in a groundmass consisting of diopside, apatite, titanomagnetite, nepheline, melilite, garnet and vishnevite crystals of various shapes, including previously undescribed skeletal and dendritic shapes, together with vesicles and residual glass. The residual glass is poor in SiO₂ (38–40 wt%), and extraordinarily rich in Na₂O (12.8–15 wt%), SO₃ (1–1.5 wt%), and Cl (0.25–0.7 wt%), as a result of rapid, non-equilibrium crystallization of groundmass phases from a CO₂-rich nephelinite melt.The Oldoinyo Lengai alkalic carbonatite lavas do not represent extreme products of the fractional crystallization of pyroxene, wollastonite, nepheline and alkali feldspar from the carbonated nephelinite melt. The most likely connection between the carbonatite and silicate magma types is one of liquid immiscibility, probably involving phonolite melt.     

Thermodynamic Analysis of Secondary Minerals Stability in Altered Carbonatites of the Oldoinyo Lengai Volcano,Northern Tanzania

Carbonatites from the Oldoinyo Lengai volcano, northern Tanzania, are unstable under normal atmospheric conditions. Owing to carbonatite interaction with water, the major minerals—gregoryite Na₂(CO₃), nyerereite Na₂Ca(CO₃)₂, and sylvite KCl—are dissolved and replaced with secondary low-temperature minerals: thermonatrite Na₂(CO₃) · H₂O, trona Na₃(CO₃)(HCO₃) · 2H₂O, nahcolite Na(HCO₃), pirssonite Na₂Ca(CO₃)₂ · 2H₂O, calcite Ca(CO₃), and shortite Na₂Ca₂(CO₃)₃. Thermodynamic calculations show that the formation of secondary minerals in Oldoinyo Lengai carbonatites are controlled by the pH of the pore solution, H₂O and CO₂ fugacity, and the ratio of Ca and Na activity in the Na₂O–CaO–CO₂–H₂O system.     

The Lava sequence of the East African Rift escarpment in the Oldoinyo Lengai – Lake Natron sector,Tanzania

A 500 m sequence of horizontal lava flows forms the Gregory rift escarpment of the western rift shoulder between Lake Natron and Oldoinyo Lengai. A detailed volcanic stratigraphy of this >1.2 Ma evolution of the EAR in Northern Tanzania is presented. The sequence is formed by several distinct rock suites, with increasing alkalinity from base to top. Alkali olivine basalts of the Waterfall Sequence at the base are followed by a basanite series, and by a range of evolved nephelinites forming the upper part of the escarpment. Numerous dykes and Strombolian scoria deposits indicate local fissure eruptions as opposed to or in addition to more distant sources. Primitive compositions within each of the series indicate variable candidates for primary magmas. The composition of the basanite suite ranges from primitive mantle melts (high Mg#, Cr, Ni) to more evolved rocks, in particular hawaiites, generated by fractionation of olivine, pyroxene and magnetite. Inter-bedded within the basanite suite, one single olivine melilitite flow with high Mg# and abundant olivine and pyroxene megacrysts is the only primitive candidate for the nephelinite suite. However, in view of the large compositional gap and marked differences in incompatible element ratios, a relation between this flow and the nephelinites remains hypothetical. The variation within the evolved nephelinite series can be partly explained by fractionation of pyroxene, apatite, perovskite (and some nepheline), while magma mixing is indicated by zonation patterns of pyroxene. The most evolved nephelinite, however, differs significantly from all other nephelinites in major and trace elements. Thus the entire sequence is petrologically not a coherent evolution, rather the result of different mantle melts fractionating under variable conditions.Carved into the rift scarp of the study area west of Engare Sero is a young explosion crater, the Sekenge Crater. Sekenge Tuffs are olivine melilitites, similar to other craters and maars of the “Younger Extrusives” on the rift valley floor surrounding Oldoinyo Lengai. Further, still younger alkaline tuffs are found on the top of the rift shoulder.     

Potassium loss during metasomatic alteration of mica pyroxenite from Oldoinyo Lengai,northern Tanzania: contrasts with fenitization

Mica pyroxenite xenoliths, occurring as the cores of nephelinite and ijolite bombs in the pyroclastic deposits of the active volcano Oldoinyo Lengai, have undergone metasomatism in which K was lost and Fe²⁺ and Ti gained. This is unlike the alkali and ferric iron addition that typifies most examples of metastomatism adjacent to peralkaline igneous rocks in carbonatite complexes.     

Carbonatite and highly peralkaline nephelinite melts from Oldoinyo Lengai Volcano,Tanzania: The role of natrite-normative fluid degassing

Oldoinyo Lengai, located in the Gregory Rift in Tanzania, is a world-famous volcano owing to its uniqueness in producing natrocarbonatite melts and because of its extremely high CO₂ flux. The volcano is constructed of highly peralkaline [PI = molar (Na₂O + K₂O)/Al₂O₃ > 2–3] nephelinite and phonolites, both of which likely coexisted with carbonate melt and a CO₂-rich fluid before eruption. Results of a detailed melt inclusion study of the Oldoinyo Lengai nephelinite provide insights into the important role of degassing of CO₂-rich vapor in the formation of natrocarbonatite and highly peralkaline nephelinites. Nepheline phenocrysts trapped primary melt inclusions at 750–800 °C, representing an evolved state of the magmas beneath Oldoinyo Lengai. Raman spectroscopy, heating-quenching experiments, low current EDS and EPMA analyses of quenched melt inclusions suggest that at this temperature, a dominantly natrite_ss-normative, F-rich (7–14 wt%) carbonate melt and an extremely peralkaline (PI = 3.2–7.9), iron-rich nephelinite melt coexisted following degassing of a CO₂ + H₂O-vapor. We furthermore hypothesize that the degassing led to re-equilibration between the melt and liquid phases that remained and involved 1/ mixing between the residual (after degassing) alkali carbonate liquid and an F-rich carbonate melt and 2/ enrichment of the coexisting nephelinite melt in alkalis. We suggest that in the geological past similar processes were responsible for generating highly peralkaline silicate melts in continental rift tectonic settings worldwide.     

Carbonatite Magmatism and Plume Activity: Implications from the Nd, Pb and Sr Isotope Systematics of Oldoinyo Lengai

New Nd (0.51261–0.51268), Pb (²⁰⁶Pb/²⁰⁴Pb: 19.24–19.26),and Sr (0.70437–0.70446) isotopic compositions from tennatrocarbonatite lavas, collected in June 1993 from OldoinyoLengai, the only known active carbonatite volcano, are relativelyuniform, and are similar to data from the 1960 and 1988 flows.Three of the samples contain silicate spheroids, one of whichhas Nd and Sr isotopic ratios similar to host natrocarbonatite,consistent with an origin by liquid immiscibility or the mixingof melts with similar isotopic compositions. Pb isotope datafor two samples of trona are inconsistent with its involvementin the genesis of natrocarbonatite. New Pb isotope data fromsilicate volcanic and plutonic blocks (ijolite, nephelinite,phonolite, syenite) from Oldoinyo Lengai are highly variable(²⁰⁶Pb/²⁰⁴Pb, 17.75–19.34; ²⁰⁷Pb/²⁰⁴Pb, 15.41–15.67;²⁰⁸Pb/²⁰⁴Pb, 37.79–39.67), and define near-linear arraysin Pb-Pb diagrams. The isotopic data for the silicate rocksfrom Oldoinyo Lengai are best explained by invoking discretepartial melting events which generate undersaturated alkalinesilicate magmas with distinct isotopic ratios. Pb isotope ratiosfrom most ijolites and phonolites are predominantly lower andmore variable than from the natrocarbonatites, and are attributedto interaction between silicate melts involving HIMU and EMIsource components and an additional component, such as lower-crustalgranulites, DMM or PREMA (prevalent mantle). Variations in Nd,Pb and Sr isotope ratios from Oldoinyo Lengai, among the largestyet documented from a single volcano, are attributed to mantlesource heterogeneity involving mainly the mixing of HIMU andEMI mantle components. Based on the new isotopic data from OldoinyoLengai and data from other East African carbonatites, and mantlexenoliths, we propose a two-stage model in an attempt to explainthe isotope variations shown by carbonatites in this area. Themodel involves (I) the release of metasomatizing agents withHIMU-like signatures from upwelling mantle (‘plume’)source, which in turn metasomatize the sub-continental (old,isotopically enriched, EMI-like) lithosphere, and (2) variabledegrees and discrete partial melting of the resulting heterogeneous,metasomatized lithosphere. KEY WORDS: carbonatite; isotopes; Oldoinyo Lengai; mantle plumes ^*Telephone: (613) 788–2660, ext. 4419. Fax: (613) 788–4490. e-mail: kbell{at}ccs.carleton.ca     

Mineralogical and geochemical characterization of ashes from an early phase of the explosive September 2007 eruption of Oldoinyo Lengai (Tanzania)

Ashes from Oldoinyo Lengai were collected four days after the onset of the recent explosive episode (i.e., on September 7th 2007). The ash is composed of poorly-vesicular natrocarbonatite droplets, vesicular microcrystalline nephelinite shards, and a mixed variety containing both silicate and carbonate minerals in variable proportions. Simple mixing calculations show that the whole-rock composition of the ashes can be explained by mixing natrocarbonatite and nephelinite magmas with a ratio of 4:1. The dominant silicate minerals are clinopyroxene, nepheline, Ti-andradite, wollastonite and alumoåkermanite. Ti-magnetite is the most common oxide mineral. This mineral assemblage is similar to that present in the 1966 eruption products. In contrast to the 1966–1967 explosive eruption where clinopyroxene is resorbed and corroded, the ashfall from September 7th contains a large amount of euhedral clinopyroxene crystals, suggesting that magma mixing was heterogeneous and incomplete in this initial stage of the eruption. This is also supported by the petrography of the ashes. The composition of the dominant carbonate minerals (i.e., gregoryite and nyerereite) and the fluidal textures of the natrocarbonatite droplets suggest mixing of higher-viscosity nephelinite and low-viscosity natrocarbonatite magmas. Characteristic carbonate minerals produced by alteration cannot be found in the ashes. This suggests limited interaction with the older, pre-existing, natrocarbonatites inside the summit crater of the volcano. The carbonate minerals show textural evidence of being partially resorbed into the hotter nephelinitic magma. At least part of this decomposition of carbonate phases (releasing CO₂ and contributing to increased explosivity) must have occurred within the volcanic edifice such that the released gas is allowed to expand during decompression.     

Isotopic composition of Sr,Nd, and Pb in pirssonite,shortite and calcite carbonatites from Oldoinyo Lengai volcano,Tanzania

Doklady Earth Sciences -     

Immiscibility in sulfate-bearing fluid systems at high temperatures and pressures

Fluid equilibria in the H₂O-Na₂SO₄-SiO₂ system were experimentally studied at 700 and 800°C and pressures of 1, 2, and 3 kbar using synthetic fluid inclusions in quartz. The obtained results indicated a heterogeneous state of fluid within the whole range of experimental parameters of this study. Sodium sulfate underwent high-temperature hydrolysis, whose products chemically reacted with quartz. As a result, a noncrystalline phase containing substantial amounts of silica was formed at a temperature of 800°C and a pressure of 3 kbar. This phase was observed in the inclusions as glass. The thermometric investigation of inclusions that trapped fluid phases immiscible under experimental conditions showed that they can, in turn, become heterogeneous at temperatures of approximately 200–400°C. Under such conditions, three or four noncrystalline phases can be in equilibrium.     

Immiscibility in Ca-amphiboles

The literature data of nine different occurrences of coexistingmineral pairs of Ca-amphibole have been studied and the bulkvectors, spanning the miscibility gap, derived. The additivecomponent is always impure Mg-tremolite accompanied by someglaucophane and cummingtonite component. The four major exchangecomponents required to describe the compositional variationin coexisting mineral pairs are the edenite (ED), tschermak's(TS), FeMg_–1 and Fe³⁺-tschermak's (FeTs) vector. Trivalentiron is postulated on the basis of excess charges in the bulkvector the size of which coincides with residuals in Al^tet,–Si, Fe and –Mg. The four cations have equal sizes,forming the vector Fe³⁺ Al^tet Mg_–1Si_–1. This distributionscheme is consistent for all the different occurrences and setsthe basis for a comparison. Deviations from the scheme wouldradically complicate the proposed exchange pattern. The ratioTS:ED in most mineral samples fluctuates between one and two.Projection of the data points in the vector space TS–EDonto the line 1ED: 2TS (Tr–Hbl) or 1ED:1TS (Tr–Prg)provides the projected tremolite content (= 1–X_Hbl or = 1–X_prg). This parameter,applied to coexisting pairs, and plotted against the ratio Mg/(Mg+ Fe) shows some characteristic features about the miscibilitygap. In the Mg-pure system the solvus is almost symmetric andlocated in the temperature range between 800 and 870C. Smallamounts (0.10 pfu) of Fe²⁺ in the M(4) -sites and replacingCa have a dramatic effect, forcing the solvus to much lowertemperatures of 650C. An increase in the ratio Fe/(Fe + Mg)causes a shift of the solvus towards more tremolitic compositionswith temperatures 500–650C. The maximum asymmetry ofthe solvus is reached where the Al-poor member (tremolite) hasa composition of =1.0 and Mg/ (Mg + Fe) 0.6. The corresponding Al-rich member has =0.5 and Mg/ (Mg + Fe) 0.4. An anomalyof the solous is observed at Mg/ (Mg + Fe)=0.8. It manifestsas a kind of highly asymmetric ‘sub-gap’ in thetremolite-rich composition range. This is explained by the partitioningof Fe²⁺ into the single M(3) -site and is characterized by athermal hump to 650–700C. KEY WORDS: tremolite; hornblende; pargasite; immiscibility; solous