桐庐和相山两类酸性火山—侵入杂岩中长石的对比研究

桐庐和相山杂岩体分别属于同熔型和陆壳重熔型火山－侵入杂岩系列,通过对这两个系列中的斜长石及碱性长石进行了对比研究。桐庐杂岩体中存在两种斜长石及两种碱性长石,而相山杂岩体中只有一种斜长石及一种碱性长石。桐庐杂岩体中的两种斜长石,一种为环带构造发育、中心绢云母化较强的斜长石,其中心成分有的已达培长石,而边缘主要为更长石,另一种为环带构造不发育的斜长石,其成分变化较小,主要为中长石;而两种碱性长石中一种为无色透明的透长石,其内部均匀分布有规则的出溶微结构,另一种为微红色的正长石,基本上看不到出溶结构,这两种碱性长石结构不同但成分基本相似。笔者认为桐庐杂岩体中存在的两种斜长石及两种碱性长石反映了岩浆的不平衡结晶作用,这种不平衡主要是岩浆上升过程中带状岩浆房的扰动引起的;而相山杂岩体中的长石反映了一种近平衡结晶的岩浆环境     

桐庐、黄山花岗岩类岩体中碱性长石的出溶和超微双晶

本文研究了桐庐和黄山两地不同成因花岗岩类岩体中碱性长石的出溶和超微双晶。结果表明,碱性长石的出溶和超微双晶同岩体的成岩温度、冷却速率以及岩体成因类型和碱性长石的化学成份有关,成岩期后的热作用对碱性长石的出溶和超微双晶产生改造和破坏。     

内蒙古正镶白旗碎斑熔岩长石特征及其岩石学意义

正镶白旗碎斑熔岩是一种特殊成因的火山岩,是岩浆沿火山通道侵出地表的产物。碎斑熔岩中钾长石主要为低透长石、高正长石、中正长石和中正微长石;斜长石则以更长石为主,其有序度低,指示了岩石高温火山成因的特征。从岩体边缘相到中心相至根部相,长石类型、化学成分和有序度具有一定的变化规律,反映了岩体不同岩相带成岩条件的差异性。     

低温碱性溶液中微纹长石溶解性质研究

对微纹长石在碱性溶液中的溶解过程及表面特征变化规律进行实验研究，温度为60℃，实验溶液分别为NaOH(pH≈8)、Na2CO3，[ρ(Na^ )=26μg／mL]和NaCl溶液，利用X射线光电子能谱(XPS)和ICP-MS测定实验前后微纹长石的表面特征及实验溶液成分变化，结果表明，随着pH值升高，微纹长石的溶解性增强；微纹长石的溶解不是被单一反应模式所控制，而是在表面控制(surface-conrtolled)、扩散控制(diffusion-controlled)和次生物相形成三种过程交替进行的结果。碱性溶液中长石表面反应分三个步骤：H^ 与长石表面碱性阳离子交换反应；OH^-联合已进入长石架状结构内部空隙，具较大结合能的Al-O键逐步破裂，并生成铝的水解产物而溶出；表面形成富Si的络合物。     

矾山杂岩体多元长石的成分、结构及其研究意义

华北早中生代矾山钾质碱性超镁铁岩-正长岩杂岩体由3个岩相带组成,本文在不同相带矿物学系统研究的基础上,重点对该区不同期次钾长石的组分和结构进行了详细的研究,其结果反映杂岩体总的具有富钾的特点,但各岩相带长石成分的差异则显示出不同期次岩浆成分上的不同。通过对长石的微量元素的进一步研究,本文首次确认了矾山地区钡冰长石和锶钡冰长石的存在。通过对不同阶段长石锶钡含量的比较研究,揭示了本区存在多次富钡流体的交代作用。该项研究对揭示本区岩石成因及其岩浆演化具有重要意义。岩浆晚期锶、钡元素的明显提高暗示:该区早中生代作为岩浆源区的岩石圈地幔有不断富集的趋势。     

我国东部碱性岩中长石研究

我国东部六个碱性岩体的长石以碱性长石为主,包括了正长石、微斜长石和钠长石;斜长石有中长石和奥长石。它们具有三种组合,即正长石-斜长石;正长石-微斜长石;钠长石-微斜长石组成的条纹长石。各组合长石在组分或结构态方面都有某些共同之处,某些岩体还体现了长石成分演化的一致趋势。本文根据长石结构态,以及二长石和长石-霞石地质温度计的应用,对岩体形成温度作了一些探讨。     

青藏高原当雄地区方沸石响岩的主要造岩矿物特征: 原生方沸石的证据

方沸石响岩是一种罕见的碱性火山岩.采用电子探针、粉晶X射线衍射、扫描电镜、拉曼光谱等研究了青藏高原当雄地区方沸石响岩的主要造岩矿物种属、共生关系和结晶顺序.研究表明, 斑晶由方沸石和长石组成.方沸石为岩浆结晶晚期形成的原生矿物; 长石均发育“次生边”结构, 中央相为斜长石, 边缘相为碱性长石.基质由碱性长石、次透辉石、钛磁铁矿和褐铁矿组成.原生矿物的结晶顺序是: 斑晶长石的中央相→斑晶方沸石+斑晶长石边缘相富钾长石→基质长石→次透辉石→钛磁铁矿和褐铁矿.利用方沸石-熔体平衡估算出方沸石结晶时岩浆的温度和压力条件分别为600~640 ℃和(5~13) ×108 Pa, 考虑到青藏高原当时已形成巨厚地壳, 认为岩浆房存在于地壳深部.      

碱性长石中的微孔及微裂隙特征研究

扫描电镜(SEM)显示长石矿物中存在不同形态的微孔和微裂隙,大小从几十纳米到几十微米不等,其表面形态主要有圆形、椭圆形和槽形.不同类型的长石微孔和微裂隙的分布不均匀,无色透明的长石比较致密,微孔和微裂隙较少;而半透明长石中的微孔和微裂隙较多,孔密度较大.N2吸附实验表明,碱性长石矿物的比表面积为1.833 m2/g ;长石矿物的BET吸附等温线为II型,吸附回线类型为H3和H4型回线,表明长石中主要为圆形孔和裂隙形孔.     

黑龙江省五大连池、科洛、二克山火山群富钾火山岩中的浅色矿物

本区富钾火山岩中浅色矿物有碱性长石、白榴石和霞石。碱性长石主要为钠透长石、钙歪长石,少数为钙钠透长石和歪长石,其光性变化与长石的地质时代和长石内部的隐条纹结构有关。白榴石的成分及产出方式主要与岩石中的SiO_2含量有关,SiO_2不饱和程度大的白榴苦橄岩的白榴石贫SiO_2;而SiO_2不饱和程度低的白榴碧玄岩和白榴玄武岩中,白榴石富SiO_2而稍贫K_2O+Na_2O,仅见于过渡相,而中心相由于水压较高和近于平衡的结晶条件,白榴石不能晶出,或先晶出后又与熔体反应形成碱性长石。     

陕南光石沟伟晶岩型铀矿床中长石矿物化学初步研究

以光石沟伟晶岩型铀矿床中相关伟晶岩中的长石为研究对象,开展系统的矿物化学电子探针研究,结果显示其中的斜长石都属于更长石、钠长石,碱性长石均为透长石,运用二长石温度计计算出了四种类型伟晶岩的结晶温度。通过研究,认为4种类型伟晶岩具有岩浆演化关系,从不含矿黑云母伟晶岩-含矿黑云母伟晶岩-二云母伟晶岩-白云母伟晶岩,随着伟晶岩岩浆演化程度的增高,成岩结晶温度不断降低,岩浆逐渐向酸性演化,同时由于黑云母等富氟矿物的产出,导致晶质铀矿的沉淀而富集成矿。     

Enthalpies and Volumes Related to K--Na Mixing and Al-Si Order/Disorder in Alkali Feldspars

In order to investigate the thermodynamic properties of alkalifeldspars, three new feldspar ion-exchange series have beensynthesized, two based on monoclinic parent materials havingintermediate degrees of Al—Si order, the other on Amelialow albite. Acid solution calorimetric measurements have beencarried out in 20?1% HF at 50?C under isoperibolic conditionson 30 members of the three series, and compared with revisedvalues for a previously reported sanidine—analbite series.Molar volumes have been determined for all feldspars, and foran additional series based on Eifel sanidine. Enthalpies of K—Na mixing (A_ex) calculated from the 50?Cheats of solution are dependent on Al—Si distributionfor both topochemically monoclinic and triclinic alkali feldspars,and in general can be expressed as where N_Or and N_Ab are mole fractions of KAlSi₃O₈ and NaAlSi₃O₈,respectively, and Z is an ordering parameter defined as twicethe difference in the mole fraction of Al in the T1 vs the T2tetrahedral sites. A_ex values for all but the most disorderedseries are maximized toward sodic compositions, and increaseboth in magnitude and asymmetry as ordering increases. For topochemically monoclinic alkali feldspar series, volumesof K—Na mixing(V_ex) are asymmetric with N_Or, but withinthe precision of present data do not depend on Al—Si distribution: Microcline-low albite feldspars appear to have volumes of mixingwith the opposite asymmetry, but expressions of for these differ somewhat among various investigators. Since no single thermodynamic mixing property is markedly asymmetricwith respect to composition, the excess Gibbs energies impliedfrom solvus data for alkali feldspars, and maximized at sodiccompositions, are apparently the result of additive effectsof subtle asymmetries in the volumes, enthalpies, and entropiesof K—Na mixing in these minerals. The thermodynamic properties of an alkali feldspar at any compositionare significantly affected by the distribution of Al and Sibetween T1 and T2 tetrahedral sites. The enthalpy of formationat 50?C of a monoclinic potassium feldspar with perfect order(Z=1) differs by 2?19 kcal/mol from one with a completely randomAl—Si distribution (Z=0), while a value of 2?86 kcal/molapplies to analagous sodium end members. ConverselyY-ordering(between T1O andT1m sites) seems to have little or no effecton the enthalpy of formation of either end member, evidencedby the fact that most of the enthalpy differences for the lowmicrocline to sanidine and corresponding low albite to analbitetransitions (1?73 and 2?79 kcal/mol, respectively) can be attributedto Al—Si exchanges between T1 and T2 sites. Observed enthalpydifferences in alkali feldspars are probably related to strainat domain boundaries, whether the domains are extremely small,or somewhat larger as in modulated structures. Neither Z-nor Y-ordering has a substantial effect on the molarvolumes of alkali feldspars.     

Optical and infra‐red studies on high‐temperature Alkali Feldspars

In ternary feldspars of essentially one phase, calcium content has a dominant influence on the optic axial angle. In such feldspars and also in binary feldspars from solvsbergite rocks, variations of cooling histories do not significantly affect the optic axial angle. In ternary feldspars which are unmixed into two or three prominent phases, Al/Si ordering has an important effect on the 2V value. A recent suggestion of several writers that in feldspars the alkali structural site may be partially occupied by (H3O)+ ions is applied as a possible way to explain a correlation observed between petrographical features of the rocks and the optic axial angles of their feldspar phenocrysts.     

An electron microscopic study of anorthoclase spherulites

Spherulites consisting of fibrous alkali feldspar and silica minerals are produced by devitrification of rhyolite glass under hydrothermal conditions. The alkali feldspars (Ab_72.5Or_23.0An_4.5, Ab_81.7Or_14.0An_4.3) in spherulites from two localities in Japan consist of triclinic anorthoclase showing fine cross-hatched twinning and monoclinic sanidine showing fine cross-hatching not attributable to twinning. The cross-hatching, which corresponds to albite and pericline twinning, is produced in the process of transition from a monoclinic to a triclinic phase. The spherulite may develop at a temperature lower than about 200°C because the co-existing silica mineral is not quartz, but metastable tridymite. According to the phase diagram of the alkali feldspars by MacKenzie (1952), the alkali feldspars should have been triclinic during growth. However, the textures show that the alkali feldspar grew as a disordered monoclinic phase. Because of the high growth rate, the Al/Si disordered structure was produced during growth and afterwards transformed into a triclinic structure with cross-hatched twinning.     

Development of perthite microstructures in the Storm King granite,N.Y.

The Storm King granite at Bear Mountain, New York contains coarse alkali feldspar mesoperthite. The microstructure of these perthites grades continuously from lamellae to isolated blebs. The K rich phase in all samples has a nerely constant composition (Õr₉₇Ab₃), but the Na-rich phase ranges from An_3.8 (lamellae) to An₂₁ (blebs). It is suggested that the more calcium-rich feldspars exsolved at a higher temperature and thus experienced a longer time at higher temperature, during which the microstructure became more bleby or spherical in response to minimization of the interfacial energy. Lack of perfect correlation of the microstructure with bulk composition, as well as variation in the Al/Si ordering, may be due to additional factors, of which water activity or concentration is the most likely.     

Two-feldspar geothermometry,geobarometry in mesozonal granitic intrusions: Three examples from the Piedmont of Georgia

The equilibrium temperatures for coexisting plagioclase and potassium feldspar pairs have been calculated for various textural varieties of feldspar from 3 post-metamorphic granites from the Georgia Piedmont; the Danburg, Siloam, and Stone Mountain plutons. Assuming an intermediate structural state for the feldspars at time of equilibration, crystallization temperatures match those expected from experimental data for quartz monzonite magmas (650 to 780° C). The variations in solidus temperature, recorded in the feldspars, may be used to estimate relative differences in depth of intrusion. Sharp reversals in plagioclase compositional trends may be caused by isothermal decreases in confining pressure associated with upward migration through the crust. In fine grained and slowly cooled intrusions, albite tends to be lost from the alkali feldspar grains, and recrystallizes as separate unzoned grains of oligoclase, thus erasing the previous thermal history. Perthite exsolution and re-equilibration within the alkali feldspar grains appears to continue down to temperatures of 400° C or so, although the zoned plagioclase does not homogenize. The recrystallization associated with changes in structural state may facilitate exsolution within alkali feldspar grains.     

Zoned ternary feldspars in the Klokken intrusion: exsolution microtextures and mechanisms

The microtextures developed during relatively slow cooling as a function of bulk composition in zoned ternary feldspars from syenodiorites and syenites in the Klokken intrusion, described in the preceding paper, were determined by TEM and their origin and evolution deduced. The feldspars normally have a plagioclase core and an alkali feldspar rim; cores become smaller and rims larger and the An content of both decrease with distance from the contact of the intrusion. The following microtextural sequence was observed. The inner plagioclase cores are homogeneous oligoclase-andesine with Albite growth twins only, but are crypto-antiperthitic towards the outer core. At first small platelets of low sanidine a few nanometres thick and up to 10 nm long occur sporadically only on Albite-twin composition planes. With further increase in bulk Or they are homogeneously distributed in the plagioclase. Thicker, through-going plates in platelet-free areas are found, which induce Albite twins in the surrounding plagioclase. The microtextures in the rims are regular cryptomesoperthitic, with (¯601) lenses or lamellae, depending on the bulk Or-content, of low sanidine in Albite-twinned low oligoclase-andesine. Albite and Pericline twins in plagioclase in an M-twin relationship, together with lenticular low sanidine, were found in only one small area. The overall diffraction symmetry of the mesoperthites is monoclinic, showing that exsolution started in a monoclinic feldspar, whereas that of the antiperthites is triclinic. The intermediate zone between the core and rim is more complex and microtextures vary over distances of a few micrometres.The cryptomesoperthites are very regular where Or-rich and probably arose by spinodal decomposition. The platelets in the outer cores arose by heterogeneous nucleation on twin composition planes and by homogeneous nucleation elsewhere. Near the intermediate zone they coarsened to give larger plates which induced Albite-twins in the plagioclase. Because of the zoning, microtextures that were initiated in areas of given composition, can propagate laterally into zones of different composition. A diagram is given showing the relationship between ternary bulk composition and the microtexture developed in coherent perthitic alkali feldspars and plagioclases from slowly-cooled rocks.CRPG contribution 730     

Nucleation on perthite-perthite boundaries and exsolution mechanisms in alkali feldspars

A transmission electron microscope study of intracrystalline boundaries between two perthites of markedly different composition in composite crystals, one a tenary mesoperthite (Or₂₆Ab₅₂An₂₂, initially a homogeneous potassian monalbite) the other a more potassic cryptoperthite (Or₆₁Ab₃₃An₆, initially a homogeneous sodian sanidine), shows that the two perthites are in nearly parallel intergrowth. Most boundaries examined were of (hkO) type; (010) boundaries are straight, whereas other (hkO) boundaries are curved or stepped. Exsolution occurred first in the potassian monalbite (mesoperthite) and was unaffected by the boundary. Subsequent exsolution in the sodian sanidine (cryptoperthite) was affected by the boundary, but for up to only a few micrometers. Exsolution occurred by heterogeneous nucleation and growth of oligoclase on and from the intracrystalline boundary. At almost the same time the rest of the volume of sanidine exsolved by spinodal decomposition. 1–2 μm from the boundary in the intervening K-rich matrix of the sodian sanidine, further exsolution occurred by homogeneous nucleation. Time — temperature — transition curves for continuous cooling have been devised to account for the unusual complexity of the exsolution texture. Except in such exceptional circumstances as the example studied, the initial exsolution in high-temperature alkali feldspars of intermediate composition, unlike other minerals, probably does not occur by nucleation, but only by spinodal decomposition.     

Structure energies of the alkali feldspars

Structural energetics of the alkali feldspars have been studied using a “lattice” or structure energy model. Electrostatic energies, U _e,for 20 well-refined, non-intergrown alkali feldspars were calculated using Bertaut's (1952) summation procedure and average about ?13,400 kcal/mol; the repulsive energies of the alkali site in each structure (～15 kcal/mol) were calculated using repulsive parameters for K-O and Na-O interactions estimated from bulk modulus data for NaF and KF and the exponential form of the repulsive potential. Using a procedure in which the position of the alkali cation was varied while the oxygen cage was kept fixed, structure energy gradients for the alkali sites of high albite and a hypersolvus Ab₄₂Or₅₈ structure were computed. In both cases, a broad structure energy well, elongated approximately parallel to c and subparallel to the observed split Na positions, was found. In both structures there is a single energy minimum corresponding closely with the observed single alkali positions. Comparison of U _e values for the alkali feldspars with different K/Na ratios shows that intermediate compositions are predicted to be less “stable” than either endmember and that the potassic end-member is predicted to be less “stable” than the sodic one, assuming that all other factors contributiong to the free energies of each phase are approximately the same. Comparison of U _e values for the high albite and low sanidine structures with different Al/Si distributions and a fixed tetrahedral framework indicates that the ordered charge distributions are 63.0 and 54.8 kcal/mol, respectively, more “stable” than the disordered distributions. Smaller, more realistic energy differences were obtained by using U _evalues averaged from four separate calculations with a +3 charge on a different T site in each and with +4 charges on the other T sites. If, in addition, the charges on cations and oxygen are reduced to half their nominal formal charges, in agreement with Pauling's electroneutrality principle and the results of recent molecular orbital calculations on silicates, the predicted electrostatic energy differences are reduced to 3.6 and 1.6 kcal/mol, respectively. These calculations also indicate that the T₁O site in the high albite structure energetically favors Al and that the Al/Si distribution determines the Na position within the alkali site.     

Mantled feldspars in alkali rhyolite from Oki-Dogo island,Japan

Summary Feldspar phenocrysts in alkali rhyolite from Oki-Dogo island in the Sea of Japan show mantled textures with cores of anorthoclase and rims of sanidine. These feldspars were examined by electron microscopy, X-ray diffraction and X-ray microanalysis. Anorthoclase first crystallized, then was partially resorbed, and finally sanidine overgrew on the anorthoclase. Saw-tooth or comb-like interfaces between the cores and rims were likely formed at the magmatic stage of resorption and overgrowth. Optically perthitic intergrowths appear in thin sections cutting saw-tooth or comb-like interfaces of the mantled feldspars. The sanidine preserves primary cryptoperthitic textures of a periodicity smaller than 10 nm, which are considered to have been produced by subsolidus exsolution after the resorption event ended. The anorthoclase has no exsolution texture under an electron microscope.

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Feldspatkristalle in Alkali-Rhyolith von der Insel Oki-Dogo, Japan

Zusammenfassung Feldspatkristalle in Alkali-Rhyolith von der Oki-Dogo Insel im Japanischen Meer zeigen ummantelte Texturen mit Kernen von Anorthoklas und Rändern von Sanidin. Diese Feldspate wurden mit Elektronenmikroskopie, Röntgendiffraktion und Mikrosondenanalyse untersucht. Anorthoklas kristallisierte zuerst, wurde dann teilweise resorbiert und schließlich wuchs Sanidin über den Anorthoklas. Sägezahn- und Kamm-ähnliche Grenzen zwischen Kernen und Rändern wurden wahrscheinlich wahrend des magmatischen Stadiums von Resorption und Überwachsung gebildet. Unter dem Mikroskop erkennt man, daß perthitische Verwachsungen durch Sägezahn- oder Kamm-artige Grenzen der ummantelten Feldspäte hinwegsetzen. Der Sanidin erhellt primäre kryptoperthitische Texturen mit einer Periodizität von > 10 nm, die als Produkte einer Subsolidus-Entmischung nach der Resorption interpretiert werden. Anorthoklas läßt unter dem Elektronenmikroskop keine Entmischungstexturen erkennen.

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With 6 Figures     

The effects of aluminum-silicon ordering on phase separation for a two-dimensional model of alkali feldspars

The time evolution of a model system for mixtures of sodium and potassium feldspars is investigated by computer simulations in order to determine what effects aluminum-silicon ordering may have on phase separation in these minerals. We assume that there is a greater energetic advantage for Al atoms to move onto preferred sites in the vicinity of Na atoms than in the vicinity of K atoms. This couples Al/Si ordering to the phase separation. We find that in such a system phase separation is slowed substantially compared to a system in which the two processes are not coupled.