Epidote Minerals Research Articles

Epidote minerals in ophiolites near Coolac, N.S.W., Australia

Epidote minerals in ophiolites near Coolac, N.S.W., Australia

Epidotic calc-silicates of Arnipol Type—a widely distributed minor sedimentary facies of the Moinian assemblage, Scotland

SummaryIn the Moinian rocks of Scotland, epidote-bearing calc-silicates, distinct from the commoner garnet-zoisite-bearing calc-silicates, occur at several widely scattered locations. The major element geochemistry of the two types of calc-silicate is very similar, and their somewhat different petrology and trace element geochemistry may be explained by the preservation of grains of detrital epidote and heavy minerals in the epidotic calc-silicates. These grains were concentrated in the original sediment and remained stable under greenschist and epidote-amphibolite metamorphic facies conditions. While the garnet-zoisite bearing calc-silicates may be derived from calcareous marls, the epidotic calc-silicate bands are probably derived from epidotic grits similar to those recorded in the Torridonian.

長門構造帯産角せん岩類

Newly found amphibolites occur as tectonic blocks and lenticular bodies in serpentinite in Nagato tectonic zone which has been known by the occurrence of orthogneisses and granites of about 400 million years in isotopic age. These amphibolites are composed mainly of amphibole, epidote minerals, and plagioclase, rarely accompanying clinopyroxene. In chemical composition, they range from basalt to andesite and resemble the high-alumina basalt series after Kuno (1960). Five amphiboles together with three epidote minerals and one clinopyroxene were analysed chemically. Of these, three amphiboles are rich in edenitic component, while two of them are pargasitic and rich in Ti and Ca than the former. Detailed field and microscopic observations indicate the common occurrence of edenitic amphibole in these amphibolites and the equilibrated mineral association corresponding to epidoteamphibolite facies to lower part of almandine-amphibolite facies. Consequently, it is suggested that the pargasitic amphiboles are the relict minerals of higher-grade metamorphism prior to the Silurian age. This is confirmed by the compositional relation between amphiboles and coexisting epidote minerals, comparatively high Al-content in relict clinopyroxene and calcic core rimmed by clear sodic periphery in plagioclase.

An Evaluation of the Provenance of Terrigenous Sediments from Offshore Vancouver Island

Modern sands and gravels from Barkley Sound, Vancouver Island, are mainly composed of igneous lithic fragments, present in sediments coarser than medium sand, and of andesine plagioclase, present in finer size fractions. Heavy mineral assemblages are dominated by hornblende, epidote, opaque minerals, and chlorite. This sediment mineralogy is mainly controlled by the composition of the source area, and to a lesser extent, by hydraulic processes of the depositional environment; effects of weathering at source and river transport are minimal. Sources include Mesozoic, eugeosynclinal basalts and andesites, late Mesozoic, post-geosynclinal granodiorite and diorite plutons, and Cretaceous to (?) late Pleistocene sediments which themselves were ultimately derived from the aforementioned igneous sources.Relict sands and gravels on the continental shelf adjacent to Barkley Sound, are similar in composition to the modern sediments and have been derived from similar igneous and sedimentary source rocks. However, relative to modern sands, relict sands have a greater variety of minor constituents, particularly heavy minerals, which in addition to hornblende, epidote, and opaque minerals, include garnet, orthopyroxene, and clinopyroxene. It is suggested the more varied mineralogy is due to: (1) a more extensive source area; and (2) introduction of additional sediment from outside the study area by modern bottom currents.

Heavy Minerals of the East-Central Bering Sea Continental Shelf

ABSTRACT A factor analysis of 51 grab samples from the east-central Bering Sea continental shelf identified three factors that account for 88% of the variation in the relative amounts of the nine major heavy mineral groups that were used as variables. The most important components for definition of the sources and processes which have determined the properties of the surface sediments in this area are epidote, garnet and opaque minerals, and clinopyroxene and weathered grains, for Factors I-III, respectively. The occurrences and textural associations of Factor I-III sediments indicate that the relative fractions of the characteristic minerals have been largely determined by sorting; proximity of source is more important along the Alaskan mainland. Throughout the area, the heavy mineral content of samples differ from those of modern Yukon River sediments. This study demonstrates the utility of factor analysis in the interpretation of relatively uniform heavy mineral data.

Fission track annealing and age determination of epidote minerals

The epidote group of minerals is well-suited for age determination by the fission-track method. The minerals occur in a large variety of rock types; fission tracks are readily etched in them; and epidote retains its tracks at high temperatures. No annealing of fission tracks in epidote was detected after 16 days at 600°C, whereas almost all tracks were annealed in 6 hours at 700°C. Extrapolation of the experimentally determined temperatures suggest that a temperature of 630°C would be required for 106 years to remove all tracks. Fission-track ages of epidote and allanite agree well with ages of coexisting minerals determined both by fission-track and other methods.

Knossos Neolithic, Part II

All the axes are of Cretan materials. The majority, fifty-nine out of seventy-one, are of altered basic igneous rock consisting of various minerals, usually with much chlorite and serpentine. Group A is called serpentine as this mineral is predominant. In appearance the axes are blue/grey/black mottled, sometimes with brown mottling also. Group B is called greenstone since the minerals epidote and chlorite are present in large quantities and the specimens have a distinct greyish-green appearance. Group C is again altered basic rock, including chlorite and serpentine, though neither predominates.There are a number of outcrops of chlorite and serpentine among the limestones of Crete, especially in the northern and southern foothills of the Mt. Ida massif.Two axes appear to be of haematite, which occurs in Crete, two of limestone and one of schist. The materials of seven others are not certain, though several are probably of altered basic rock (see The Catalogue).The materials agree with those of other stone axes from Crete, for serpentiniferous basic rock and greenstone examples have been picked up in various parts of the island. Greenstone and haematite were the predominant materials of the Late Neolithic axes from Magasá near Palaikastro.

Stability and relations of the Al-Fe epidotes

SummaryThe various (P, T) stability fields of iron-free zoisite have been deduced for systems containing excess silica and water with Ca : Al ratios ranging from 1 : 2 (anorthite) to 3:2 (grossular). Zoisite is a possible phase in all systems with Ca:Al lower than 3:2, attaining its maximum stability at the zoisite (Ca:Al = 2:3) and prehnite (Ca:Al = 2:2) compositions.The consequences of varying the Al : Fe ratio are next examined. Zoisite with 4 % pistacite is stable to 525° at 2 kilobars, compared with ∼585° for clinozoisite and 620 to 630° for epidote (Ps35) at the same pressure. Increasing iron content also stabilizes epidote minerals relative to their low-temperature and high-pressure decomposition products.Examples of natural zoisite-clinozoisite, zoisite-epidote, and clinozoisite-epidote assemblages are described. It is concluded that zoisite-epidote mixtures result from the disproportionation of clinozoisite outside its own stability field, but within those of zoisite and epidote. The assemblage zoisite-clinozoisite is probably not stable, but further evidence is needed on this point. The assemblage clinozoisite-epidote is stable below 550° C, at which temperature the solvus in the Al-Fe series closes.

Heavy Minerals of Pre-Morrison Jurassic Rocks, Lucero Uplift, Northwestern New Mexico: ABSTRACT

A heavy-mineral study was made of the following formations in the Lucero uplift: Triassic: Chinle Formation and Wingate Sandstone; Jurassic: Carmel Formation, Entrada Sandstone, Todilto Formation (not analyzed), Summerville Formation, Bluff Sandstone, and Morrison Formation; Cretaceous: Dakota Sandstone. Emphasis was placed on the pre-Morrison Jurassic rocks. Heavy minerals identified from these rocks consist dominantly of zircon, garnet, tourmaline, rutile, and several opaque minerals, and very minor epidote, biotite, staurolite, and apatite. This limited suite of mostly rounded, stable heavy minerals indicates that the rocks concerned are multicycle sediments, and agrees with the conclusion previously derived from utilizing other stratigraphic tools in studying these rocks. Thus, in this respect, the usefulness of heavy minerals as a stratigraphic tool is demonstrated. Furthermore, the pre-Morrison part of the section, in which relatively abundant amounts of magnetite-ilmenite occur, is readily distinguishable from the Morrison Formation in which these minerals are virtually absent. When the pre-Morrison Jurassic rocks are viewed in sequence, it appears that the Carmel and lower Entrada are characterized by low zircon-garnet and high magnetite-ilmenite content, the upper Entrada by a high zircon-garnet and low magnetite-ilmenite content, and the Summerville and Bluff by a high zircon-low garnet and variable magnetite-ilmenite content. Nevertheless, with the exception of the upper Entrada, the differences in per cent distribution of the heavy minerals are not sufficient to unequivocally characterize any single pre-Morrison unit. Thus, in this respect the heavy minerals are not as useful a stratigraphic tool as they might be if the suite were mineralogically more varied. Prior to the determination of the heavy-mineral content by using standard techniques of counting individual grains, an attempt was made to estimate the heavy-mineral content. A comparison of the results of the two methods shows that, even with a simple suite of easily identifiable heavy minerals, an estimated per cent distribution is not reliable and does not justify the apparent saving in time. End_of_Article - Last_Page 273------------

Stability of Epidote Minerals

A NUMBER of silicate minerals have proved difficult to synthesize by the commonly employed methods involving hydrothermal crystallization of glasses or reactive oxide mixes of suitable composition. Failure to synthesize a phase may be caused by the experimental conditions not being within the region where the phase is thermodynamically stable. But, in some cases, failure simply reflects a large nucleation barrier to the formation of the stable phase so that metastable phases which nucleate with ease form, and may persist indefinitely. Several workers1–3 have considered this problem in relation to the epidote minerals, and the consistent failure of low-pressure synthesis could indicate that these phases may be stable only under relatively high-pressure conditions. Geological occurrence does not entirely support this proposition. The results presented here indicate that with these minerals nucleation is a sluggish process at low pressures, and if this step is by-passed, growth is readily achieved from the phases which normally form in synthesis. This sluggish nucleation is possibly related to the structure of the minerals.

Epidote minerals in the Sanbagawa metamorphic rocks from eastern part of the Kanto Mountains

Epidote minerals: clinozoisite, pistacite and piemontite, are widely distributed in regional metamorphosed rocks, especially in green metamorphic rocks in the eastern part of the Kanto Mountains. Here, (1) the composition variation of epidote minerals with increasing grade of metamorphism roughly estimated from their optical angle, (2) the chemical, optical and X-ray properties of a clinozoisite and a pistacite are shown.

FORMATION OF EPIDOTE IN TANGANYIKA

Minerals of the epidote group in Tanganyika are commonly produced during cataclasis of lime-bearing rocks; invariable accession of the hydroxyl radical during dislocation assists their formation. Epidote minerals are not produced from plagioclase by the action of water, even at high temperatures and pressures; shearing stress or the presence of other material is required. Saussuritization follows, and is often a result of, uralitization. Metasomatism, progressive metamorphism, and granitization of lime-bearing rocks can give rise to epidote minerals. Epidote and clinozoisite are stable in the greenschist facies and the albite-epidote amphibolite facies, but are unstable in the almandine-diopside amphibolite subfacies of the amphibolite facies. Zoisite is unstable in the greenschist facies, but is stable in the albite-epidote amphibolite facies and throughout the amphibolite facies. Lime metasomatism, which is responsible for epidotization, is largely a result of displacement by the alkalies of lime from lime-bearing minerals. Epidotization often accompanies ferruginization and sulfide mineralization. Much epidote in the anatectic granites is the result of contamination; primary magmatic epidote is unlikely to occur. The epidote minerals form between 300°C and 500°C. High hydrostatic pressures favor the formation of zoisite. Under conditions of shearing stress monoclinic epidotes are formed. The epidotes are not stress minerals; under metasomatic conditions they are frequently formed without stress.

METAMORPHISM AND ASSOCIATED MINERALIZATION IN THE PHILIPSBURG REGION, MONTANA

Intrusive contacts of several granodiorite bodies, with calcitic, dolomitic and shaly limestones are described in detail. Isochemical metamorphism included recrystallization and formation of minerals of the pyroxene hornfels facies; it was succeeded by several phases of alteration with addition of material. In the shaly and silicic limestones, addition of iron and smaller quantities of silica and magnesia altered biotite-calcite marble, through several phases, to a banded andradite-diopside tactite. Fluo-hydrothermal introduction of much iron subsequently altered this tactite to one composed predominantly of magnetite and pargasite; simultaneously magnetite and forsterite replaced the pure carbonate formations. Coarse, concentrically zoned garnet was deposited at some intrusive contacts. In the succeeding phase, soda-rich hydrothermal solutions locally replaced nearly all types of rock with scapolite or calcite; the iron was re-concentrated as magnetite at some places in the scapolite. In a final phase, epidote, andradite, and other minerals, derived from the surrounding rock, filled the joints. Pyrite, then other sulfides and gold, also locally followed the Scapolitization. Fractures controlled all phases of the metamorphism, except probably the early reconstitution, although metamorphism was generally more pronounced near intrusive contacts. Most of the controlling structures are older than the intrusives. I n the shaly limestone at Philipsburg, thin aplites were emplaced along the bedding by a combination of replacement and sloping.

On the nature of withamite from Glen Coe, Scotland (With Plate IV.)

In 1824 Henry Witham discovered a beautiful red epidote mineral in the lavas of Glen Coe, Argllshire, which was described by Brewster under the name withamite. An analysis was made by Coverdale of obviously highly impure material, but later a more complete one was carried out by Heddle on carefully hand-picked mineral. Heddle admits, however, that his analysis shows ‘by no means a satisfactory agreement with the composition of epidote’, and from his figures it would seem very likely that his analysed material was also impure. In the Glen Coe Geological Survey Memoir no further data were recorded of this mineral, Heddle's analysis and description merely being quoted.The writer visited Glen Coe and collected material in a cutting on the new road, approximately 5½ miles west of King's House (Geological Survey, sheet 53), where it is fairly well exposed. It occurs in a fine-grained, rather altered andesitic rock, as narrow veinlets up to 120 mm. in length and 50 mm. in width ; also as infillings of vesicles. The veinlets may be straight or highly irregular; in the former case they possibly follow joint-planes, for the rock will often fracture along such a plane, producing one or two surfaces covered with the red epidote mineral. The vesicles vary greatly in size from approximately 0·3 to 70 mm., and in the larger ones calcite is a common associate.

Garnet-bearing Sands of the Northumberland Coast

It has long been known that garnet and other heavy minerals occur in many of the sedimentary beds of the North of England. Among such, where visual proof is easy, are the Inghoe Grits, particularly at Shaftoe, and the Dunstanburgh sandstone at Cullernose Point. Hutchings examined washings of the shales at Seaton Sluice and found zircons in large numbers, along with angular colourless garnet, rutile, anatase, tourmaline, sphene and barytes. In a later study by the same author of the sedimentary beds metamorphosed by the Whin Sill, it was shown that small garnets are very plentiful, especially in the calcareous shales, and are accompanied by augite, hornblende, epidote, sphene, idocrase, wollastonite and other minerals. Kellett made an extended investigation of the heavy minerals in the Coal Measures of County Durham and found 0·4 per cent of these in the sandstone under the Hutton seam (this is the only datum of Kellett's strictly comparable with our results). Zircon proved to be the commonest mineral in these beds; next in order came rutile and tourmaline, while garnet was quite subsidiary.

II.—On Scottish Rocks containing Orthite

From America and from many different localities on the continent of Europe the occurrence of Orthite or Allanite as an accessory mineral in crystalline rocks of different kinds has been frequently described. But so far it does not seem that its occurrence in Great Britain has been recognized, or at any rate I have failed to find any mention of it. Quite recently two instances of Scottish orthitebearing rocks have come under my notice which I wish to describe briefly in the present communication. Both are in acid holocrystalline rocks—granites and gneisses—and in both it is associated with the closely allied mineral epidote, as is usually the case. For specimens of the granite of Fell Hill I am indebted to Mr. James More, jun., M.Inst.C.E., Edinburgh, who sent sections of this rock for microscopic examination to Professor James Geikie, Edinburgh University. Although only two in number, the sections contain at least eight recognizable crystals of orthite, besides smaller pieces about the identity of which there might be room for doubt, and some of these are fortunately so cut as to render possible not only a certain identification of the mineral, but also a fairly complete account of its optical properties.