LEIDSE GEOLOGISCHE Vol. 249—259 1-4-1967 MEDEDELINGEN, 39, pp. published separately
Mineralogy and petrograghy of some tin, lithium and beryllium bearing albite-pegmatites near Doade, Galicia, Spain
BY
B.J. Hensen
Abstract
The petrography and mineralogy of some Hercynian albite-pegmatites near Doade, Galicia, Spain is described.
The mineral assemblage consists of albite, K-feldspar, quartz, muscovite, spodumene, petalite, cassiterite, beryl, columbite-
tantalite, montebrasite, apatite, eosphorite-childrenite, zircon, tourmaline, garnet, pyrite, bertrandite, ferrisicklerite, heterosite
and cookeite.
A tentative of these minerals is From the between and paragenetical sequence given. relationships petalite spodumene approxi-
mate initial temperature-pressure conditions of600°C. and 2,5—3,5 kb. can be derived. The possible cause of the high sodium content and of the fine grainsize of the bulk of the pegmatites is discussed. A genetical relationship of the pegmatites with the
medium to coarse grained Hercynian two-mica granites is suggested.
Resumen en español
Se describe la la de hercinianas de albita de petrografia y mineralogia algunas pegmatitas cerca Doade, Galicia, Espaňa.
Las contienen los minerales pegmatitas siguientes: albita, feldespato alcalino, cuarzo, muscovita, espodumena, petalita,
cassiterita, berilo, columbita-tantalita, montebrasita, apatita, eosphorita-childrenita, zircon, turmalina, granate, pirita, heterosita cookeita. bertrandita, ferri-sicklerita, y
Se da serie tentativa de De deducir una paragenética estos minerales. las relaciones entre petalita y espodumena se puede
condiciones iniciales de de 600°C. kb. La del elevado temperatura y presion aproximadas y 2,5—3,5 causa posible porcentaje
de albita del fino de la de Se los y grano mayor parte la pegmatita son discutidos. propone una relación genética con hercinianos de dos micas de medio granitos grano y grueso.
Contents 600 m. wide and about 15 km. long (Fig. 1) and have
thicknesses ranging between 0,5 —6 m. At approxi- Introduction 249 mately one km. to the W. of this zone an Hercynian
two-mica granite out. Its contact is roughly The country rock 250 crops parallel to the schistosity of the mica schists. The Petrography of the pegmatites 251 relationship between this granite and the pegmatites
is discussed below. Summary of paragenetic relationships 256 Most of the veins have been mined for cassiterite and Genesis of the pegmatite 257 some for beryl. Mining developments are mainly open but shallow Conclusions 259 cut some underground workings are
present. Acknowledgements 259 of the Most the study was confined to pegmatite bodies Doade where detailed References 259 near (Fig. 2) sampling
and subsequent mineralogical investigations were
carried out. Two mines have been developed in this
of the Parabellon in the INTRODUCTION part pegmatite zone, namely N. and Alcebedo in the S.
Field-work carried In the N. vertical section reveals the was out by the author in the summer open cut a shape this of 1964 on some pegmatites occurring near the border of the main pegmatite vein (Plate I Fig. 1). In of section vertical vein its the provinces Pontevedra and Orense in Galicia, a dyke-like suddenly changes N.W. 186 and 153 of the almost horizontal and thickens and Spain. Topographical maps dip to considerably
Instituto Catastral de were used further W. it losing most of its thickness Geografico y Espana steepens again scale of The is similar in the on an enlarged 1 : 25.000. (Fig. 3). pattern underground
The studied veins in of the Alcebedomine to the former pegmatites occur as garnet- workings according NNW- of this mine. the W. bearing quartz-mica schists that have a vertical manager Thus, a stepwise dip to
SSE striking schistosity. The veins, which run parallel seems to be general. The with the wallrock Inclusions to the schistosity, are not continuous and are of contacts are sharp.
variable thickness. All veins are confined to a zone of schist in the pegmatite are rare. Some large patches 250 B. J. Hensen
2. of the Doade Thicknesses of Fig. Geological map area.
the veins are exaggerated.
be an axial plane to small scale folds that have sub-
horizontal Numerous bands axes. quartz occur gen- erally forming boudins parallel to the schistosity and occasionally following the above mentioned small
scale folding. Close to the contact with the pegmatites the schists are very rich in tourmaline.
1. after Mineralogy Fig. Simplified geological map Parga-Pondal (1963) constituent. Quartz. Quartz is always a major and unpublisheddata ofthe Department ofPetrology it shows undulose extinction. Different and Mineralogy, Leiden University. Encircled Invariably of of them to numbers refer to the locations of handspecimens of types quartz, some probably belonging different have been found. Small which the micas were analysed for trace elements. generations, quartzes (Read late-Hercynian instead of post-Hercynian) with their long dimension oriented parallel to the while in the schistosity occur in the mica-rich bands of schist enclosed in which are completely or partly bands quartz bigger equidimensional grains occur. pegmatite have not been rotated or otherwise dis- However, in some thin sections of the mica schists placed to any extent. Irregular apophyses ofpegmatite the these grains have an elongated habit, elongation in the schist are relatively rare but seem to be slightly being parallel or at a small angle to that of the smaller more abundant in the thick, irregular flat-lying parts grains in the mica-rich bands, and in others they are of the veins pegmatite (Plate I, fig. 2). strongly undulose, showing amoeboid intergrowth
In both the N. and the S. a vein of milky is quartz produced by recrystallisation ofcrushed zonesbetween found in the immediatevicinity ofthe main pegmatite the grains. vein. The dimensions of this vein 2 quartz are m. Muscovite. This mineral is relatively fine-grained thick and several tens of metres long (Fig. 2). to and has a preferred orientation parallel the schisto- crenulation sity. Some recrystallised cleavages are THE COUNTRY ROCK the foliation observed, showing that present was The schists that constitute the earlier A small quartz-mica country preceded by an schistosity. proportion rock third ill-defined which is have a vertical NNW-SSE striking schistosity. of the micas indicate a plane This of amoeboid schistosity has been observed in some places to parallel to the direction elongated quartz Mineralogy and Petrography of some Albite-Pegmatites 251
PETROGRAPHY OF THE PEGMATITES
The pegmatites are leucocratic. Albite is the main
constituent. and muscovite K-feldspar, quartz are found in varying amounts. Petalite, spodumene and ofthe minerals difficult most accessory are to recognize macroscopically because of their white colour and
fine grainsize. Exceptions are cassiterite, columbite-
tantalite and beryl. The in pegmatites vary grainsize. Only K-feldspar in 5 None occurs relatively large crystals up to cm. of the other minerals exceeds the grainsize of 2 cm. Furthermore, the grainsize distribution is inhomo-
In the wall is geneous. places a layering parallel to marked by variations in grainsize and changes in
mineralogical composition (Plate I, fig. 3). The for layering, however, cannot be traced any great
distance. Irregular variations in grainsize occur even
within a thin section. Fine fractions have grainsizes of have 0,05—0,15 cm., coarser parts 0,3—1,5 cm.
while the K-feldspar-rich bands and patches vary from 1,5—4 cm. A of 2 —10 thick is contact zone cm. generally present.
It consists of fine-grained muscovite and quartz. Very fine grains of cassiterite and apatite have been found
in this zone. The first albite crystals adjoining the
contact zone are often oriented with their longest axis
normal to the wall.
Mineralogy Fig. 3. Sketch of the pegmatite vein at Parrabellon. profile Albite. Albite is the most abundant constituent of Outcrop of the vein is inserted. map the which contain 80 pegmatites, occasionally up to %
ofthis mineral. It is white to colourless and the longest
dimensions of the generally lath shaped crystals do
described above. Late cross-muscovites occur. These not exceed 1 cm.
also show deformation in the form of bending and Microscopically, several varieties of albite may be kinking, similar to that observed in all of the above recognised. Crystals of all varieties have large negative
mentioned varieties. extinction angles (in sections perpendicular to X the
Biotite. This mineral is less abundant than between Y and the traces of between musco- angle (010) vary 14 and albite vite. Its pleochroism is red-brown (Z, Y) to yellow 17°, indicating a nearly pure to pure The fabric is the for Albite is but Carlsbad- (X). arrangement same as composition). twinning common albite and muscovite with which it is frequently intergrown. pericline twinning are relatively rare.
Cross-biotite has not been observed. The varieties are as follows:
Small albite orientation Garnet. Small poikiloblastic crystals (1 —3 mm.) of i) crystals with random are contain small included in and show corroded garnet are present. They elongated K-feldspar out-
quartzes that are mainly parallel to the external lines. These probably crystallised at a very early foliation. Mica-rich bands bend around the crystals. stage. Tourmaline. Small, euhedral, needle shaped crystals ii) Relatively big crystals that often display graphic with of (1 mm.) have a pleochroism of yellow-brown (O) to intergrowth non-perthitic K-feldspar are colourless (E). When zoned the pleochroism of the later origin. The K-feldspar is usually about but in cores is blue-green (O) to colourless (E). The c-axes 5—10 % rare cases may reach 50 %
of the crystals are parallel to the schistosity, being (Plate I, fig. 4). As indicated below this albite
randomly oriented in this plane. In thin sections in variety commonly accompanies petalite and which the show amoeboid The subhedral euhedral quartzes intergrowth, slight spodumene. to crystals
deformation of the tourmaline has occurred in the are lath shaped and exhibit irregular twinning,
form of bending accompanied by fracturing at right various patches having lamellae of different
angles to the c-axis. thickness. Wedge shaped twins are common, Accessories. unidentified in that bent. Zircon, apatite, anatase, especially crystals are
opaque mineral (lath shaped parallel tot het schist- iii) The fine grained (0.2 —7 mm.) sugary type of
osity). albite is quantitatively the most important one. 252 B. J. Hensen
The euhedral to subhedral crystals are lath- spodumene is common. Only albite and quartz have albite shaped and display a fine lamellar twinning. been noticed as inclusions.
Together with muscovite and quartz the sugary The second variety, interstitial K-feldspar, commonly
albite corrodes and replaces K-feldspar phenocrysts shows crosshatching and Carlsbad twinning and is not
along crystal boundaries, cleavage planes and perthitic. It is only found in small quantities. This
irregular veins. The albite crystals in these veins K-feldspar seems to have crystallised at the same time have random orientation. This process leads in or later than the sugary albite.
to the almost of the Both varieties have in thin places complete replacement a cloudy appearance section few of K-feldspar, a patches K-feldspar with owing to alteration to kaolinite. orientation the former Adularia is the third parallel indicating presence variety and abundant in a few
ofthat mineral hand-specimens collected in the western pegmatite near with Granophyric intergrowth quartz is locally very Parabellon. Euhedral crystals occur in small cavities abundant in both the bigger albites and particularly together with quartz but the crystals in the solid rock the saccharoidal of vermicular have euhedral type. Separate swarms a hemimorphic appearance showing inclusions within be outlines one grain may distinguished by wedgeshaped on one side only. Diagonal their different orientations. The of is fine optical presence simple twinning common. Very irregular
inclusions does influence the Crosshatch be observed in quartz normally not twinning (?) can parts habit of the albite (Plate II, fig. 1). Sometimes the where the alteration to kaolin is less pronounced.
is confined to the rims of 2Vx m 70—80°. with adularia granophyric quartz crystals, Together aggregates which have corroded outlines. it of fine and Mostly, however, greenish muscovite, idiomorphic quartz occurs irregularly, showing no preference for either bertrandite occur. of the albite core or outer parts crystals. Frequently, Quartz. Quartz is a major constituent whose of albite contains the vermicular in thin sections only part an crystal percentage varies from 25 to 60. The
quartz and this area is separated from the normal anhedral grains commonly show undulose extinction. albite a the from the Alcebedo undulose by sharp boundary. Although grano- Quartzes area are strongly with be due and in phyric intergrowths quartz may to some places amoeboidintergrowth ofelongated 1 izationof simultaneous crystal albite with that mineral grains occurs indicating deformation. No euhedral
described the observed relation- after have as by Mayer (1965), pseudomorphs high temperature quartz ships seem to indicate that some of the myrmekitic been detected. of intergrowths are due to the replacement albite by Apart from the small parallel oriented euhedral
quartz. quartzes that occur with cloudy albite and adularia,
Veins of normal albite in a earlier formed sugary granophyric-rich replacing minerals, prismatic quartz rock do favour time described not a relationship as crystals have been observed in veins and small by Maijer in which granophyric albite crystallizes cavities. later than the It is that the As indicated in sugary type. thought above, milky quartz occurs two large also between the apparent contradictory observations, quartz veins near the Parabellon and Alcebedo second and third caused the fact that varieties, are by open cuts. the paragenetic relationships not only vary in time Muscovite. Fine-grained (0,5 —5 mm.) muscovite is but also in found the The space. throughout pegmatite. average per- fourth of A possible variety irregularly twinned, cloudy centage is estimated to be 5 to 15. albite which with occurs together adularia and Especially in spodumene-rich specimens in which euhedral is formed under quartz probably hydro- quartz predominates both muscovite and albite are thermal conditions as proposed by Maijer (1965). This rather rare. Most muscovite occurs with saccharoidal found in thin section. type was only one All varieties albite and quartz. show of deformation in the form of and One mica signs bending was observed to have a central part with
undulose extinction. = 2V 0° and a with 2V = 40°. This X marginal part X The of in could that the K-feldspar. percentage K-feldspar the mean mica is composed of different pegmatite probably does not exceed 15 but is difficult structural polymorphs (Deer, Howie and Zussman, estimate because of its mode of to occurrence. 1962). Three varieties be The can distinguished. first occurs X-ray powder patterns of micas from both Parabellon in white to light pinkish of to 5 in and Alcebedo with crystals up cm. were compared values given by length. Euhedral crystals are not common but when Levinson (1953). They show some resemblance to the present are mainly concentrated in patches and bands. monoclinic 2-layer Li-muscovite, but the Li 0 content 2 Crosshatch is twinning very rarely observed in portions appears to be relatively low. of the and Carlsbad crystals twinning is absent. Trace-element analysis of Sn, Mn, Li and B contents
Perthite either as fine and in Table 1. Muscovite from the may occur strings patches are given Parabellon which are probably due to exsolution of mine has a Li O of Micas from Na-feldspar, a 0,13 %. Campanarias or as a coarser albite twinned in veins and have Li 0 = and those from Alfonsin type patches 2 0,05 % which might be due to albite. Re- Li Q = 0,23 replacement by 2 %.
muscovite and with is placement by albite, quartz ± Graphic intergrowth quartz occasionally Mineralogy and Petrography of some Albite-Pegmatites 253
TABLE 1 Trace analyses of muscovites granulated, cloudy zones containing spodumene and
quartz, that occasionally cut across euhedral spodu- Li 0 MnO Sn B 2 mene crystals, are most likely due to deformation.
Offsetting along these zones has not been observed. Parabellon 0,13 0,04 0,073 0,01 Spodumene is not altered. Campanarias 0,05 0,02 0,053 0,01 Petalite. (Li(Na)Si AlO ) In contrast with \lfonsin 0,23 0,06 0,061 0,01 4 lO this mineral encountered in spodumene, was only very
Analyses by H.M.I.B. Arnhem. few thin sections of specimens collected in the eastern vein of the Alcebedo mine. encountered. K-feldspar is often replaced by muscovite Macroscopically its colour is white. The anhedral in particular along grain-boundaries. Very fine- grains with grainsizes varying from 0,05 —1,5 cm. of muscovite in the in albite-rich rocks. Alteration grained, greenish aggregates occur occur along cleavage
to adularia-rich rocks. They are probably of a late planes yellowish-brown, fibrous montmorilloniteis feature. Lamellar is origin. a typical twinning rare.
Deformation of muscovite in the form of bending and Near the margin of a large crystal small albite in- kinking is common. In the more strongly deformed clusions occur exhibiting thin, myrmekitic rims. This of is in oriented parts the pegmatites muscovite concentrated crystal also encloses differently petalite grains, trails that follow a twisting course between the feldspar that become more abundant towards the margin, mosaic of small —2 crystals. grading into a (0,5 mm.) petalite
In hand-specimen these rocks have a coarse phyllonitic grains. Along grain boundaries replacement by texture similar to that found in parts of the Hercynian graphically intergrown spodumene and quartz is two-mica granites (Plate IV, fig. 4). The rocks part observed (Fig. 4). The breakdown of petalite to along irregular planes which are mica-rich and display spodumene and quartz has been demonstrated ex- slickensides. perimentally by Stewart (1963).
o occurs only in The textural seem to indicate that petalite Spodumene. (LiAlSi 2 6) Spodumene relationships where it constitute with the albite. the veins of the Alcebedo area may was formed before and together
25—35 of the rock. The white to colourless formed later. % crystals Spodumene was noticed are fine-grained (0,5 —3 mm.) and have a prismatic The pink clay mineral that had been previously habit. basal sections and others or platy Euhedral, eight-sided by Ypma (personal communication) of with simple (100) twinning are numerous although proved to be the alteration product petalite. It sub- anhedral outlines encountered. in the E. vein and found in commonly to are occurs in small nests was In thin section sheaf-like 10 in one aggregates radiating greater quantities (in nests up to cm. diameter) thin section from from a vein have been noticed (Plate 11, fig. 2). in a mine near Presqueira (Fig. 1). A
2V * 65—70° and Z/c = 23—25°. this contains petalite. An X-ray powder Z locality
Spodumene often shows graphic intergrowth with photograph proved that the mineral was dioctahsdral, of the quartz. The relative proportions intergrown minerals from in vary some quartz droplets spodumene of The to small isles spodumene surrounded by quartz. often and quartzes are relatively big strongly undulose, not infrequently displaying amoeboid intergrowths.
Several spodumene grains with different orientations "sieve often occur in individual quartz grains giving structure" (Plate II, fig. 3).
Spodumene replaces K-feldspar, although a fine graphic intergrowth observed locally on the boundary between the two minerals might indicate simultaneous crystallization. Anhedral, symplectitic spodumene
(spodumene + albite?), together with albite and muscovite, that is encountered in veins of K-feldspar, has been formed later (Plate II, fig. 4). Against albite, and rarely against K-feldspar, the spodumene displays symplectitic rims (Plate III,
Fig. 1). Wholly symplectitic, anhedral grains as described above occur interstitial to the first generation albite, slightly corroding it along grain-boundaries.
The second mineral in the symplectites often appears
to be quartz. Only in a few instances in rims against albite grains, albite could be proved to be the inter-
In one thin section this was found grown phase. type Fig. 4. Petalite, showing cleavage, replaced by a graphic in albite-rich rock. as dispersed grains an Finely intergrowth of spodumene (stippled) and quartz. 254 B. J. Hensen
the montmorillonite anhedral to thus belonging to group. (See outlines. Grainsizes vary from 0,3 3 mm. chemical in Table with analysis 2). Beryl occurs together sugary albite, quartz, section of Plate III, fig. 2 shows a thin a pink mont- muscovite and cassiterite. morillonite in which relics of In thin section of the altered aggregate petalite, a hydrothermally part of lamellar The colour the W. vein of the Alcebedo displaying twinning, occur. pink area, a grain was en- of the mineral is its moisture dependent upon content countered that vaguely exhibited two cleavages at because when dried it loses its colour. right angles. An X-ray powder photograph of this
grain revealed a distortionofthe normal beryl pattern. The cell calculated the TABLE 2. Chemical analysis of montmorillonite parameters were using d-values
of the and reflectionsand = A (304) (431) are a 9,159 Si0 44,89 and c = A. with values 2 9,142 Compared given by A1 0
31,15 = = 2 3 Deer, Howie and Zussman (1962) i.e. a 9,188 A, c Fe 0 0,44 2 3 9,189 A and the A.S.T.M. X-ray, powder data file FeO 0,00 i.e. consider- (card 9-430), a = 9,215 A, c = 9,192 A, a MnO 0,07 able shrinkage of the lattice is indicated. MgO 1,24 In the same thin section and in others made from CaO 0,16 the Na 0 same hand-specimen, beryl is found only in 2 0,07
K 0 0,03 small amounts while the mineral bertrandite 2 very
Ti0 0 2 0,00 (Be Si (OH) ) is rather abundant. 4 2 7 2
p*o 0,07 As indications of alteration of to this mineral 5 beryl Li 0 0,3 2 have been found it seems possible that the occurrence
Loss on ignition 22,34 of the anomalous cell parameters is due to incipient
alterationof beryl to bertrandite. By the loss ofAl and 100,76 Si and the addition of H 0 the lattice 2 beryl might Miss A. Giesbert shrink before it loses its Analyst: slightly hexagonal symmetry and becomes that has rhombic bertrandite, a pyramidal Howie and Zussman this alteration Deer, (1962) report symmetry (Solovyeva and Belov, 1961).
to be typical for petalite occurrences. Three other alteration products have been found, the To date petalite has not been recorded from Galicia first two listed below also occurring with bertrandite. N. The identification of All three or Portugal. petalite was minerals are mostly fibrous and occur in confirmed radial by X-ray powder photography. aggregates. They occur along grain-boundaries Cassiterite. values indicate that the and Mining occasionally completely replace beryl. These are: cassiterite of the does A biotite-like mineral weight-percentage pegmatite i) with a pleochroism of dark exceed Cassiterite is found not 0,2. dispersed through brown or green-brown (Y, Z) to pale yellow (X), the subhedral and 2V 20° pegmatite as anhedral, occasionally = approximately, r euhedral crystals up to 0,5 cm. in size. birefringence, the mineral is black. In the Macroscopically heavy ii) a chlorite-like mineral with an orange-yellow concentrate mineral dipyramidal crystals with occa- colour, 2V = 40° low bire- Z approximately, sionalsmall faces prism were observed. In thin section fringence and anomalous interference colours, and sections with lozenge-shaped diagonal twin planes iii) a clay mineral with very low birefringence and are common. 2V = 30° Z approximately. Pleochroism is red-brown (E) to yellow to colourless Columbite-tantalite. In the heavy mineral concentrates A marked bands (O). zoning by showing an oscillating subhedral to euhedral platy and prismatic crystals of intensity of pleochroism is common. The zoning is columbite-tantalitehave been found. The crystal faces often not to to irregular, outer zones being parallel inner are striated parallel their length and have a black and sometimes them off. lustre with streak is zones cutting a purple tinge. The dark brown Inclusions of quartz, muscovite and albite are found, to black. in the the mineral rarely large quantities, giving cassiterite a Microscopically is opaque, probably due poikiloblastic habit. to the fact that it is Fe-rich. Very small inclusions The of paragenetic assemblage consists of both albite columbite-tantalitewere noticed in the margin and the and muscovite. the outside of cassiterite of types, especially sugary type, quartz on crystals. Intergrowth In the spodumene-rich rocks cassiterite is rare or bigger grains with cassiterite has been observed absent. It does not occur as inclusions in K-feldspar. macroscopically. In the small of cassiterite In the from the contact-zone very grains are spodumene-rich specimens S., found together with muscovite and quartz. columbite-tantalite has been found to be more Beryl. Beryl is a common and often ratherabundant abundant than cassiterite in a heavy mineral con- accessory constituent. Its percentage equals and not centrate, but the total percentage of heavy minerals this infrequently exceeds that of cassiterite. In hand- was very low in case. specimen it has a pale green to yellowish colour. The X-ray pattern is in accordance with that of Crystals are mainly prismatic and have subhedral to columbite-tantalite. Mineralogy and Petrography of some Albite-Pegmatites 255 Montebrasite. Montebrasite Garnet. A few of this mineral have been found (LiAI(OH)PO 4) grains occurs in very small quantities both in the N. and S. in the heavy mineral concentrate of the northern mine. veins. The colourless in size forms euhedral grains are 0,1 to 1 mm. It very small, pinkish, crystals. and have subhedral, platy or anhedral habit. Lamellar a 0 = 11.57 A, n = 1,81. These properties indicate is almandite twinning typically developed. One grain displayed a garnet with high spessartite and content, of almandite. This is in accordance two systems twinning approximately at right angles having spessartite > to each other (Plate 111, fig. 3). with the values given by Schneiderhohn (1961) for In the southern veins the mineral with in occurs together garnets pegmatites. spodumene, in which it is occasionally included. In Pyrite. In the heavy mineral concentrate of the the Parabellon veins it is found with southern mine small amounts of together sugary very fine-grained albite, muscovite, quartz, cassiterite and beryl, all of pyrite were found. The identity of this mineral was which verified may replace K-feldspar. by X-ray photography. to Winchell and Winchell the Bertrandite. 5i 0 Bertrandite was According (1956) (8e 4 2 7(0H) 2) measured value of 2V = 72° indicates that the It forms small z 71, only recognized microscopically. mineral is the F-free end member of the isomorphous colourless crystals with sub- to euhedral outlines. It amblygonite-montebrasite series. Similar montebrasite exhibits two perfect cleavages at right angles and a has been described by Maijer (1965). Three alteration typical heart-shaped, simple twinning (Plate 111, described be These twins mentioned products by Maijer can recognized. Fig. 4). heart-shaped (Oil) are i) A light green to colourless mineral with very low by Troger (1959). Relief and birefringence are very birefringence, similar to those ofmuscovite. 2V = 70, 71° (measured) x ii) a fibrous chlorite-like mineral. Both i) and ii) occur and r < v weak. along grain-boundaries and Direct alteration of beryl to bertrandite could only iii) a mineral with a higher birefringence than be found in a few places. Generally the bertrandite of montebrasite, occurring along cleavage-planes. occurs in clusters euhedral, hemimorphic crystals, As the alteration of the montebrasite is that the mineral not very indicating probably crystallized pronounced no determinationof the optical properties from hydrothermal solutions. It is almost invariably of these materials could be made. found together with adularia, both in the W. veins, Apatite This mineral is only found in small amounts. where this mineral is very abundant locally and in with the where vein It forms small, seldom euhedral crystals often a E. veins, only a very small containing adularia and bertrandite observed in albite- cloudy appearance. was an It has probably formed at different times during rich specimen. Bertrandite is not altered. of the in the verified the of this the crystallization pegmatite. It occurs X-ray powder photographs identity wall zone and in the albite-rich rocks. It is slightly more mineral. common in the eastern altered Ferri-sicklerite. Ferrisicklerite hydrothermally rocks, (Li(Fe.Mn)PO 4) where it is associated with adularia. only occurs in the Alcebedo mine. This mineral, AIP0 H that has been described from several Eosphorite-childrenite. ( (MnFe) 4(OH) 2 . 2O) Li-pegmatites, This mineral has been encountered in some minute is probably a hydrothermal alteration product of clusters of anhedral grains in thin sections of the veins triphylite (Mason, 1964). of Parabellon mine. the mineral Colour is dark brown black. In thin section the Microscopically, very to a is colourless and has a moderate birefringence and dark red brown to orange brown pleochroism is 2V = 44° The is parallel observed. The anhedral show one good and X (measured). optical plane grains the rather The relief is each other. The to poor (100) cleavage. high one poor cleavage at right angles to have could and axial dispersion r < v strong. These properties grains a straight extinction. An axial figure indicate that the composition lies near the eosphorite not be obtained. An X-ray analysis provided values end of the series. very similar to those given by Bjorling and Westgren The mineral has been reported from two other local- (1938) for this mineral in the Varutrask pegmatite ities in Galicia (Ave Lallemant, 1965 and Hensen, (Sweden). See Table 3. Note that values under d A these authors. 1965). Both of these occurrences are in Hercynian = 1,35 had not been recorded by leucocratic muscovite-granite. The anhedral grains are dispersed through the Zircon. Zircon is found in heavy mineral concen- spodumene-rich parts of the pegmatite. Grainsize is euhedral from is abundant in the W. trates as small, brown-yellow, crystals, 0,05 to 0,5 cm. It most having tetragonal-bipyramidal symmetry. In thin vein of the Alcebedo-mine. The mineral readily alters heterosite if the section they are metamict (cloudy) and show de- to exposed to atmosphere. composition haloes. X-ray powder pattern of these Triphylite which has not been found in this pegmatite crystals confirmed them to be zircon. has been reported by the author in an aplite near Tourmaline. Tourmaline is It Bueu 2Vz = very rare. was only (Hensen, 1965). Optical properties are a observed once in narrow vein containing very little 5—10°, axial dispersion r>v . marked, high reliefand cassiterite other accessories. The small low The data and no pris- birefringence (< quartz). X-ray powder matic crystals show a pleochroism of yellowish brown are consistent with those given in the A.S.T.M. X-ray (O) to colourless (E). powder data file for sicklerite (card 13-338). 256 B. J. Hensen TABLE 3. data Ferri-sicklerite and Heterosite. X-raypowder for have acute, wedge-shaped forms and striated crystal Fe-Kα diam. 90 radiation, camera mm. faces (Plate IV, fig. 1). the those Optical properties are same as given by Ferri-sicklerite Heterosite Winchell and Winchell (1956). 2V varies from 0 to d(A) I d(A) I Z 70°. Axial dispersion r > v is distinct. n * 1.57. A y basal section divided into six bi-axial sectors around 5,06 s 4,88 s an uni-axial centre as described the above named 4,37 vs 4,30 vs by authors observed in of the In other 3,84 m 3,74 m was one crystals. 3,49 s 3,46 s crystals either this phenomenon is only partly devel- 2,96 vs 2,92 vs or fan-like extinction exhibited. oped patterns are 2,75 m 2,69 w X-ray data are in accordance with those given by the 2,49 VS 2,43 s A.S.T.M. X-ray powder data file (card 10-397). 2,43 m 2,42 W With a crystal reflection goniometer two zones of 2,33 w 2,32 w weak faces in relatively reflecting were detected one 2,24 w 2,25 vw crystal (Plate IV, fig. 2). The small numberofmeasured 2,17 \v 2,21 vw crystal faces did not ananswer to the 1,86 w 2,16 m supply crystallo- w of the 1,75 1,97 vw graphic symmetry crystal. However, an X-ray 1,67 w 1,84 vw rotation photograph, rotating the crystal around its 1,61 m 1,81 w zone-axis of most prominent (zone 4 faces) gave the 1,48 w 1,72 w result presented in Plate IV, fig. 3. An almost random 1,46 \v 1,68 w orientation of fibrous is in- probably components 1,35 m 1,63 vw dicated by the photograph. Some preferred orientation 1,28 w 1,57 w is expressed by slightly thicker and more blackened 1,25 w 1,55 w of parts the lines, which show a symmetrical pattern. 1,16 vw 1,51 w Part of the of different colour- 1,15 vw 1,46 m crystals are composed a less mineral. Striations the faces 1,13 vw 1,41 w on crystal continue 1,12 vw 1,33 \v uninterruptedly across mineral boundaries. The fact 1,10 vw 1,30 vw that this mineral appears to be quartz, makes it very 1,09 w 1,24 vw that the cookeite probable crystals are partial pseudo- 1,04 vw 1,23 vw after morphs quartz. None of the measured interfacial 1,02 vw 1,21 vw found angles are among the normal crystal faces of 1,01 WW 1,19 vw quartz, listed by Frondel (1962). 0,99 w 1,10 w Mn oxide. Black on surfaces and 0,98 vw 1,09 w (Fe) coating along of albite and muscovite are 1,07 vv cleavage planes very abundant in of the parts pegmatite. Microanalysis Heterosite. This mineral is the (Fe, Mn)PO ) as 4 proved Mn the most important element. Iron is weathering product of ferrisicklerite. It after shows, only of minor importance. One exception of the thin black has been a coating removed, a bright Alcebedo mine Mn trace and P gave Fe positive, colour. In two at purple places cleavages, approxi- positive. This is similar to the results of heterosite. mately 90°, could be observed. The hardness of the These gave Fe positive, Mn positive and P positive. mineral is low and it has a purple streak. Under the thin microscope cleavage fragments have a purple SUMMARY OF PARAGENETIC RELATIONSHIPS colour and are weakly birefringent. Pleochroism as described by Quensel (1957) has not been observed. On the basis of the microscopic investigations, According to Quensel the above mentioned black described above, several consecutive phases in the is an H Fe-Mn coating 2O-rich, phosphate. crystallisation history of the pegmatite can be distin- analysis of heterosite provided values that are X-ray the between the guished. Although age relationships slightly different from those given by Bjorling and different phases could not definitely be established (1938). The d-values are given in Table 3. Westgren of the each and some phases may largely overlap Around the small of heterosite masses an orange- other, the paragenetic assemblages as given below are brown powdery decomposition material is found. An regarded to be significant. The phases are listed in the photograph showed only a small number of X-ray order of formation. lines in those of very vague part corresponding to heterosite. Pegmatitic phase Cookeite. Si AlO The mineral + minor amounts of albite (variety (i) ) (LiAl 4(OH) 8 3 10) a) K-feldspar cookeite encountered in fraction and that included in the former has been a ground quartz are (< 0,4 mm.) of two hand-specimens, from the b) albite (variety (ii) ) + petalite albite Alcebedo mine. It could not be detected in thin c) spodumene + quartz ± (variety (ii) ) sections albite muscovite + of these rocks. It occurs as transparent, light d) sugary (variety (iii) ) + quartz euhedral of 2 in that minerals. green, crystals 0,5 to mm. size, i cassiterite, beryl + accessory and 257 Mineralogy Petrography of some Albite-Pegmalites This assemblage constitutes the bulk of the of above named composition can exist at tem- below 625° C. pegmatites. peratures iii) The pressure-temperature conditionsof the system Hydrothermal phase influence the formation of either petalite or adularia, bertrandite, ferri-sicklerite, cookeite, spodumene independently of the composition of albite (variety (iv) ). the melt. It has been found that the petalite- Supergene phase feldspar assemblage is the high temperature-low clay minerals, heterosite. pressure equivalent of the feldspar-spodumene- Under quartz assemblage. equilibrium-conditions These paragenetic relations are summarisedin Table 5. petalite is stable between 550°—680° C. at 2 kb. and 635°—680° C. at 4 kb. Below both minimum TABLE 5. Summary ofpargenetical relationships temperatures petalite reacts to spodumene 4- 2 quartz. phase Hydro- Super- Pegmatitic It would then that the of both seem presence petalite b d thermal a c gene and spodumene and the reaction rims of spodumene phase phase in the rocks under and quartz around the petalite Albite discussion indicate that the must have X XX X XXX X temperature X X been close the K-feldspar XX X to phase boundary. Stewart further found that the solid dissolved Quartz X XXX XX X (1963) Muscovite X X XXX X in the gas phase coexisting with the melt contains more Spodumene (A) XX X indications for silica than the melt. As other an escape Petalite (A) of from the exist the gas system (see below), quartz- Casisterite X veins be veins occurring close to the pegmatite may Beryl X explained as having been formed from these solutions. Columbite- and Tuttle elaborate on the tantalite X Jahns (1963) genetic between and Montebrasite X relationships pegmatites aplites. They the relative enrichment in sodium of the Apatite X X emphasize Both the Eosphorite- fine-grained parts of layered pegmatites. childrenite X difference in grainsize and in chemical composition of Zircon X aplitic and pegmatitic phases is explained by a Tourmaline X in segregation process in which potassium preference Garnet X to sodium is extracted from the liquid by a vapour. Pyrite X this Experimental evidence has given proof for process Bertrandite X also for the that Ferri- and supposition phases crystallized from the are much coarser than those crystal- sicklerite (A) X vapour Heterosite from the melt. (A) X lized directly Cookeite (A) X Several phenomena that have been observed during Chlorite(-like) "Bodies of the present study fit their category aplite mineral X X with or fine-grained pegmatite very large phenocrysts Biotite-like mineral X X or megacrysts" remarkably well. The following Pink montmorill- properties have to be noted: onite (A) X X and i) Anhedral to euhedral phenocrysts of perthite Mn, Fe oxide X other minerals. and ii) Groundmass consisting mainly of quartz The pegmatitic phase is subdivided into four subphases that albite, with a sugary appearance and grainsizes coincide with those mentioned in the text. from mm. in 0,2 —5,0 (A) indicates found Alcebedo area only. iii) Fine layering of bands with different composition (probably flow-layering). The layered ground- mass is commonly traceable into homogeneous GENESIS OF THE PEGMATITE rock both laterally and in directions normal to the The tends be Recent experimental work on the crystallization layering. layering to parallel to behaviour of Li-containing melts by Stewart (1960, the walls. 1963) indicates the following: Although the phenocrysts in this case are relatively i) the saturated feldspar-silica liquidus is lowered by small (up to 5 cm.) and merely consist of perthitic of it must be in mind that small the presence Li-bearing components, K-feldspar, kept only a of number of zoned vertical section is accessible for ii) the bulk compositions a large, investigation. and unzoned pegmatites liein a region ofminimum Maijer (1965) reports bigger phenocrysts of K-feldspar from, the melting temperatures of the system albite-quartz- and also certain other minerals otherwise, The Li 0 content ofthese rocks and similar spodumene-water. 2 mineralogically texturally very pegmatites, from He refers varies from 1,2—1,8 %. An homogeneous magma the Amarante area, N. Portugal. to a 258 B. J. Hensen study by Cotela Neiva (1954) of pegmatites near Mineralogy Cotela is Cabracao, N. Portugal. Neiva showed that The mineralogy of this two-mica granite very chemical analyses on specimens taken at different similar to that given by Floor (1966) and is as follows: intervals between 0 and 70 m. below the surface K-feldspar. (30—35 %) The K-feldspar is indicate a rapid decrease of potassium with depth. perthitic and has small inclusions of plagioclase that the fine in all show decalcification rims. It has marked microcline However, apparent uniform, grainsize a outcrops visited in the pegmatitezone, which have cross-hatching. maximum differences in elevation of approximately Plagioclase. (25—30 %) Albite composition 100 does not indicate differentiation of this —An is m., any (An 9 10). Although zoning inconspicuous, within these limits. seriticized indicate that kind cores seem to a zonary It is possible that in this case the mechanism of distribution with a more lime rich core originally "pressure quenching" (Jahns and Tuttle, 1963) that existed. results from the of the from the In undulose. escape vapour phase Quartz. (40 %) places strongly system, has played an important role in the crystal- Muscovite. (10 %) Fairly thick lepidoblasts having lization history of the melt. irregular intergrowths with biotite. The of into the wall rocks does not Biotite. —7 lower than escape potassium (5 %) Slightly percentage taken muscovite. Pleochroism: seem to have place to any great extent as no red-brown (X, Z) to light K-feldspar and only minor amounts of late cross- yellow (Y). muscovite have been found in the schists within the Both minerals show signs of deformation in the form of and the pegmatite zone. kinking bending. The Li-pegmatites from Lalin (see Fig. 1) described Accessories. zircon, apatite, indeterminate opaque by Parga-Pondal and Cardoso (1948) are coarse- mineral. The observations indicate relation- grained and have prismatic spodumene crystals up to following seem to a 30 in size. It is that in that the between this and the cm. probable case gas ship granite pegmatites: ofveins towards pressure during crystallization has been higher. No i) The stepwise dip the granite. is from these An The of in the petalite reported pegmatites. analysis ii) occurrence phyllonitic texture of a bulk of this Na 0 = This feature indicates that the sample pegmatite gave 2 pegmatite. emplace- = of took before that of the K 0 Li O = SiO = ment pegmatite place 6,56 %, 2 0,92 %, a 1,20 %, s A1 0 = and to the south 72,61 %, 2 3 17,68 (Parga-Pondal Cardoso, coarse-grained biotite-granites (Fig. 1). the K 0 content of this is The ofthe veins is thought 1948). Although 2 sample emplacement pegmatite for have taken the time the probably not representative the whole pegmatite, to place at about same as the Na 0 seems to be intrusion of the The aside" high 2 percentage significant. granite. "shouldering have The absence of fluorite and topaz in the pegmatites of the countryrocks by the intrusion may is indicate facilitated the of the thought to a deficiency in F. The occurrence uprise pegmatitic magma which of the mineral montebrasite instead of amblygonite along a dilation zone in the schist, runs and the absence to of lepidolite which is a stable Li- parallel the contact. silicate under 640° C. in the of K and H O A chemical relationship cannot be proved. Trace presence a from and F are most likely due to this deficiency. analyses of a small number of samples of micas Schneiderhohn Table The numbers (1961) mentions the fact that only the granite are given in 4. sample of few occurrences are known in He with those on 1. The Li 0 content very topaz Spain. correspond Fig. 2 concludes that F must have been in the biotite is not and does not to relatively rare very high seem for between pegmatitic rest melts, while B was very abundant. In provide a proof a genetical relationship this of the has case most B escaped into the wallrock pegmatites and granite. to form tourmaline. These tourmaline-rich very TABLE 4. Trace analyses of biotite and muscovite from 2-mica contact zones are known from the contacts of many other granite Hercynian granite and pegmatite occurrences in Galicia and also appear to be common in Portugal Sample (Maijer, 1965). MnO no. Mineral Li,0 % % Sn% B% For several reasons given below, a genetic relation- with ship the two-mica granite to the W. is suggested. 1 muse. 0,13 0,007 0,009 0,01 This is medium two-mica biot. 0,115 0,01 granite a to coarse grained 0,51 0,27 2 muse. 0,15 0,03 0,013 0,01 granite. It is an homogeneous, intrusive granite of biot. 0,51 0,39 0,040 0,01 Hercynian age (Floor, 1966). Quite frequently the 3 muse. 0,08 0,007 0,007 0,01 of this have cataclastic granites type undergone a late biot. 0,41 0,45 0,080 0,025 which them deformation, gives a phyllonitic texture 4 muse. 0,13 0,01 0,039 0,01 (Ave Lallemant, 1965; Hensen, 1965). The coarse- biot. 0,51 0,47 0,058 0,01 grained, biotite granites which out to the S. do crop 5 muse. 0,10 0,007 0,019 0,01 not show this of deformation and could type also, by biot. 0,34 0,27 0,022 0,01 means of intrusive relationships, be proved to be than the former. H.M.I.B. Arnhem. younger Analyses by Mineralogy and Petrography of some Albite-Pegmatites 259 CONCLUSIONS Cotelo Neiva, J. M., 1954. Pegmatites com cassiterite e tantalite — columbite da Cabracao, Ponte de Lima — From the above facts the conclusions following are Serra de Arga. Mem. Not. Mus. Lab. Min. Crist., derived: No. 61 36, pp. Deer, W. A., R. A. Howie & J. Zussman, 1962. Rock- i) The pegmatitic assemblages most likely crystallized forming minerals, Vols. 3 & 4, London, Longmans & from a melt genetically related to the two-mica 705 Green, pp. granites outcropping to the W. Floor, P., 1966. Petrology of an aegirine-riebeckite gneiss- The initial of ii) temperature crystallization was bearing part of the Hesperian Massif: The Galineiro and around 600° C. between Leidse probably at a pressure surrounding areas, Vigo, Spain. Geol. Med., 36, 1—203. 2.5 and 3.5 kb. pp. Frondel, C, 1962. Dana's of mineralogy, Vol. Ill: A that removed system iii) segregation process preferentially & Silica minerals. New York, Wiley Sons, 334 pp. potassium from the melt led to the high albite Hensen, B. J., 1965. Petrologie en structurele geologie van content of the pegmatites. het westelijk gedeelte van het schiereiland Morrazo, The of the bulk of the N.W. Doctoraal iv) fine, aplitic, grainsize prov. Pontevedra, Galicia, Spanje. of Petrology and pegmatite is either due to the escape of the gas scriptie (unpublished), Department from the Leiden University. phase system producing a "pressure Mineralogy, Jahns, R. H. & O. F. Tuttle, 1963. Layered pegmatite- quench" or to the fact that albite crystallized intrusives. Min. Soc. of America. Spec, 1, from melt rather than aplite paper, a a vapour phase. Possibly pp. 78—92. both processes operated to give the observed A. 1953. in the mica Levinson, A., Studies group; relation- result. ships between polymorphism and composition in the Am. 1078— muscovite-lepidolite series. Miner., 38, pp. 1112. ACKNOWLEDGEMENTS Mason, B., 1964. Some iron-manganese phosphate minerals and their alteration products with special reference to much indebted I am to Dr. P. J. M. Ypma, Dr. G. material from Varutrask. Geol. Foren. Forhandl. L. Krol and Mr. L. J. Fick for supervision of the work 117—175. Stockholm., 63, pp. which was carried out for the second M. Sc. thesis Maijer, C, 1965. Geological investigations in the Amarante in the Dept. of Petrology, Mineralogy and Crystallo- region (N. Portugal) with special reference to the cas- of Leiden Dr. P. Hartman is graphy University. siterite-bearing albite pegmatites. Thesis, University of acknowledged for his kind interest and help with the Amsterdam. X-ray investigations and Mr. R. O. Felius for the Parga-Pondal, J., & G. M. Cardoso, 1948. Die Lithium- X-ray determination of ferri-sicklerite. To Professor pegmatite von Lalin in Galizien (Prov. Pontevedra, Schweiz. Min. Petr. Mitt., 28, 324—334. B. Nashar the author wishes his Spanien). pp. to express gratitude — 1963. Mapa petrografico estructural de Galicia. Inst. for critically reading and correcting the English Geol. Minero de Madrid. y Espafia, manuscript. Quensel, P., 1957. The paragenesis of the Varutrask The N.V. Billiton is thanked for the Maatschappij of its mineral pegmatite including a review assemblage. trace element carried out in the Arkiv Min. 9—125. analyses analytical Geol., 2, pp. of the N.V. Hollandse laboratory Metallurgische Schneiderhohn, H., 1961. Die Erzlagerstatten der Erde, II, Industrie G. Fischer. Billiton, Arnhem. die Pegmatite. Stuttgart, 720 pp. L. P. & N. V. 1961. The structure Last but not least I want to acknowledge the invaluable Solovyeva, Belov, crystal of bertrandite. Ac. Sci. help of the technical staff of the Leiden Geological Doklady U.S.S.R., 140, pp. 1086—1089. Institute in completing this study. D. 1960. The NaAISi 0 Stewart, 8., system LiAlSiO, 3 8 H O at 2000 bars. Int. Geol. 21 2 Congress, sess., 17, 15—30. REFERENCES part pp. 1963. Petrogenesis and mineral assemblages of lithium New Av6 Lallemant, H. G., 1965. Petrology, petrofabrics and rich pegmatites. Abstr. Geol. Soc. Am. Meeting 159. structural geologyofthe Sierra de Outes-Muros region (La York, p. Leidse W. 1956. der Corufia, Spain). Geol. Med., 33, pp. 147—175. Troger, E., Optische Bestimmung gesteins- Bjorling, C. O. & A. Westgren, 1938. Minerals of the Varut- bildenden Minerale, I. Stuttgart, E. Schweizerbart, rask studies of varulite 147 pegmatite IX, X-ray triphylite, pp. and their alteration products. Geol. Foren. Forhandl. Winchell, A. N. & H. Winchell, 1956. Elements of optical 19—54. 4th ed. New & 551 Stockholm., 64, pp. mineralogy, II, York, Wiley Sons, pp. PLATE I Fig. 1 Vertical vein with irregular flat-lying body (centre). Fig. 3 Layering in fine-grained albite-quartz-muscovite Parabellon mine N.N.W. marked both in and open cut looking rock, by changes mineralogy grainsize. The layering is cut off by a small vein containing albite a coarser-grained (nat. size). Fig. 2 Irregular apophysis of pegmatite in schist, occurring Fig. 4 Graphic intergrowth of albite (2nd variety) and of the in the flat-lying part of the pegmatite (Parabellon) K-feldspar. Spodumene (lower part picture) corrodes the albite along its grain boundaries (25 X ). PLATE II thin a perthitic from in outwards muscovite and radiating albite with spodumene spodumene of x). aggregates symplectic (25 x). of (25 K-feldspar Sheaf-like vein Veinlet 2 4 Fig. Fig. grains quartz X). spodumene (60 albite several vermicular Several individual the across quartz and quartz. per cutting origin and x). occur veins (25 spodumene quartzreplacement them orientations to of The a albite. had related different have be intergrowth to with to Granophyric seem appears Graphic grains 1 3 Fig. Fig. PLATE III X). shaped (60 montmorillonite heart right) pink showing (upper in adularia twinning, bertrandite. and lamellar of (left) crystals muscovite exhibiting hemimorphic Also petalite, twinning. of x). Relics (50 Euhedral simple 2 4 Fig. Fig. the Dark each At Spodu- to occurs. variety). quartz. normal rim is twins (2nd grey of albite symplectitic Dark sets x). two x). with thin (40 with a (60 twinning. contact minerals grain quadrant in K-feldspar lamellar Note two hand fine are the relief. spodumene with right albite high between upper in shows in Symplectitic contact patches Montebrasite mene other 1 3 Fig. Fig. PLATE IV is phyll- part X). (30 white penetrative Milky cookeite the faces. with showing crystal common schist in of in vein zones face section. two crystal pegmatite with one Horizontal crystal having crosscutting Cookeite quartz Thin onitisation. 2 4 Fig. Fig. was appear, arrange- preferred crystal crystals The some radiation. X). Most 2. symmetrical by (15 fig. (Ni) in caused faces. quartz The Kα be Cu crystal of pictured faces). may 4 partly of crystal. striated crystal lines (zone the of with consist the axis of to within crystals photograph zone parts a flakes examination, of cookeite rotation around thicker of close Typical X-ray rotated orientation on ment 1 3 Fig. Fig.