A second wave of hydrothermal activity produced the minerals of Class III. This mineralization represents a lithium-manganese-phosphorus phase in the ascending magmatic waters. At Buckfield seven minerals were formed, the majority of which are very rare. The phase itself is unusual in pegmatite formation and it is found elsewhere in this country developed to any degree only at Branchville, Connecticut.¹⁷ Pegmatites containing phosphate minerals in which iron predominates over manganese are found in France,¹⁸ and in Rabenstein, Bavaria.¹⁹

     The least FeO previously reported was 3.99 per cent, on a different specimen from Branchville.²¹ The Buckfield material with but 2.96 per cent FeO is the nearest of any lithiophilite so far analyzed to the theoretical end point of the triphylite-lithiophilite series.

     At Buckfield lithiophilite occurs frozen to the pocket walls, which generally consist of massive quartz. A change in the character of the solutions following the deposition of lithiophilite made this mineral unstable, and it was extensively attacked and replaced by later minerals.

     Beside its well known occurrence at Branchville lithiophilite has been reported from Tubbs Farm, Norway, Maine; Faires Mine,²² Cleveland County, North Carolina; and Pala, California. A specimen from the latter locality in the Harvard collection shows lithiophilite with sicklerite, a more hydrous lithium-iron-manganese phosphate.

TRIPHYLITE

      Triphylite is represented in the Buckfield collection by a single isolated fragment measuring five by five by two and a half centimeters, blue in color.

     Owing to its chemical similarity with lithiophilite it is assumed that triphylite belongs in Class III as well. As no other minerals were present in the specimen its paragenesis could not be determined further. Probably a very temporary increase in the amount of iron in the ascending solutions was responsible for the formation of a small amount of triphylite at Buckfield.

     Triphylite has a wider geographical range than lithiophilite. It has been found at Mount Mica, Stoneham, and Peru, Maine; Grafton, New Hampshire; Norwich, Massachusetts; Rabenstein, Bavaria; and Keityo, Finland. At several of these localities it is associated with spodumene.

RHODOCHROSITE

     The rhodochrosite of Class III has the pink color typical of the mineral. It appears occasionally in distinct, small crystals showing the forms M (4041); f (021), and v (211), but most of it is in anhedral masses up to ten centimeters across. A black manganese oxide coats much of the material.

A chemical analysis by Miss Vassar of the Buckfield rhodochrosite belonging to this generation follows:

FeO       2.97 per cent
CaO       trace
CO₂     38.43
MnO    58.79
          100.19 per cent

     This is the only appearance of a primary carbonate. For a short period the proportions of CO₂ to P₂O₅ in the ascending solutions must have been such that the former displaced the latter as the negative radical.

     No specimens could be found containing rhodochrosite and lithiophilite adjoining, so the relative age of these two minerals is in doubt. Like lithiophilite, rhodochrosite has been partially dissolved by subsequent solutions, and replaced and coated by later minerals.

     Rhodochrosite is rare among the pegmatites of central Maine. A small quantity was found at the Towne quarry, Auburn, and at the Berry quarry in Poland. At Branchville this mineral was common with the manganese phosphates found there.

EOSPHORITE

     The long yellow prismatic crystals of eosphorite make it the most striking mineral in Class III. A picture of a group of such crystals is shown on Plate B. The mineral is orthorhombic with the direction of elongation taken as the vertical axis. The faces of the prism zone are striated vertically. Forms and habit of the crystals are identical with those of the type material from Branchville, Connecticut.

The refractive indices of this eosphorite are as follows: α = 1.629, β = 1.650, γ = 1.658.

These are slightly below those given for a higher iron variety from Branchville.²³

SiO₂          0.90 per cent
Al₂O₃      22.37
MnO       29.94
FeO          1.38
H₂O±      15.34
F             trace
P₂O₅       29.89
               99.82 per cent

     Buckfield eosphorite follows lithiophilite in closely approaching the theoretical end point of the series. Its content of FeO, 1.38 per cent, is much lower than that in any of the childrenite-eosphorite analyses published in Dana or Doelter. The Branchville eosphorite contained 6.62 per cent FeO.

     With the introduction of eosphorite chemically combined water becomes important. This and the succeeding minerals in Class III contain fair amounts of water.

     Determination of the relative age of eosphorite is complicated by two features of the mineral. One is its strong crystallization power which tends to mask its true relationship with associated minerals. The other is its unusually high resistance to attacking solutions, which have in some instances removed the neighboring minerals to such an extent that eosphorite will rest upon and appear later than younger minerals.

     Although on the whole eosphorite deposition followed the formation of rhodochrosite there was a slight overlap of the two. This is shown on one specimen where a large crystal of eosphorite started to grow out from a rhodochrosite mass only to have the latter add a little to its size, burying the stump end of the eosphorite crystal a fraction of an inch. This and other examples of eosphorite crystals lying on and growing out from rhodochrosite masses prove the slightly later age of eosphorite.

      Connecticut, and Poland,²⁴ and Hebron, Maine. Specimens from Hebron have been called childrenite, but after an optical study Larsen stated that the mineral was probably eosphorite.²⁵  Childrenite has been found at Mount Mica and at four English localities, one in Cornwall and three in Devonshire.

     Eosphorite from Mount Mica has not heretofore been described. Specimens in the Harvard laboratories contain this mineral in small radiating needles coated by dahllite. It is embedded in massive siderite. Under the microscope the Mount Mica eosphorite is pale yellow green, whereas the Buckfield material is colorless. The color and slightly higher indices show that the Mount Mica eosphorite is higher in iron than that from Buckfield, but it is not childrenite because the axial angle for violet is greater than that for red.

FAIRFIELDITE

     Straw colored plates here and there among the Class III minerals were found to consist of small crystals of fairfieldite. These are triclinic, have one perfect cleavage, and in thin plates are absolutely colorless and transparent.

     Crystals of simple form were seen but proved to be too dull for measurement. The largest crystal, shown in Plate III, A, and nearly 3 centimeters long, is entirely coated with dahllite.

     Fairfieldite from the zone of oxidation is white and opaque and often coated with manganite. The nature of the alteration may be seen under the microscope. A white opaque mineral which may be kaolin has commenced to attack the fairfieldite, working first along the cleavage cracks and from them laterally into the mineral. Sometimes a yellowish to olive green alteration product is present but chemical and optical tests on this mineral were hopelessly complicated by the unfailing presence of residual fairfieldite.

     The refractive indices on the Buckfield fairfieldite were determined as follows: α = 1.633, β = 1.641, γ = 1.652.

PLATE III

A. Dahllite coating various minerals of Class III. The large coated crystal in the center is fairfieldite. The stalactite looking forms are crystals of eosphorite coated with dahllite. Buckfield.

B. A crust of lepidolite. Formerly a vein, this fragment has been isolated by the solution of the host. The crust is under 3 millimeters in thickness. Note the exceedingly thin secondary veins projecting from the much larger main vein. Buckfield.

     Fairfieldite is a hydrous phosphate of calcium and manganese, with iron generally replacing a little of the manganese. The iron in the Buckfield material was very low. An analysis by Miss Vassar follows:

SiO₂       1.07 per cent
FeO       1.00
CaO     29.77
H₂O±     9.94
P₂O₅    37.79
MnO    19.68
            99.25 per cent

The lowest percentage of iron in previously analyzed fairfieldite was 3.42 per cent, in a specimen from Branchville.

     This is the only appearance in Class III of calcium in more than a trace. In fact, fairfieldite is one of the few minerals in the entire suite to contain this element. The calcium that was present in the ascending solutions during the other periods was usually taken care of by apatite, which does not appear in Class III.

     Fairfieldite was deposited after eosphorite. In some instances eosphorite crystals cut through fairfieldite and appear later in age, but a close examination shows that the cleavage directions in the fairfieldite on opposite sides of the eosphorite are never parallel. The latter does not divide single individuals, but merely lies between separate crystals, of later age.

     The type locality for fairfieldite is Branchville, Connecticut. Sandberger described an occurrence of this mineral, first under the name of leucomanganit, at Rabenstein, Bavaria.²⁶ Oberpfalz, Germany, is another reported locality.

REDDINGITE

     Reddingite, a rose pink to reddish brown mineral, crystallizes in octohedroids of the orthorhombic system. It is usually granular, and ranks with rhodochrosite as one of the commonest minerals in Class III. It may be distinguished from that mineral by its lack of cleavage.

     The Buckfield reddingite is pleochroic. This serves to distinguish it in the thin section from lithiaphilite. It also has a much higher birefringence. The indices follow:

α = 1.643.           X=colorless.
β = 1.648.           Y= pinkish brown.
γ = 1.674.            Z =pale yellow.

The indices of the Branchville reddingite average .008 higher. A greater percentage of iron in the latter material probably accounts for this difference.

     Reddingite is a hydrous manganese phosphate, 3 MnO, P₂O₅, 3H₂O, and as in fairfieldite a small amount of iron replaces the manganese. An analysis of the Buckfield reddingite by Miss Vassar is given below:

Insol. (SiO₂)        0.30 per cent
FeO                     2.19
MgO                    0.35
CaO                     0.73
H₂O±                 13.14
P₂O₅                  34.54
MnO                  48.15
                           99.40 per cent

The Branchville reddingite contained 4.70 per cent FeO.

     Reddingite is definitely later than all the other minerals in Class III. Previous to its deposition the ascending waters became active solvents and removed in part the preceding minerals (except the extremely insoluble eosphorite) forming cavities which later became lined with reddingite. The mineral also replaces, as veinlets in the lithiophilite are very common.

     Heretofore reddingite has been known only at Branchville, and there in very small quantities. The abundance at Buckfield of this rare mineral is remarkable. The very rare crystals found with idiomorphic form are identical in habit with the type crystals from Branchville.

MINERALS OF CLASS IV

     Class IV is divided into two periods of mineralization. In the earlier period veins of lepidolite were formed in the zone containing Class II minerals, while veins of rhodochrosite were formed in the zone of Class III minerals. Both are clearly later than the same species with which they are thus associated. The confinement of these vein minerals to zones containing earlier deposits of the same minerals suggests that they did not result from an influx of new material at this stage, but were formed by solution and redeposition of material already in place. The waters bringing this about were probably hypogene as we know from the presence of later minerals that hydrothermal activity had not yet ceased.

     The later and main period of Class IV mineralization closed the hydrothermal activity at Buckfield. It represents a lithiasilica phase in the ascending solutions. Cookeite, quartz, apatite and herderite were deposited in the order named. The first two appear in great abundance. The latter are of very minor importance. These four minerals represent a well defined group, and were deposited so closely together that they overlapped.

     The vein minerals were deposited largely in fissures through earlier minerals. Cookeite and quartz also filled open spaces created by the solution of pre-existing minerals. The filling was far from perfect, as great porosity exists in the later Class IV specimens and in some the open spaces may be measured in inches. Volume by volume replacement was less common and was mostly confined to the alteration of vein lepidolite into cookeite.

     The Class IV minerals are quite evidently late in age, as shown by the cross cutting relationship of the lepidolite and rhodochrosite veins and the superimposition of quartz and cookeite on the minerals of the earlier classes. In some specimens such typical Class II minerals as cleavelandite, lepidolite, and milky quartz are coated by cookeite and quartz.

     In other cases removal of earlier minerals was far more complete. Often only a few bits of the difficultly soluble minerals are left in patches in quartz and cookeite as remnants of an earlier period. Lepidolite, columbite, manganotantalite, and kaolinized spodumene occur in this manner.

     In one specimen quartz and apatite are replacing rhodochrosite and amblygonite of Class III.

     A final phase of this sort in the deposition of pegmatite minerals is the rule rather than the exception in central Maine. At Greenwood it was an outstanding feature.

LEPIDOLITE

     In Class IV lepidolite occurs exclusively in veins. This habit easily distinguishes it from the earlier hydrothermal and magmatic types. The veins are lilac in color, and cut minerals of Class II, especially spodumene. The separate individuals in the veins are very minute. They vary sufficiently in orientation so the vein has no well-defined cleavage.

     In the majority of cases the veins have been isolated by the complete removal of surrounding minerals. The result is an irregular sheet of massive lepidolite ranging from paper thin up to one centimeter in thickness. These are found buried in kaolin in the bottom of the pockets. Very often minor veins of much smaller dimension extended out from the large veins into cleavage and other cracks in the surrounding minerals. In that case a great host of excessively thin shelves jut out from the side of thicker slabs (Plate III, B). Their rough parallelism suggests that they were formerly veins extending into a highly cleavable or platy mineral, such as cleavelandite.

     One specimen contains as remnants of Class II mineralization a few quartz crystals and many small books of lepidolite. The latter are connected to each other by paper thin isolated veins of lepidolite. The plane of the vein is at right angles to the plane of the basal cleavage in the book lepidolite. Larger veins cut across the specimen in a few places. These likewise have a multitude of smaller offshoots. The secondary veins all have the same general strike, although often joining or diverging down the dip. No trace of the mineral or minerals serving as host for this deposition remains.

RHODOCHROSITE

     Small veins of rhodochrosite cut through the minerals of Class III. These are obviously later than the massive rhodochrosite and phosphate minerals of that class. In many instances the earlier minerals have been removed by solution and dike-like forms are the result. These isolated veins are roughly parallel to each other except for an occasional vein which intersects the others at a low angle.

     Deposition of rhodochrosite continued during the period of solution of the neighboring minerals and as a result the projecting veins are lined with small pink rhodochrosite crystals, showing that ascending hot waters dissolved and redeposited rhodochrosite over a relatively long period.

COOKEITE²⁷

     Cookeite is an abundant and rather conspicuous mineral at Buckfield. Large specimens were obtained coated with brownish aggregates of plates shown schematically in Fig. 2. The substance is so soft that the slightest touch broke the edges of these plates leaving a light yellow mark very much as the hand leaves a mark when it touches the bloom of a plum. The color also varies to pale green and pink. It may readily be distinguished from lepidolite by lower refractive indices and positive optical character. Under the microscope the plates of cookeite appear hexagonal as in Fig. 3. Some of the plates have a uniaxial center, e.g., BEFGHK, but others lack this central core. The uniaxial character is probably due to the aggregate effect of three plates at 60° to each other, the axial angle being largest midway in each segment. For example, in the segment ACEB the axial angle is largest at D with the plane of the optic axis parallel to the side AC; approaching A the angle gets smaller until on the line AB the axial angle is 0, then swings 60° and becomes parallel to AM, enlarging until it reaches the position L. The segments are striated perpendicular to their outer edges. The axial angle reaches a maximum of about 80 degrees.

     Some of the plates have narrow rims which are dark colored and may be due to an alteration of cookeite. One of these borders was large enough to note that the index was slightly less than the index of the cookeite.

Fig. 2. Cookeite.

    The indices of cookeite, determined by daylight, are: α = 1.576 ±.003; β = 1.579±.003 (seems to be constant in value); γ = 1.597±.005.

     The cookeite was separated by heavy solutions for analysis. The specific gravity of the clean sample was then determined by the pycnometer as 2.670; (according to Brush the sp. g. is 2.70, according to Penfield 2.675).

FIG. 3. Basal plate of cookeite showing optical observations and pseudohexagonal habit.

     Chemically cookeite is very similar to lepidolite. The chief differences are a greater percentage of water and correspondingly smaller amounts of fluorine and potash in cookeite. An analysis of Buckfield cookeite by Miss Vassar follows:

SiO₂        34.81 per cent
Al₂O₃      45.90
Fe₂O₃       0.72
CaO          trace
Na₂O   }  0.52
K₂O    }
H₂O±     14.87
F              0.13
MnO       trace
Li₂O         3.59
            100.54 per cent

     The green cookeite often occurs lining solution cavities and cleavage faces in microcline. Later quartz crystals rest upon the cookeite. Plate I, A pictures a cavity coated in this manner. Solution and removal of microcline coated by quartz and cookeite may leave behind a mass of empty shells. Plates IV, A, and IV, B illustrate two stages in this process. Lepidolite, cleavelandite, and early hydrothermal quartz minerals of Class II are all coated locally with a later incrustation of cookeite. In one specimen cookeite had replaced spodumene, preserving the structure of that mineral.

PLATE IV

A. First stage of cookeite-quartz mineralization. White masses of microcline (best seen at lower right) becoming coated with cookeite and quartz. Buckfield.

B. Last stage of cookeite-quartz mineralization. Crusts of cookeite and quartz alone remain. Microcline and other older minerals have been completely removed. Buckfield.

     A great deal of cookeite is disseminated through and intergrown with the quartz of Class IV. Here it is usually pink or yellow, sometimes cream colored. It is on the whole earlier than the quartz, and appears to be undergoing replacement by quartz in a few places.

     One other occurrence of cookeite was observed at Buckfield. In this the isolated veins of lepidolite previously described have been partially altered into cookeite. Sometimes the secondary veins are completely altered, while the outer edges only of the main vein will be changed from lepidolite to cookeite. This is alteration in situ, cookeite resulting from hydration of lepidolite.

     Cookeite is usually considered an alteration product of either tourmaline or lepidolite. Examples of the alteration of tourmaline into cookeite are rare at Buckfield, but this change is well shown in specimens from Hebron and Mount Mica. Alteration, or hydration, of lepidolite into cookeite has just been described. But there is much cookeite at Buckfield and elsewhere in central Maine that cannot be accounted for by either of these processes. Bastin ²⁸ says, in referring to cookeite: "Abundant as a coating in most of the pockets in the coarser pegmatites. Not an original constituent, but secondary and due to water deposition." But, much of the Buckfield cookeite is definitely earlier than the abundant and intimately associated quartz, which is in turn earlier than apatite and herderite. It would seem then that this cookeite is of hydrothermal origin. The low temperatures prevailing during this final period of hypogene deposition permitted the formation of cookeite rather than the less hydrous lepidolite.

QUARTZ

     Quartz is the commonest mineral in Class IV. Some large masses resulting from this period of deposition consist of over ninety percent quartz. The crystals themselves are small, never exceeding two and a half centimeters in length. They are clear and absolutely colorless. Even the most massive specimens are extremely porous, as the crystals do not fit together very closely. These large quartz aggregates, consisting of a multitude of small individuals, are pink and yellowish in color, due to the presence of flakes of cookeite throughout the mass.

     The clear and colorless character and the small size of the quartz crystals usually distinguish them from those belonging in Class II. The latter are more often milky than colorless. When they are colorless distinction must be made on a basis of. association. The practically invariable presence of cookeite is a good criterion of later age.

     Sometimes the quartz crystals of Class IV project in parallel groups, from thin shelves. The latter are likewise parallel to each other and are usually composed of finer grained quartz. They may also contain cookeite, especially toward the center of the slab.

     Specimens illustrating this structure represent the last of a sequence of interesting events. In the first place a pocket was formed containing the typical minerals of Class II, such as lepidolite and cleavelandite. Further hydrothermal activity brought about the formation of lepidolite veins, connecting the lepidolite books. The roughly parallel structure of these veins may have been due to the platy character of minerals traversed, such as cleavelandite. Whatever the host it was soon removed by the dissolving activities of the ascending solutions. Lower temperatures and quantities of water brought about the hydration of the vein lepidolite into cookeite. Then silicification became dominant and the cookeite was partially or wholly replaced by quartz. From lepidolite to cookeite to quartz, then, is the history of some of these veins. During and after their last replacement they served as a base for many quartz crystals growing laterally into the open spaces between the shelves.

     Silicification as a final stage in pegmatite mineralization had a much greater development at Greenwood, and has been noted as an important phase at Auburn.

APATITE

     Apatite is unimportant among the minerals of Class IV at Buckfield. It is very clear, and pale green or light blue in color. Crystals are characteristically tabular parallel to the base, and show the forms m(100), x(101), r(102) and s(111).

      Class IV apatite is distinguished from earlier types by its habit and associations. It commonly lines cavities in the cookeite-quartz aggregates and where these cavities are tabular in shape the crusts on the opposite walls meet and simulate a vein. The pegmatites at Mount Apatite and Greenwood have produced many beautiful specimens containing apatite of this type.

HERDERITE

     But few herderite crystals were found associated with the Class IV minerals. These were small, light brown, and clear. The herderite of Class II was translucent rather than transparent.

     Where found herderite was always associated with apatite in small openings in the cookeite-quartz masses.

MINERALS OF CLASS V

     Most of the minerals in the last group occur as bristles, stalactites, crusts, or films associated with open spaces. They are without doubt secondary as in every case they consist of elements obtainable very near at hand and their method of deposition is typical of minerals formed by ground water activity. Many of the minerals themselves are characteristic of supergene deposition.

     Due to their secondary origin these minerals are found scattered throughout the pegmatite wherever the parent primary minerals are intersected by the zone of oxidation. Thus certain amounts of kaolin may be found where microcline, belonging to Class, I, lies close to the surface. Kaolin also results from the decomposition of spodumene, a Class II mineral. Cleavelandite, of the same class, is prone to alter into montmorillonite under near-surface conditions at Buckfield. The greatest variety of secondary minerals is found where the Class III minerals reach the zone of oxidation. The presence in the primary minerals of this class of MnO, P₂O₅ and CO₂ has been responsible for the formation of psilomelane, manganite, an unknown manganese phosphate, rhodochrosite, and dahllite.

      Owing to the scattered positions in the pegmatite of the secondary products no sequence of mineralization can be applied to the class as a whole. It is probable that more than one mineral was being deposited in different places at one time. A fairly definite sequence for the Class III secondary products is possible, however. Several specimens show psilomelane altering into manganite. Likewise manganite alters to an unknown manganese phosphate. Rhodochrosite is later than manganite, but dahllite was formed before it.

KAOLIN

      A surprisingly small amount of the Buckfield feldspar is kaolinized. But spodumene was very susceptible to this alteration and fresh pieces of spodumene are rare. Many more specimens are found in which this mineral has been completely kaolinized. Usually the original structures of the spodumene are preserved, giving a true pseudomorph of kaolin after spodumene. Lepidolite veins formerly cutting spodumene crystals are found buried in a mass of kaolin in the pocket debris.

     Other minerals sometimes incipiently altered to kaolin are amblygonite, fairfieldite, cleavelandite, and topaz.

MONTMORILLONITE

     Cleavelandite in the zone of oxidation is sometimes found partially altered to a pink, clay-like substance. This is one of the clay minerals, known as montmorillonite. The bright pink color of the fresh material pales upon exposure, probably due to partial dehydration.

     Under the microscope this material is seen to consist of an irregular aggregate of small scales or plates. The mineral is colorless in very thin plates, but becomes pink and opaque with an increase in thickness. Optical data are: Biaxial negative, 2V= almost 0, β = 1.513.

     Montmorillonite is a very indefinite species. The optical data above more nearly coincide with those given for leverrierite,²⁹ another hydrous aluminum silicate.

PSILOMELANE

     Hard, black botryoidal crusts coating rhodochrosite proved to be psilomelane. This mineral is limited in its occurrence to the zone of weathering and even there it is quite subordinate to manganite, with which it is associated. The primary mineral from which psilomelane was derived was usually rhodochrosite.

MANGANITE

     Manganite is one of the commonest of the secondary minerals. It is widespread in the surface zone, even forming a fine film on quartz and cleavelandite. Thicker deposits are found on the minerals actually undergoing alteration, such as rhodochrosite and lithiophilite. In a very few specimens manganite coated psilomelane, preserving the botryoidal character of that mineral. In this case it was quite evidently derived from psilomelane, but it is very doubtful that all the manganite passed through a psilomelane stage. In one instance the alteration about the edges of dahllite into manganite was observed.

RED MANGANESE PHOSPHATE

     In a few specimens botryoidal surfaces of manganite (pseudomorphous after psilomelane) were coated by a thin, velvety, brick red crust. The latter very evidently resulted from alteration of the manganite, as the radiating fibers forming the rounded surface are red on that surface, but darken toward the interior, where they become indistinguishable from the black manganite.

     The fibers are extremely fine, and so soft and brittle that they powder immediately when an attempt is made to work with the material. The quantity present was insufficient for a chemical analysis. A bead test proved the presence of manganese, also a phosphate reaction was obtained with ammonium molybdate.

    Microscopic examination was hindered by the small size of the particles and their platy character. The mineral is biaxial and probably negative. The plates due to a good cleavage are perpendicular to X. Further optical data follow:

α = 1.71,           2V=large;
β = 1.73,             Y =yellowish brown;
γ = 1.74,              Z =deep red brown.

     It was at first thought that the mineral might be sicklerite, a hydrous lithium manganese phosphate. The indices of both lie between 1.71 and 1.75. Likewise, the pleochroism is somewhat similar for both. But comparison with the type sicklerite from Pala, California, brought out the following differences: 1. Birefringence greater for sicklerite. 2. Sicklerite gives good lithium flame, while unknown gives none. 3. Sicklerite readily soluble in nitric acid. Unknown but difficultly soluble. 4. Sicklerite is much harder.

RHODOCHROSITE

     Ground water activity at Buckfield produced a third generation of rhodochrosite. In Class III this mineral appeared in masses, in Class IV in veins and druses, and in Class V in minute bristles or quills projecting out from the surfaces of earlier minerals. These much elongated crystals vary in color from white to bright yellow. A few may be seen in Plate II, B.

     The commonest primary minerals coated are rhodochrosite and reddingite. One small crystal of rhodochrosite has a pompadour of white secondary bristles. These project upward from about the top of a steep rhombohedron on the primary crystal. The secondary rhodochrosite fibres are parallel in crystallographic orientation to the host.

     These white or yellow quills of rhodochrosite are without doubt supergene in origin. They were deposited even after the manganite. In one specimen a somewhat spherical mass of manganite is covered with secondary rhodochrosite fibers like spines on a burr.

DAHLLITE

     Dahllite, a calcium phosphate-carbonate, appears in yellowish green crusts coating various minerals of Class III. The coating is most striking when eosphorite is the host. Prismatic crystals of that mineral are often completely covered and the terminations rounded off, so that pseudostalactites result. Solution remnants of fairfieldite and rhodochrosite are similarly coated by crusts of dahllite. These may be seen on Plate III, A.

     Under the microscope the yellowish green crusts are seen to consist of parallel fibers normal to the surface. The mineral is uniaxial and negative, and the optical properties of the Buckfield material checked up quite closely with published data on dahllite.³⁰

     A great variety of quartz crystals, and some magnificent specimens, have been quarried at Greenwood. These have been found either lining the cavity walls or loose in the pocket sand. Crystals vary in length from one to thirty centimeters. The large crystals are usually milky, the smaller ones clear or smoky. Many small pockets were barren save for one or two small clear or smoky quartz crystals. In the larger pockets groups were the rule. Some of these consist of several odd sized crystals in irregular orientation. Others, of truly spectacular appearance, consist of a parallel aggregate of smoky or milky quartz crystals (Plate VII, B). Besides the uniform orientation, these individual crystals are remarkably consistent in size. They vary from one to four centimeters in length, over an entire specimen, but any two adjacent crystals are usually quite similar in size. These parallel groups may be frozen on to massive quartz of the pocket wall, or they may cluster on a single large quartz crystal. In the latter case crystallographic directions in the individuals of the aggregate are parallel to similar directions in the host. In some. instances the large crystal is incompletely developed on one side, and a multitude of smaller crystals have so arranged themselves as to tend to complete the crystal form.

     The separate crystals in a cluster usually merge together on prism faces. Sometimes the groups are tabular, due to addition in one plane only. At other times growth has been in all directions, and striking turret-like forms are the result.

     There is some evidence that the earlier crystals to form were milky in color, and the later ones smoky. In some instances smoky crystals give way to milky crystals as one penetrates into the cluster. One slab two feet long is covered by a host of small parallel quartz crystals. Those on the outside are smoky, but farther in they become milky.

     The ascending solution deposited quartz over rather a long period as shown by the succession of clusters on many specimens.

OTHER MINERALS

     The cleavelandite at Greenwood does not differ from that found at Buckfield. It is typical of early hydrothermal activity at both localities. Large masses were found in and about the pockets in the pegmatite.

     Lepidolite was formed during one period only at Greenwood. Here it occurs either in beautiful lilac colored books up to one centimeter wide and an equal thickness, or in massive aggregates of fine plates, lavender, white, or even pale green in color. Some of these aggregates were thirty centimeters or more through. Both types of lepidolite are commonly associated with cleavelandite.

     Tourmaline likewise has two modes of occurrence. It is sometimes found in green crystals radiating rather irregularly from the pockets. Although fairly clear these crystals are never gem worthy. Most of the individuals are from five to eight centimeters long, but an occasional one is twice that length. Later silicification has in most cases partially destroyed the crystals. In the second occurrence tourmaline appears in green transparent crystals. When found these are usually loose in the pocket sand. Some are doubly terminated and thinner than a needle. As at Buckfield many of the pocket tourmalines are gem worthy.

     The largest specimen of spodumene collected at Greenwood is fifteen centimeters wide and thirty long and this is but a fragment of a larger piece. The mineral is decidedly replacive, exhibiting an irregular contact toward the older magmatic minerals. It is pale green, and some pieces are translucent. Kaolinization is incipient only.

PLATE V

A. Amblygonite nodule. White core is amblygonite, surrounded by varicolored crusts of kaolin. Greenwood.

B. Amblygonite nodule sawed in two. Note fairly sharp contact between amblygonite core and kaolin rim. Greenwood.

FIG. 4. Amblygonite crystal.

     Amblygonite occurs in very striking forms. One crystal found by Mr. Noyes and shown in FIG. 4 is a complete individual 15 cm. long, 8 wide and 5 thick. It is flattened parallel to the dome h(01) and shows the forms c(001), a(100), z(10), h(01), and e(01). Most of the amblygonite however occupies the centers of nodules composed of alteration products in concentric arrangement. Six or more of these remarkable specimens were obtained, the largest an oval mass about fifteen centimeters in diameter. Some of them were completely altered to kaolin or sericite and are described in detail on a later page. Others show a core of fresh glassy amblygonite with polysynthetic twinning. The boundary of the amblygonite towards the crust is irregular but the bands of the crust are sharp and smooth, each a few millimeters thick and variously colored dull green, brown and white. The nodules are either embedded in lepidolite and cleavelandite or in a mass of pure white plastic kaolin. Plates V, A and V, B show two of these nodules, one having been sawed through the center to reveal its structure.

     Cassiterite was found sparingly, in part in irregular masses or stout pyramidal crystals embedded in cleavelandite, in part in crystals of unusual form attached to a surface of cleavelandite plates. One of these crystals is shown in FIG. 5 and is remarkable in showing practically only a ditetragonal pyramid as its bounding form, doubly terminated and as symmetrically developed as a model. The largest of these crystals was not more than four millimeters across. The same rare form, μ  (766) is shown in a somewhat similar figure by Jeremejew, of cassiterite from Siberia reproduced in Goldschmidt Atlas der Krystallformen Vol. IX, plate 89, figure 227.

FIG. 5. Cassiterite

     Other minerals of the pocket zone at Greenwood are pale green to colorless beryl, mostly deeply etched, rare embedded crystals of dark green gahnite or zinc spinel, columbite in masses up to a centimeter across, and herderite, bertrandite and hamlinite, the last three in minute amount only, in implanted crystals.

SECOND HYDROTHERMAL PHASE

     The second and last hydrothermal phase was characterized by enormous amounts of silica, with smaller amounts of lithium and phosphorus. As a result quartz is the outstanding mineral. Cookeite and apatite are common, but are quite subordinate quantitatively. The minerals resulting from this period of activity are confined to the swelling in the pegmatite, and in many portions of this zone their deposition has been accompanied by the absolute obliteration of earlier minerals. Cookeite was deposited with quartz, but apatite is distinctly later. Its single crystals and druses rest upon quartz and cookeite.

QUARTZ AND COOKEITE

     Many of the quartz-cookeite specimens from Greenwood are indistinguishable from those found at Buckfield. The quartz crystals are small and colorless or milky. Cookeite appears in scales and flakes disseminated through the quartz aggregates. It varies in color from white to pink.

     The first quartz to be deposited formed as druses on pre-existing minerals. If crystals were available these were completely coated by a layer of small remarkably uniform quartz crystals. At other times this coating was deposited on mineral aggregates, or as a lining on the walls of solution cavities. Deposition was accompanied and followed by intense dissolving activity. In every case the material coated was removed. If the host was an euhedral crystal the coating retained the outline of that crystal. Sometimes, instead of being coated, the original mineral was completely replaced by fine grained quartz and cookeite. These two types of pseudomorphs are described and discussed under a separate heading. But, if the host was an ill-defined aggregate merely an irregular crust resulted from its removal. Many specimens of both pseudomorphs and crusts were found in the cavity zone of the pegmatite.

     Quartz and cookeite continued to be deposited after much of the pre-existing material had been dissolved and removed. Having a great abundance of space at its disposal this quartz merely piled up on itself, its individuals assuming little or no similarity in orientation. It is this type of aggregate that gives certain portions of the pegmatite their high porosity.

QUARTZ PSEUDOMORPHS

CHARLES PALACHE AND K. K. LANDES

     The quartz pseudomorphs taken from the Greenwood ledge in numerous specimens by Mr. Noyes when the large pockets were first opened are among the most interesting minerals which it yielded. They are of two types and both were described by Warren,³¹ the one as a pseudomorph after phenacite, the other as after topaz. It was largely in the hope of solving the riddle of their nature that ownership of the Greenwood ledge was secured for the Harvard Museum and the later excavation there carried on. Many new specimens of both types were then secured but none approaching the beauty of the early groups; nor was the hope of finally determining their nature fulfilled although a new interpretation is offered on a later page.

     According to Mr. Noyes the large pocket (see pages 363 and 400) was completely encrusted either with quartz crystals or with the hollow pseudomorphs or both. He saved many single specimens and large slabs showing groups such as are reproduced in Plate VI. During our quarrying hundreds of specimens showing broken or imperfect pseudomorphs of the same kind were thrown over the dump. They vary in size from a centimeter or less in diameter to more than half a meter. Whatever the primary mineral may have been it was certainly developed in very great abundance in all the cavernous parts of the ledge. Of the embedded, solid pseudomorphs only a few have been found. The largest, described by Warren and now in the Harvard collection, has a diameter of about thirty centimeters and weighs twenty eight pounds.

     The two types of quartz pseudomorphs, one due to replacement and the other to incrustation, also differ from each other crystallographically. Two minerals, then, played the part of host. No traces other than external form remain of either of these original minerals, but some of the pseudomorphs contain cleavelandite, which was evidently replacing the missing minerals. Both types of pseudomorph have been found on the same specimen. It is evident that the original minerals were associated together and preceded cleavelandite in formation. However, their euhedral form and aggregate arrangement point toward deposition in open spaces, rather than crystallization from the primary pegmatite magma. The conclusion is that the original minerals of the pseudomorphs were deposited rather early during the first hydrothermal period.

     Examples of the replacement type of pseudomorph are shown in Plate VI, A and will be described first. These are composed of fine grained quartz and cookeite with an occasional remnant of cleavelandite. Crystal faces are plentiful and definite. Five specimens of this type of pseudomorph have been found, and the same groups of faces occupy each.

Contact Measurements of Quartz Pseudomorphs.

     Under the impression that the form was rhombohedral Warren³² described the original mineral as phenacite, but the definite isometric character of the pseudomorph rules out this mineral.

     The most plausible guess as to the original mineral is analcite. This is supported by the following facts:

     1. It is the only known mineral of exactly similar habit. One crystal of analcite in the Holden collection from the Tyrol is a perfect model of the pseudomorph. On page 596 of Dana's System of Mineralogy a drawing of an analcite crystal (Fig. 3) from the Cyclopean Islands (near Catania, Sicily), could just as well be labelled a pseudomorph from Greenwood.

     2. The original mineral was deposited during the first hydrothermal period when we know the solutions were sodic.

     3. Although hydrous and a zeolite, analcite is known elsewhere as an igneous mineral, so temperature considerations cannot. rule it out here.

     Points unfavorable to the analcite hypothesis are:

     1. The large size of the pseudomorphs. Nine inch cubes as opposed to three inch crystals from the Tyrol.

     2. Analcite is more often associated with rocks of intermediate or basic character.

     No other possibilities suggest themselves; the cube precludes garnet, the trapezohedron precludes fluorite.

     The second type of pseudomorph resulted froze incrustation by drusy quartz, followed by the solution and removal of the host. Pictures of this type are given on Plate VI, B. Some are hollow, others are partially filled with later quartz or earlier clevelandite. The form is prismatic, terminated bluntly by faces at ninety degrees to the axis of elongation. The end faces, giving the cross section of the form, are sometimes diamond shaped, at other times six-sided. The apparent orthohombic symmetry in many of the forms suggested to Warren that the original mineral was topaz.³³

      The ground water solutions at Greenwood had little to work on that was susceptible to their activity. No suite of fairly soluble manganese phosphates was present to give a wide range of secondary products as at Buckfield. Supergene mineralization was practically confined to the kaolinization of feldspar and amblygonite.

     In the case of feldspar the masses of that mineral near the surface, or adjacent to the highly porous portions of the pegmatite, have been altered to some degree. At times montmorillonite resulted from this decomposition.

     The nodules of amblygonite appear to have been converted into kaolin by concentric zones working inward. In the nodules pictured (Plate V) the outer crusts are kaolin and sericite, but the core has not as yet succumbed to this alteration. In other instances the center of the nodule is white and compact, with a conchoidal fracture. Here kaolinization has been more complete, A green crust also surrounds the kaolin core. Analyses by Miss Vassar of the green crust and the white core follow: