PETROLOGY AT GOOSE CREEK — SHANNON. 59 - Proceedings of the United States National Museum

in figure 7,

which

is of a crystal

between

crossed nicols resting

on

the pinacoid a (100), the broad flat face.

The

colors of a typical

example

are indicated on the drawing.

Thisstructureisnot

due

to twinningbutis

due

rather todifference in composition resultingin varying birefringence.

The

variationis

probably inthe

amount

of the iron-epidote molecule,thisbeinggreat- est inthe

end

sectors

and

gradually^decreasing

from

centertooutside in all sectors.

The

extinction is parallel to the sides of the crystals

and

the axial plane across the length, this tabular face being nearly perpendicular to the obtuse bisectrix.

The

crystals

vary

in color, in ordinarylight,fi'om colorless to pale greenish yellowwith noticeable pleochroism, the color being distrib-

uted in the

same

pattern as the birefringence. This coloris visible in the

end

sectors of the thicker crystals

and

in the inner tips of the side sectors.

The

epidote is biaxial, negative

(

—

), with

2V

large.

The

indices

vary

with the zoning.

One

crystal gave, at the outer edges of the side ^ sectors,

which

is the portion of mini-

mum

birefringence

and

probably of

minimum

index,

«=

1.748, j3= 1.754.

This is typical "hour-glass struc- ^{'^}

ture,"

which

is described

by

Iddings-*

as follows:

Differences in the molecular attractions 2^

in different directions in a crystal also show

"themselvesintheconstitution ofsomemixed crystals or crystals of isomorphous com- pounds. Itappears asthough certain mole-

culesintheisomorphous serieshavea greatertendency to attach themselves in onedirectionthan another;that isthey are more strongly attracted to certain faces of the mixed crystal than to others. The crystal then differs in composition insegments builtup oflayers parallel to such faces, which

may

show themselvesin differences ofcolor or refraction. In some minerals the segments are pyramidal withthe apexesofthe pyramidstowardthe centerofthe crj-stal,

and the bases atthesurface. Insections ofsuchcrystalsthe reversedpyramids sometimes suggestthe shape of an hour-glass, hence the term hour-glass struc- ture. Thecommonest examplesofsuchstructure arefoundin augite in certain basaltic rocks.

While

allofthe epidoteof thisshearzone

and

thatofthespecimens collected

by

Merrill

and Wherry

are of thispeculiar type,the epidote ofthe miarolitic cavities

and

that associated with the axinite

were

Fig.8.—Epidote. Projection of"hour GLASS" CRYSTALS ON ^ft(010) SHOWING OPTICAL ORIENTATION.

'*J P.Iddings. RockMinerals,p. 72,NewYork,1906.

60 PROCEEDINGS

THE NATIONAL MUSEUIM.

Vol.

not of this type.

A few

hour-glass epidotes were seen, however, entangled with the

amphibole

of bluish smears

on

cracks in albitic pegmatite (see p. 53).

Paragenetically the epidote is

contemporaneous

with chlorite, asbestiform

amphibole and

alsowith the earliest prehnite,indicating that its formation covered a considerable period.

Angles _of crystals of "hour-glass" epidote. Measured with elongation ₍₆ axis) vertical.

CRYSTAL^1.

Form.

ART.2.

PETROLOGY AT

GOOSE

CREEK SHANNON. ₆₁ South

of the central part of the quarry face an apKte dike

from

2 cm. to.3 cm. wide is cut

by

later

seams

filled with diopside

and where

these diopside

seams widen

they have a central layer of pur-

plish-gray axinite. In places the axinite

seams

contain

minute

vugs linedwith acute

wedge-shaped

axinite cr^^stals of purplish-brown

color.

The

centralportionofthese vugs is filled with cottony white hornblende

which

contains

embed- ded

perfect crystals of axinite

and

a fewlong prismaticcrystals of epidote.

One

suchaxinite-lined cav- ity

was

filled withawhite mineral which,

when examined

microscopi- cally,

was

foundtocontain

numer-

ous included colorlesshornblende

fibers. This white mineral

which

is biaxial, positivewith

2V

small,

r>v

pronounced, has a perfect cleavage perpendicular to the acute bisectrix

and

is doubtless apophyllite. Itsrefractive index,

i3is1.538.

The

axinite crystal

which was measured was

one of the perfect individuals suspended on hornblende fibers. It has the

form and

habit

shown

in figure 9

and gave

the following

measurements

Fig.9.—Axinite. Showing common h.\bit of CRYSTALS FROMQOOSE CREEK. ORTHOGRAPHIC ANDCLINOGRAPHIC PROJECTION ON C(010)"

Measurementsofaxinite froyn Goose Creek.

Form.

62 PEOCEEDINGS

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VOL. 66.

The

faces of the crystals are all lustrous

and

brilliant but the prismatic planes are striated vertically

and somewhat rounded by

oscillation.

The

optical properties of this axinite are: biaxial, negative (

—

2V medium

^large,

r<v

moderate,

a=

1.673, /3= 1.680,

7=1.684,

—

a =.011. In the granular portion of the taxinite

seam

chalcopyrite. apparently contemporaneous, is intergrown with theaxinite.

Paragenetically the axinite is placed as con-

temporaneous

with chalcopyrite, epidote,

and

hornblende,

and

earlier than apophyllite.

a yn

Fig.10.—Prehnite.Typeicrys- tal. Orthographic crystal drawings.

bril-

The

scarcity of quartz is rather a nota-

i)le feature of the

Goose

Creek assemblage of vein minerals. This mineral, so

common

elsewhere in association with zeolites in trap-

pean

rocks,

was

seen onlyoncein allthe spec- imens collected. In this instanceitoccurred as

combs

ofprismatic crystal

grown

out

from

either wall of a vein.

The

crystals rested

upon

a layer of chlorite.

The

individual quartz crystals average 2 millimeters long

by

1 millimeter thick

and

are transparent, colorless,

and

liant.

They have

the

common

^habit,

hexagonal prism terminated

by

a symmetrical hexagonal pyramid.

The

vein

between

the quartz

combs

isfilled with coarse granular datolite w^hich preserves

molds

of the quartz crystals

when

they are

broken

out.

The

wholevein averages about 1 cen- timeter wide.

A

^single^{small cube}^of

galena

was

seeninthe^chloriteunder- lying the quartz. Paragenetically

the quartz is laterthan chlorite

and

^fi«-

h

^-prehnite, ^type 2crystalelon-

\.i.L\j K.^Kj.i.i,LMu io

GATED ON THE 6 AXIS SHOWING CRYSTAL

galena

and

older than datolite. habitandstriation ofc (ood.

Prehnite is the

most abundant

^of the vein minerals

and

occurs in a variety offorms.

In the lot ofmaterial collected

from

this locality in 1915

by Doc-

tors Merrill

and Wherry,

the prehnite exhibits theordinaryform, pale green

columnar

crusts with botryoidal to ill-defined

cockscomb

surface. This ^prehnite ^rests

upon

a layer of

somewhat

weathered

Dalam dokumen Proceedings of the United States National Museum (Halaman 59-63)

PETROLOGY AT GOOSE CREEK — SHANNON. 59

which

between

on

The

example

due

due

The

amount

end

and

from

The

and

The

vary

same

end

and

The

—

2V

The

vary

One

which

mum

and

minimum

«=

which

by

may

While

and

by

and Wherry

and

were

60 PROCEEDINGS

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A few

amphibole

on

contemporaneous

amphibole and

PETROLOGY AT

CREEK SHANNON. 61 South

from

by

seams

and where

seams widen

seams

minute

wedge-shaped

The

which

embed- ded

and

One

was

when examined

was

numer-

which

2V

r>v

and

The

which was measured was

form and

shown

and gave

measurements

62 PEOCEEDINGS

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The

and

CREEK SHANNON. ₆₁ South