to explain the principle in view.
ISREGULARITIES OF CBTSTALS.
Before concluding this subject, a few remarks may be
added on the irregularities of ciystals.
Crystals of the same form vary much in length, and in the
size of corresponding faces.
The same mineral may occur
in very short prisms, or in long and slender prisms : and
some planes may be so enlarged as to obliterate others ;
a few figures of quartz crystals will illustrate these pecu-
79 80 81 . 6S 83
Figure 79 is the regular form of the crystal. Figure 80 is
the same form with some feces very much enlarged, and
oUiers yerj small. Figure 81 is a very short prism and
pyramid of quartz, such as is often seen attached to the
surface of rocks ; and figure 82 is a similar form very much
elongated. Notwithstanding all these variations, every angle
What are some of the irregularities o^oiyatala ?
46 STEUCTtJBE OF MINEKAIS*
of inclination remains the same : and this is a general &ct
in all crystals, that whatever distortions take place, the angles
lure constant. Greater diversity is given to the shapes of
crystals by these simple variations, without multiplying the
number of distinct forms. Figure 83 is a tapering prism of
the same mineral, with a minute pyramid at the apex. The
fiices of this pyramid have exactly the same inclinations as
those of figure 79.
The constancy of the angles shows that the fundamental
form of the crystal, or, in other words, the form of its mole-
cules, is constant, amid all these variations of size and shape.
Crystals have sometimes curved &ces. The faces of
diamonds are usually convex, and some crystals are almost
84 spheres. Figure 84 is one of these diamond
crystals. It is the same form as is represented
in figure 45. For cutting glass, they always
i select those crystals that have a natural curved
edge, as others are much inferior for the purpose
' and sooner wear out. In figure 85 a different
kind of curvature is represented. It is a curved rhombohe-
85 dron, in which the opposite faces are parallel in
# their curving : it is a common form of spathic iron
and pearl spar. The latter mineral from Lock-
port, New York, is always curved in this way.
Still more singular curvatures are sometimes
met with. In the manmioth cave of Kentucky,
86 leaves, vines and flowers are beautifully imita-
Some of the " rosettes" are
> and consist of curving leaves,
ful shapes. The frostings on
dnter are oflen miniature pic-
nd vines with rolled tendrils,
the many singular results of
3n the cool mornings of spring
3 climate, twigs of plants are
id encircled by fibrous icy
hich are attached vertically to
are formed during the night,
>n aflerthe appearance of the
What is said of carved crystals ? What of curved crjBtallizationB of
gypfBum ? 6( ice 7
XSABVRBIESNT OP CBT8TALS. 47
ON MEASUBINO ANGLES OF CRYSTALS.
As the angles of crystals are constant, minerals, as has
been stated, may oRen be distinguished by measuring these
angles. This is done by means of instruments called goni'
ometerSj a term meaning, literally, angle-measurers.* These
are of two kinds ; one is called the common goniometer, the
other the reflecting goniometer.
The common goniometer depends on the
very simple principle that when two straight .
lines cross one another, as A E, C D in the
annexed figure 87, the parts will diverge
equally on opposite sides of the point of in-^^
tersection (O) ; that b, in mathematical language, the angle
AODisequaltotheangleCOE,andAOC is equal toDOE.
The instrument in common use is here represented.
It consists of two arms, ah^cd, moving on a pivot at o : the arms
open and shut, and their divergence, or the angle they make
with one another, is read off on the graduated arc attached.
In using it, press up between them, the edge of the crystal
whose angle is to be measiured, and continue opening the arms
thus till the inner edges lie evenly against the faces that include
How are the angles of crystals measured 1 Explain the principle of
the common goniometer from the figure. Explain the common goni-
ometer and its use.
* From the Greek gonu, angle, and tMiron, measure.
48 BTBVCTUSB OF MINSRALK*
the required angle. To insure accuracy in this respect, hold
the instrument and crystal between the eye and the light,
and observe that no light passes between the arm and the
applied &ces of the crystal. The arms may then be secured
in position by tightening the screw at o ; the angle will then
be measured by the distance on the arc from k to the left
or outer edge of the arm c d, this edge being in the line of
0, the center of motion. As the instrument stands in the
figure, it reads 45**. The arms have slits at ^ A, n p, by
which they may be shortened so as to make them more con-
venient for measuring small crystals.
In some instruments of this kind the arc is detached from
the arms. When this is the case, after the measurement is
made and the screw at o tightened, the arc (which has the
shape o£a fb inthe annexed figure, except that from a to ft
is a solid bar) is adjusted to the upper edge of one of the
arms, bringing the marie at o, the center, exacSy to the center of
divergence of the arms. The angle is then read ofifas before.
With a little ingenuity the student may construct a goni-
ometer for himself that vnll answer a good purpose. A semi-
circle may be described on mica or a glazed card, of the
shape in figure 88 : it should then be divided into halves at
/, and again each half subdivided into nine equal parts.
Each of these parts measures 10 degrees ; and if they are
next divided into ten equal parts, each of these small divisions
will be degrees. The s^mi-circle may then be cut out, and
is ready for use. The arms might also be made of stiff card
for temporary use ; but mica, bone or metal is better. The
arms should have the edges straight and accurately parallel,
and be pivoted together, l^e instrument may be used like
that last described, and will give approximate results, suffi-
ciently near for distinguishing most minerals. The ivory
rule accompanying boxes of mathematical instruments, having
upon it a scale of sines for measuring angles, wiU answer
an excellent purpose, and is as con-
venient as the arc. The annexed | S k VW ? \\*/ /j^ ^^^
figure will illustrate the mode of ^ ^^^^\Tif%^<
using it. The scale is graduated h*"
along the margin, the middle point
marking 90**, and the divisions
either side 10 degrees (as in the figure) andl also single de-
How 18 it used when the arms are detached 1 How may a temporary
goniometer be made 1 How may a seale of sines be used ?
XBABXTRBMBNT OF CHTITAL8. 40
grees. The arms are so applied to the scale, that the center
of motion is exactly at the extreraitj of the middle line,
marked 90; aad the leg crossing the scale (or £hat edge of it
in the line o£ the center of motion) will then indicate bj its
^sition over the graduated margin, the angl^ desired.*
In making such measurements it is important to remember
1. An angle ADD (figure 87} and A O C, together,
equal 180^ ; so that if A O C be measured, A O D is ascer*
tained by subtracting A O C from 180**.
2. In a rhomb or rhomboid, hab and a ( a, to-
gether, equal 180° ; taid one may be ascertained
by subtracting the other from 180°. If an obtuse
angle of a rhombic prism has been measured and
found to be 110°, and the acute angle on measurement is as-
certained to be 60°, the student should add the two together
to find whether the sum is 180° ; for if not, there is some
error in the measurement, and it should be repeated* 110^
added to 60° makes 170°, showing in this case an error
3. In any polt^mi^ the sum of the angles is equal to twice
as many right angles as there are sides less two. Let the
number of sides, for example, be 6 : 6 less two is 4 ; and
the angles together equal twice 4, (or 8,) right angles, which
is equivalent to 8 X 90° =72p°. If we have a prism of six
sides, and wish to ascertain the angles between these sides,
the angles should be measured successively, and the whole
added together to ascertain whether the measurements are
correct. If the sum is 720°, there is good reason to confide
in them. Crystals are at times a little irregular ; and this"
should be looked to, as part of the apparent error may at
times be thus accounted for. This general principle and the
Whftt three points must be obflcrred in making measurements 1
* Another mode for approximate results consists in holding the crys-
tal with the two feces (whose inclination is to be measured) in an
exactly vertical position over a piece of paper : then place a small rule
parallel, as near as the eye can judge, to one face, and draw a line ; next
do the same for the oUier face. The angle between the two lines,
measured either by an arc or the ivory rule just mentioned, is the
desired inclination. With practice, much skill may be acquired in
sach trials. They may be made with microscopic crystals under a
50 8THUCTCJRE OF MINEBALS.
preceding, which is only a simpler case of the same, are of
great importance in the measm^ments of crystals.
Reflecting Goniometer. The reflecting goniometer aflbrds
a more accurate method of measuring crystals that have
luster, and may be used with those of minute size. The
principle on which this instrument is constructed will be un-
derstood from the annexed figure (fig. 90) representing a
90 crystal, whose angle a b c ia required.
A ^ The eye, looking at the face of the
m *^ / ^^ crystal h c, observes a reflected image
^v/ ^y^^ ofm, in the direction P n. On revolving,
^^m^ the crystal till a ^ has the position of
n^ x^^^y i c, the same image will be seen again in
•^''^^'^ the same direction P n. As the crystal
is turned, in this revolution, till ah d has the present position
of h c, the angle d, h c measures the number of degrees
through which it is revolved. But d h c, subtracted from
180^, equals the angle of the crystal a b c. The crystal is
therefore passed in its revolution through a number of de-
grees, which, subtracted from 180°, give the required angle.
This angle, in the reflecting goniometer of Wollaston, is
measured by attaching the crystal to a graduated circle which
revolves with it, as here represented (fig. 91.)
91 A B is the graduated cir-
The wheel, m, is at-
ed to the main axis, and
es the graduated circle
ther with the adjusted
tal. The wheel, n, is
lected with an axis
ch passes through the
a axis, (which is hollow
he purpose,) and moves
ely the parts to which
crystal is attached, in
r to assist in its adjust-
t. The contrivances for
„ idjustment of the crystal
are at p, q, r, *. To use the instrument, it must be placed on
a small stand or a table, and so elevated as to allow the ob-
server to rest his elbows on the table. The whole, thus
Explain the principle of the reflecting goniometer. Explain the mode
of using the instrument.
XEABURSMSXT OF CRYSTALS. 51
fimily arranged, is to be placed in front of a window, distant
from the same &om six to twelve feet, and with the axis of
the instrument parallel to it. Preparatory to operation, a
dark line must be drawn below the window near the floor,
parallel to the bars of the window ; or, what is better, on a
slate or board placed before the observer on the table.
The crystal is attached to the movable plate, 9, by a piece
of wax, and so arranged that the edge of intersection of the
two planes forming the required angle, shall be in a line with
the axis of the instrument. This is done by varying its
situation on the plate, ^, or the situation of the plate itself or
by means of the adjacent joints and wheel, r, s, p, as will be
readily understood from the instrument.
When apparently adjusted, the eye must be brought close
to the crystal, nearly in contact with it, and on looking into
a face, part of the window will be seen reflected, one bar of
which must be selected for the trial. If the crystal is cor-
rectly adjusted, the selected bar will appear horizontal, and
on turning the wheel, », till this bar, as reflected, is observed
to appioach the dark line below, seen in a direct view, it will
be found to be parallel to this dark line, and ultimately to
coincide with it. If there is not a perfect coincidence, the
adjustment must be altered until this coincidence is obtained.
Continue then the revolution of the wheel, n, till the same
' bar is seen by reflection in the next &ce, and if here there
is also a coincidence of the reflected bar with the dark line
seen direct, the adjustment is complete ; if not, alterations
must be made, and the first face again tried. A few succes-
sive trials of the faces, will enable one to obtain a perfect
The circle A B is usually graduated to half degrees, and
by means of the vernier, v, minutes are measured. Afler
adjustment, 180^ on the arc must be brought opposite 0, on
the vernier. The coincidence of the bar and dark line is
then to be obtained, by turning the wheel, n. When ob-
tained, the wheel, m, should be turned until the same coinci-
dence is observed, by means of the next face of the crystal.
If a line on the graduated circle now corresponds with on
the vernier, the angle is immediately determined^ by the
number of degrees opposite this line. If no line corresponds
with 0, we must observe which line on the vernier coincides
with one on the circle. If it is the 18th on the vernier, and
' the line on the circle next below on the vernier marks 125^,
52 BTRUCTURB OF MINERALS.
the required angle is 121"* 18' ; if this line marks 125"^ 30',
the required angle is 125^ 48'.
Some goniometers are furnished with a small polished re*
flector, attached to the foot of the instrument below the part
8j q^ which is placed at an oblique angle so as to reflect a bar
of the window. The reflected bar then answers the purpose
of the line drawn below the window, (or on a slate,) and is
more conveniently used.
. Other modes of adjustment for the crystal, are also used ;
but they will explain themselves to the student acquainted
with the above explanations, and need not here be dwelt
HASSrVB MINERALS, OR IMPERFECT CRYSTALLIZATIONS.
Massive or imperfectly crystallized minerals either consist
of fibers or minute columns, of leaves or laminsB, or of grains :
in the Jirst, the structure is said to be columnar ; in the
second^ lamellar ; in the third, granular. We have a fitmiiiar
example of the lamellar structure in slate rocks and many
minerals that occur in masses made up of separable laminae.
The fibrous or columnar structure is common in seams of
rocks, and sometimes in incrustations covering exposed sur*
faces ; the material of the seam or crust is made up of mi-
nute fibers or prisms closely compacted together, produced
by a rapid crystallization on the supporting sur&ce. The
granular structure is well seen in loaf sugar and statuary
1. Columnar Structure. The following are explana-
tions of the terms used in describing the different kinds of
Fibrous; when the columns are minute and lie in the
same direction ; as gypsum and ashestus* Fibrous minerals
very commonly have a silky luster: a fibrous variety of
gypsum, and one of calc spar, have this luster very strongly,
and each is often called satin spar*
Reticulated ; when the fibers, or columns, cross in various
directions, and produce an appearance having some resem-
blance to a net.
Stellated ; when they radiate from a center in all direc-
tions, and produce a star-like appearance. Ex. stilbite^
What kinds of structare exist in massive minerals ? Explain the dif-
ferent varieties of columnar structure, fibrous ; reticulated, &c.
IHFERFXCT CBTSTALLIZATIONB. 58
Radiatedj divergent; when the crystals radiate from a
center, without producing stellar forms. Ex. quariZj grai^
2. Lamellae Stbttctuee. In the lamellar structure, the
laminae or leaves may be thick, or very thin ; they some-
times separate easily, and sometimes with great difficulty.
When the laminae are' thin and separate easily, the struc-
ture is said to be foliaceous. Mica is a striking example,
and the term micaceoua is often used to describe this
When the laminae are thick, the term tabular is ofien ap-
plied ; quartz and heavy spar afford examples.
The laminae may be elastic^ as in ndcA, flexible, as in talc
or graphite, or brittle, as in diallage.
' Small laminae are sometimes arranged in tteUar shapes ;
this occurs in mica.
3. Granulab Steucttjeb. When the grains in the
texture of a mineral are coarse, it is said to be coarsely gran*
tdar, as in granular marble ; when ^e, finely granular, as
in granular quartz ; and if no grains can be detected with
the eye, the structure is described as impalpable, as in
Granular minerals, when easily crumbled by the fingers,
are said to he friable.
Imitative Shapes. — ^Massive minerals also take certain
imitative shapes, not peculiar to either of these varieties of
structure. The following terms are used in describing imi-
Globular; when the shape is spherical or nearly so : the
structure may be columnar and radiating, or it may be con-
centric, consisting of coats like an onion. When they are
attached, they are called implanted globules.
Reniform; kidney-shaped. In structure, they are like
Botryoidal ; when a sur&ce consists of a group of rounded
prominences. The prominences or globules usually consist
of fibers radiating fi-om the center.
Mammillary ; resembling the botryoidal, but consisting of
Filiform ; like a thread.
Adcular ; slender like a needle.
Explain the varieties of lamellar structure ; of granular structure ; the
several imitative shapes, globular ; reuiform, &c.
Digitized by VjOOQ IC
54 STBITCTUBB OF MINERALS.
Stalactitic ; having the form of a cylinder, or cone, hang-
ing from the roofs of cavities or caves. The term stalactite
is usually restricted to the cylinders of carbonate of lime
hanging from the roofs of caverns : but other minerals are
said to have a stalactitic form when resembling these in their
general shape and origin. Chalcedony and brown iron ore
are often stalactitic.
Brusy ; a surface is said to be dru^ when covered with
Amorphous ; having no regular structure or form, either
crystalline or imitative. The word is from the Greek, and
means without shape,
A pseudomorphous* crystal is one that has a form which is
foreign to the species to which the substance belongs.
Crystals sometimes undergo a change of composition from
aqueous or some other agency, without losing their form ;
for example, octahedrons of spinel change to steatite, still
retaining the octahedral form. Cubes of pyrites are changed
to red or brown iron ore.
Again : crystals are sometimes removed entirely, and at the
same time and with equal progress, another mineral is sub-
stituted ; for example, when cubes of fluor spar are trans-
formed to quartz. The petrifaction of wood is of the same
Again : cavities left empty by a decomposed crystal, are
refilled by another species by infiltration^ and the new
mineral takes on the external form of the original mineral,
as a ftised metal the form of the mould into which it is cast.
Again : crystals are sometimes incrusted over by other
minerals, as cubes of fluor by quartz ; and when the fluor is
afterwards dissolved away, as sometimes happens, hollow
cubes of quartz are left.
The first kind of pseudomorphs, are pseudomorphs by aL
teration ; the second, pseudomorphs by replacement /* the
What is a psendomorphous crystal ? What is the first, the second,
the third and the fourth mode of pseudomorphism 1 What are they
* From the Greek pseudes, false, and tnorphe, form.
LUSTBR OF MINEBAL8. 56
iVird, pseudomorphs hy inJUtraiion ; the fourth, pseudomarphs
Pseudomorphous crystals are distinguished bj having a
different structure and cleavage from that of the mineral
imitated in form, and a different hardness, and usually little
A large number of minerals have been met with as pseu-
domorphs. The causes of such changes have operated very
widely and produced important geological results.
CHAPTER III.— PHYSICAL PROPERTIES OF
CHARACTERS DEPENDIirO ON LIGHT.
The characters depending on light are of Jive kinds, and
arise from the power of minerals to reflect^ transmUj or enUi
light. They are as follows :
1. Luster; 2. Color; 3. Diaphaneity; 4. Refraction;
90. The luster of minerals depends on the nature of their
sur&ces, which causes more or less light to be reflected.
There are different degrees of intensity of luster^ and also
different kinds of luster,
a. The kinds of luster are six, and are named from some
fiimiliar object or class of objects.
1. Metallic : the usual luster of metals. Imperfect me-
tallic luster is expressed by the term suh-metalUc,
2. Vitreous : the luster of broken glass. An imperfect
vitreous luster is termed suh^vUreous. Both the vitreous and
sub-vitreous lusters are common. Quartz possesses the
former in an eminent degree ; calcareous spar oflen the lat-
ter. This luster may be exhibited by minerals of any color.
3. Resinous : luster of the yellow resins. Ex. opal, zinc
4. Pearly : like pearl. Ex. talc, native magnesia, stil-
bite, &;c. When united with sub-metallic luster, \he term
metalUC'pearly is applied.
How are -pseudomorphous crystals distinguished ? What characters
depend on light 1 Explain the varieties of luster, metallic, vitreous, dec
• This subject is farther treated of by the author in the Amer. Jour,
of Science, vol. xlviii, pp. 66, 81, 397.
56 PHYSICAL FSOPERTIES OF BONERALS.
5. SUky : like silk ; it is the result of a fibrous strnctore*
Ex. fibrous carbonate of lime, fibrous gypsum, and many
fibrous minerals, more especially those which in other forms
have a pearly luster.
6. Adamantine : the luster of the diamond. When 8ub«
metallic, it is termed metallic-adamantine, Ex. some varie-
ties of white lead ore.
h. The degrees of intensity are denominated as jR)llows :
1. Splendent : when the surface reflects light with great
brilliancy, and gives well defined images. Ex. Elba iron
ore, tin ore, some specimens of quartz and pyrites.
2. Shining : when an image is produced, but not a well
defined image. Ex. calcareous spar, celestine.
3. Glistening: when there is a general reflection from
the surface, but no image. Ex. talc, copper pyrites.
4. Glimmering: when the reflection is very imperfect,
and apparently from points scattered over the surfiice. Ex.
A mineral is said to be duU when there is a total absence
of luster. Ex. chalk.
In distinguishing minerals, both the external color and the
color of a sur&ce that has been rubbed or scratched, are
observed. The latter is called the streaky and the powder
abraded, the streak-powder.
The colors are either meiaJlie or non-metallic.
The metallic are named after some familiar metal, as
copper-red, bronze-yellow, brass-yellow, gold-yellow, steel-
gray, lead-gray, iron-gray.
The non-metallic colors used in chai-acterizing minerals,
are various shades of white, gray, black, blue, green, yellow,
red and brown.
There are thus snow-white, reddish- white, greenish-white,
milk-white, yellowish-white ;
Bluish-gray, smoke-gray, greenish-gray, pearl-gray, ash-
Velvet-black, greenish-black, bluish -black ;
Azure -blue, violet-blue, sky-blue. Indigo-blue ;
Emerald-green, olive-gi-een, oil-green, grass-green, apple-
green, blackish-green, pistachio-green (yellowish) ;
What is observed respecting color 1
COLOR OF MINBRALB. 57
Sulphur-yellow, straw-yellow, wax-yellow, ochre-yellow,
honey-yellow, orange-yellow ;
Scarlet-red, blood-red, flesh-red, brick-red, hyacinth-red,
rose -red, cherry-red ;
Hair-brown, reddish-brown, chesnut-brown, yellowish-
brown, pinchbeck-brown, wood-brown.
A play of colors : this expression is used when several