MINERAL CONSTITUENTS OE METEORITES. 
203 
A slide cut for the microscope from a fragment of the nodule was found to exhibit a 
section of one of the crystals of this mineral cut very nearly parallel to the plane of 
symmetry. Two of the edges bounding this section were parallel, the one to a series of 
lines running through the crystal corresponding to its cleavage-planes, the other to 
certain bands that are constantly present in this augite, generally parallel to the plane 
0 01, apl formed of a white doubly refracting silicate, no doubt of the enstatite next 
to be described, intercalated in microscopic layers through the augite. These two edges 
represent the planes 10 0 and 0 0 1 as seen in a section nearly parallel to the plane of 
symmetry. They gave a normal angle of 0 01, 1 0 0 = 75° 15'. In diopside this angle 
is 73°-59. 
Light traversing this section of the crystal between crossed Nicols is at its maximum 
of extinction when polarised in a plane parallel or perpendicular to a line, making with 
0 01 an angle very near to 22° 45', and with 1 0 0 an approximate angle of 52° 30'. 
In diopside the second mean line makes corresponding angles of 22° 5' and 51° 6' with 
these normals. These measurements w T ere made by an eyepiece goniometer fitted to 
the microscope, and having a fixed spider-line to indicate the plane of polarisation, while 
a rotating line is employed to measure on a graduated circle the inclinations of the edges 
and other directions in the section. 
A section made parallel to the plane 10 0 exhibited one of the optic axes on the limit 
of the field of view in a Norremberg’s polariscope. The plane containing the optic axes 
is perpendicular to the edge [10 0, 0 0 1], and the optical character in the centre of the 
field is negative. 
In all the above respects the mineral accords with diopside. 
When looked through in a direction nearly normal to 0 0 1 or 0 1 0, or indeed in any 
direction parallel to the zone circle [001,01 0], the crystals show a remarkable dichroism, 
which is, however, especially conspicuous when the direction is nearly normal to the 
plane 0 01. 
If the section is parallel to the plane of symmetry, light polarised in a plane perpen- 
dicular to the principal section containing the acute mean line and the axis of symmetry 
is transmitted, of a pale pink lilac ; when the crystal is turned 90°, so as to bring the 
same principal section into parallelism with the plane of polarisation, the transmitted 
tint is bluer, exhibiting a pale slate-blue or lavender. 
The plane 10 0 presents a somewhat facile cleavage, much more readily obtained than 
cleavages which are also met with on the planes of the form 110, the latter being 
interrupted and uneven. The plane 1 0 0 is also conspicuous for a remarkable metallic 
lustre, recalling that seen on some kinds of diallage, but of a fine golden hue. 
Two analyses of this mineral by the method already described, the silica being distilled 
as silicium difluoride and determined as potassium fluosilicate, gave the following 
results : — 
