362 Brush and Dana—Fairfield County Minerals, 
twinning-plane must make with a (100) an angle of either 51° 
(toward 010) or 39° (toward 010). This condition is equally 
well satisfied by the plane 270 (100,270 = 51° 4’), or by 270 
(100,270 = 39° 38’) As this supposed twinning-plane has 
so complex a relation to the other planes of the crystal, it is 
probable that this coincidence is only accidental. 
Optical properties. 1 seat fragments of fairfieldite parallel 
to the two cleavage planes were examined in the stauroscope, 
with the following eagle :—The planes of tiplitiy thewtoda 
intersect the ee plane @ (100) in lines which make angles 
of 40° and 50° respectively with the edge a|. One optical 
axis was baile. on the edge of the field in converging light, 
ny lying in the vibration-plane making an — of 50° 
A, B 
P.O; 38°39 39°62 
F 5°6 7 
MnO 15°55 12°40 
CaO 28°85 30°76 
Na,O 0-73 0°30 
K,0 0°13 ige™ 
H,O 9°98 9°67 
Quartz P31 0°55 
100°56 100°30 
Stale ratios ees the oxides calculated from these analyses are 
as 
A. B. 
P,0, 270 270 1 279 279 «1 
FeO 078 "097 
MnO 219 175 
CaQ “515 825 3°06 549 826 2°96 
Na,O 012 
20 --- 
H,O “554 “554 2°05 *D3T “637 1-93 
The = P, O.: RO: BOs 829 answers to the formula 
R,P,0,+ 2 aq. Tf here R = Ca:Mn+Fe = 2:1 and the 
ratio ve a Fe be also 2:1. The formula requires :— 
