Anorthite

Habit
Gray tabular crystals of anorthite, not more than 1.25 inches in greatest diameter, were found embedded in white limestone. Their edges and angles are rounded and their faces are dull, yielding poor measurements. They have striations on the base caused by pericline twinning, and they contain included grains of calcite and graphite and droplets of water.

Figure 73 Crystal of anorthite tabular to the base, showing the c(001), b(010), m(110), M(110). Fowler quarry.
	Figure 74 Prismatic crystal of anorthite showing the c(001), b(010), m(110), M(110), y(201), p(111), and o(111). Fowler quarry.

Optical properties
The average extinction angle on the base is 37° ; 2V near 90° ; r > v (perceptible); a = 1.1573, b = 1.579, g =1.585.

Composition
An analysis of some of the crystals by Warren showed a molecular ratio of alumina to lime to silica almost exactly that demanded by theory—1 : 1 : 2. Hence the mineral is unusually pure anorthite with but a trace of soda.

**Analysis of anorthite**
	1	2	3
SiO₂	40.16	43.04	0.706
Al₂O₃	34.89	37.39	0.367
CaO	18.26	19.57	0.349
Na₂O	trace
CaCO₃	5.30
H₂O	1.69
C (graphite)	0.18
	100.48	100.00

1. Anorthite. Franklin. C. H. Warren (186), analyst.

2. Same, recomputed to 100 percent after deducting unessential constituents.

3. Molecular ratio computed from 2.

Occurrence
The anorthite crystals were discovered by E. P. Hancock and were described by Warren (186), from whose paper the facts and data given above were taken. They were found in limestone of the Fowler quarry south of Franklin, near the contact of a pegmatite dike.


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