Specimen data for Anorthite.

Tom Mortimer catalog number: 1775

Identification method: EDS analysis.


The anorthite species is listed for this area. Anortite is the pure calcium end member of the albite-anorthite series. Pure anorthite is quite rare. Bytownite is a now discredited term previously used for feldspars containing 10 to 30 percent albite. The EDS plot above shows a small sodium (Na) response. The actual sodium content of the specimen is higher than might be inferred by the Na peak due to the weak EDS detector response for light elements such as sodium.
Reference: Hitchcock, Geology of New Hampshire Vol. 3, pg 90, "Anorthite ... most notable specimens are found in the diabase at East Hanover (NH). There a rock occurs that is filled with crystals of this species...often an inch in length and breadth. The rock is so full of them that it is an anorthite porphyry."
This specimen is from that area of East Hanover.
A diabase is a volcanic basalt rock, a dark plutonic gabro, medium to fine textured.

A second (polished grain) EDS analysis was done on this feldspar with a detector with excellent light element response.

The Atoms Per Formula Unit (APFU) computed from the Atomic percents of this analysis gave a chemistry of:
(Ca_0.66Na_0.33K_0.04)Al_1.6Si_2.27O₈ - normalized for 8 oxygen. Ideal anorthite chemistry is: CaAl₂Si₂O₈
Deer, Howie and Zussman in Introduction to Rock Forming Minerals states "When some sodium replaces calcium in the anorthite structure there must be an equivalent replacement of Si for Al ..." This chemistry nicely shows that !
Although this feldspar is very calcium dominant, it is quite distant from a pure anorthite end member. This specimen should be classified as labrodorite-bytownite. Perhaps Hitchcock's sample came from a different area of east Hanover.

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