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MinIdent-Win - Get a copy of MinIdent-Win at http://www.micronex.ca/index.php?idx=10

Formula: [Na]0.3-0.5[Ca]0.5-0.7[Al]1.5-1.7[Si]2.3-2.5O8 Status: name is IMA approved or traditional Kretz abbreviation: Pl Level: Sub-species First Described in 1780 Parents: plagioclase-series Symmetry: Triclinic Space Group: P1 Mean Atomic Number: 11.6 Diffraction Values: 3.180, 3.760, 3.210, 3.200, 2.520 ICDD (TM) Number: 18-1202

Minimum Maximum Average Std. Dev. a (A) 8.165 8.176 8.171 b (A) 12.853 12.872 12.863 c (A) 7.095 7.115 7.105 Alpha 93.420 93.600 93.510 Beta 115.930 116.500 116.215 Gamma 89.700 90.700 90.200 Volume 668.252 668.119 668.241

Minimum Maximum Average Std. Dev. n(Alpha) 1.553 1.563 1.558 n(Beta) 1.556 1.568 1.562 n(Gamma) 1.562 1.573 1.566 Max. birefringence0.009 0.010 0.009 2V Gamma 74 87 78 Optical Sign: +ve

C(Alpha) Colourless C(Beta) Colourless C(Gamma) Colourless Dispersion R>V

Minimum Maximum Average Std. Dev. Mohs 6.0 6.5 6.0 Vickers 771 943 771 Density 2.67 2.72 2.70 0.01

Total Min Anal. Min Average Anal. Max Total Max Average Coordination Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Atomic H 0.0000 0.0000 0.0148 0.0582 0.0582 0.0398 Li 0.0000 0.0000 0.0005 0.0030 0.0030 0.0002 O 45.1273 46.0013 47.0890 47.5179 49.2281 8.0000 Na 2.2478 2.2478 3.2655 4.1321 4.3441 0.3861 6 7 Mg 0.0000 0.0000 0.0447 0.2412 0.2412 0.0050 Al 14.3638 14.3638 15.5116 16.4060 17.2812 1.5628 4 6 Si 23.0460 23.0460 24.9037 26.2607 26.4369 2.4099 4 K 0.0000 0.1079 0.3589 0.9298 0.9298 0.0250 9 Ca 7.2255 7.2255 8.4129 9.7198 10.4675 0.5706 6 7 Ti 0.0000 0.0000 0.0152 0.0659 0.0659 0.0009 Mn 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 Fe 0.0000 0.1127 0.4452 0.8953 0.8953 0.0217 Rb 0.0000 0.0000 0.0002 0.0008 0.0008 0.0000 Sr 0.0000 0.0000 0.1598 1.7588 1.7588 0.0050 9 Ba 0.0000 0.0000 0.0287 0.3583 0.3583 0.0006 9 Total 100.2507 13.0274

Atomic proportions calculated for O = 8.0

Page 1 - 04/03/2010 MinIdent-Win - labradorite Get a copy of MinIdent-Win at http://www.micronex.ca/index.php?idx=10

Compilation based on 2 general and 17 sample records Values in italics are calculated from the minimum and maximum values. Other data are from the sample and general records.

Lustre Pearly, Vitreous Aggregation Massive, Compact, Granular Habit Tabular Tenacity Brittle Fracture Conchoidal, Subconchoidal, Uneven {001} Excellent, {010} Good Surface Colour Colourless, White, Pale Grey, Grey, Dark Grey, Bluish Grey, Dark Bluish Grey, Greyish Blue, Dark Greyish Blue, Greenish Grey Colourless, White Fluor. Unspec Blue, Yellowish Green Other lumin. Cathodoluminescent: Pale Blue, Blue, Dark Blue, Red, Yellowish Green

Comp. Plan. Comp. Surf. Twin Plane Twin Axis Notes {010} [001] Penetration, Contact {021} {021} Contact {0-21} {0-21} Contact {001} {001} Contact {010} {010} Polysynthetic [010] Parallel, Polysynthetic [100] Parallel, Polysynthetic

Notes on hand specimen data: Commonly exhibits "labradorescence, a play of colours due to submicroscopic, film-lke exsolution.

Polymorphs: "High" & "low" structures occur.

Remarks: Usually colourless, white, greyish, bluish grey or greenish, often showing a distinctive play of colours ("labradorescence"). It is transparent to translucent and the is vitreous. Labradorite is brittle with an uneven fracture, excellent {001}, very good {010} and good {110} cleavages. Simple and polysynthetic twinning are ubiquitous. It commonly forms tabular crystals that are flattened parallel to {010}, but is usually massive or granular.

Occurrences: A common constituent of anorthosites, norites, basalts and gabbros as well as other igneous rocks. In the metamorphic environment it occurs in gneisses derived from basic rocks.

Localities of samples used in compilation: Hawk's Bay, Labrador; Ottawa; Quebec; Canada. Tigerton and Merrill, Wisconsin; Shelby, North Carolina; Millard County and Clear Lake, Utah; Summit of San Marcos Mountains, California; St. John's Point, Ardglass, Colorado; Grand Marais, Minnesota; Chester County, Pennsylvania, U.S.A. New Amalfi, Matatiele, South Africa. County Down, Northern Ireland. Middle Latin Valley, Roman magmatic province; Stromboli, Aeolian Islands; Monte Rosso, Linosa, Sicily, Italy.

References: Eur. J. Mineral. v.15, p.665-679. Can. Min. v.45, p.1443-1469. Deer et al. (1963) v.4, p.121- 165. Rev. Mineralogy v.2. Roberts et al. (1974) Encycl. Mins. Phillips & Griffen (1981) Opt. Min.

Page 2 - 04/03/2010 MinIdent-Win Labradorite (plagioclase )

Dorian G.W. Smith Copyright © 2000, Micronex Ltd. Caption: The basal cleavage surface of a massive piece of labradorite has been oriented to show the trace of the polysynthetic twinning that is so characteristic of the plagioclase . At a different angle, this same specimen shows the labradorescence that is also common in the range of compositions traditionally known as labradorite. Locality: Nain, Labrador, Canada. Keywords: labradorite; plagioclase-series; feldspars; polysynthetic twinning; basal cleavage; Nain; Labrador; Canada; tectosilicates; framework silicates Acknowledgements: From the collections of the University of Alberta (specimen no. 5562). Photography by Frank Dimitrov and Dorian Smith.