Jump to ContentJump to Main Navigation
Show Summary Details
More options …

American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian

IMPACT FACTOR 2018: 2.631

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
Source Normalized Impact per Paper (SNIP) 2018: 1.103

See all formats and pricing
More options …
Volume 102, Issue 10


Protoenstatite: A new mineral in Oregon sunstones with “watermelon” colors

Huifang Xu
  • Corresponding author
  • NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin 53706, U.S.A.
  • Email
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Tina R. Hill
  • NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin 53706, U.S.A.
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Hiromi Konishi
  • NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin 53706, U.S.A.
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Gabriela Farfan
Published Online: 2017-10-02 | DOI: https://doi.org/10.2138/am-2017-6186


Al-Fe-bearing protoenstatite was discovered in Oregon sunstones with unusual pleochroic/dichroic red to green coloration using high-resolution transmission electron microscopy (HRTEM) and X-ray energy-dispersive spectroscopy (EDS). The empirical formula calculated on the basis of 6 O apfu is (Mg1.17Fe0.43Al0.26Ca0.03Na0.10Ti0.01)Σ2.00(Si1.83Al0.17)Σ2.00O6. The protoenstatite has a space group of Pbcn; its unit-cell parameters refined from selected-area electron diffraction patterns are a = 9.25(1), b = 8.78(1), and c = 5.32(1) Å. Protoenstatite nanocrystals are quenchable to low temperature. The crystallographically oriented nanocrystals of protoenstatite and clinoenstatite in association with copper nanocrystals are responsible for the unusual green and “watermelon” coloration of the labradorite gemstone.

Keywords: Oregon sunstone; labradorite; new pyroxene; clinoenstatite; protoenstatite; HRTEM; native copper; dichroic; Nanominerals and Mineral Nanoparticles

Special collection papers can be found online at http://www.minsocam.org/MSA/AmMin/special-collections.html.

References cited

  • Angel, R.J., Chopelas, A., and Ross, N.L. (1992) Stability of high-pressure clinoenstatite at upper-mantle pressures. Nature, 358, 322–324.Google Scholar

  • Cameron, M., and Papike, J.J. (1981) Structural and chemical variations in pyroxenes. American Mineralogist, 66, 1–50.Google Scholar

  • Chen, C.-H., and Presnall, D.C. (1975) The system Mg2SiO4-SiO2 at pressure up to 25 kilobars. American Mineralogist, 60, 398–406.Google Scholar

  • Dallwitz, W.B., Green, D.H., and Thompson, J.E. (1966) Clinoenstatite in a volcanic rock from the Cape Vogel area, Papua. Journal of Petrology, 7(3), 375–403.Google Scholar

  • Hofmeister, A.M., and Rossman, G.R. (1985) Exsolution of metallic copper from Lake County labradorite. Geology, 13(9), 644–647.Google Scholar

  • Ijima, S., and Buseck, P.R. (1975) High resolution electron microscopy of enatatite I: twinning, polymorphism, and polytypism. American Mineralogist, 60, 758–770.Google Scholar

  • Johnston, C.L., Gunter, M.E., and Knowles, C.R. (1991) Sunstone Labradorite from the Ponderosa Mine, Oregon. Gems and Gemology, XXVII, 220–233.Google Scholar

  • Jones, S.A., Burlitch, J.M., Duchamp, J.C., and Duncan, T.M. (1999) Sol-gel synthesis of protoenstatite and a study of the factors that affect crystallization. Journal of Sol-Gel Science and Technology, 15, 201–209.Google Scholar

  • Lee, S., and Xu, H. (2016) Size-dependent phase map and phase transformation kinetics for nanometeric iron(III) oxides (γ → ε → a pathway). The Journal of Physical Chemistry C, 120, 13316–13322.Google Scholar

  • Murakami, T., Takeuchi, Y., and Yamanaka, T. (1982) The transition of orthoenstatite to protoenstatite and the structure at 1080°C. Zeitschrift für Kristallographie, 160, 299–312.Google Scholar

  • Peterson, N. (1972) Oregon “Sunstones”. The Ore Bin, 34(12), 197–215.Google Scholar

  • Schmitz, S., and Brenker, F.B. (2008) Microstructural indications for protoenstatite precursor of cometary MgSiO3 pyroxene: A further high-temperature component of comet Wild 2. The Astrophysical Journal, 681, L105, .CrossrefGoogle Scholar

  • Shiraki, K., Kuroda, N., Urano, H., and Maruyama, S. (1980) Clinoenstatite in boninites from the Bonin Islands, Japan. Nature, 285, 31–32.Google Scholar

  • Smith, J.V. (1969) Crystal structure and stability of MgSiO3 polymorphs: Physical properties and phase relations of Mg-Fe pyroxenes. Mineralogical Society of America Special Paper, 2, 3–29.Google Scholar

  • Smyth, J.R. (1974) Experimental study on the polymorphism of enstatite. American Mineralogist, 59, 345–352.Google Scholar

  • Smyth, J.R., and Ito, J. (1975) The synthesis and crystal structure of a magnesium-lithium-scandium protopyroxene. American Mineralogist, 62, 1252–1257.Google Scholar

  • Stewart, D., Walker, G., Wright, T., and Fahey, J. (1966) Physical properties of calcic labradorite from Lake County, Oregon. American Mineralogist, 51, 177–197.Google Scholar

  • Tribaudino, M., Nestola, F., Camara, F., and Domeneghetti, M.C. (2002) The high-temperature P21/c-C2/c phase transition in Fe-free pyroxene (Ca0.15Mg1.85Si2O6): Structural and thermodynamic behavior. American Mineralogist, 87, 648–657.Google Scholar

  • Wenk, H.R., Joswig, W., Tagai, T., Korekawa, M., and Smith, B.K. (1980) The average structure of An 62–66 labradorite. American Mineralogist, 65, 81–95.Google Scholar

  • Xu, H.F., Lee, S., and Xu, H.W. (2017) Luogufengite: a new nano-mineral of Fe2O3 polymorph with giant coercive field. American Mineralogist, 102, 711–719.Google Scholar

  • Yang, H. Finger, L.W., Conrad, P.G., Prewitt, C.T., and Hazen, R.M. (1999) A new pyroxene structure at high pressure: Single-crystal X-ray and Raman study of the Pbcn-P21cn phase transition in protopyroxene. American Mineralogist, 84, 245–256.Google Scholar

About the article

Received: 2017-05-09

Accepted: 2017-06-29

Published Online: 2017-10-02

Published in Print: 2017-10-26

Citation Information: American Mineralogist, Volume 102, Issue 10, Pages 2146–2149, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2017-6186.

Export Citation

© 2017 by Walter de Gruyter Berlin/Boston.

Comments (0)

Please log in or register to comment.
Log in