|♦ Gem Variety of Feldspar Group
Sunstone is not a mineral species but a gem name for a variety of feldspar showing aventurescence, and this name was originally for oligoclase from India and other locality containing iron oxide mineral such as hematite.
Andesine that is commercially named sunstone (photo 6) has been on the market since one or two years ago.
Feldspar is a mineral that forms a solid solution with a wide range of composition. Plagioclase series, belonging to triclinic crystal system, forms a solid solution series from an end member on the Ca side, andesine, to an end member on the Na side, albite, and feldspar consisting of the intermediate chemical composition are subdivided into six varieties (fig 1).
Andesine sits between oligoclase and labradorite, so that its RI and properties also fall between their values. This andesine has orange-red colour, but it does not show aventurescence so that it is not appropriate to call it sunstone.
Moonstone is also not a mineral species but a name of a gem variety showing blue to white schiller by interference and scattering of light caused by alternate layered structure of orthoclase and albite, which is an end member of a plagioclase solid solution series.
A gemstone called grainbow moonstone" (photo 7) appeared on the market in the middle of 1990s. This is commercially called moonstone, but mineralogically it is actually labradorite in feldspar group, and it does not fall in the category of moonstone described above.
Another gemstone called gblue moonstone" (photo 8) appeared on the market recently. Despite the name, it was mineralogically proved to be a plagioclase containing anorthite constituent under 10mol% after compositional analysis with X-ray fluorescency. A feldspar showing iridescence with this chemical composition is called peristerite. *4
Please refer following gGEMMOLOGY", or visit to GAAJ website at www.gaaj-zenhokyo.co.jp.
*1 gGEMMOLOGY" September 2000, November 2004
*2 gGEMMOLOGY" May 2004, February 2005
*3 gGEMMOLOGY" February 2005, June 2005
*4 gGEMMOLOGY" February 2003, June 2003, May 2005