Tremolite is a member of the amphibole group in silicate minerals with the chemical formula of Ca₂Mg₅[(OH)Si₄O₁₁]₂. The crystals are generally arranged in radial, fibrous or slender prismatic shapes.

In a Zambian emerald, irregular jumbles of transparent columnar colorless crystals caught the author’s eyes, which were further identified as tremolite by Raman (figure 1). Tremolite is mostly colorless, but sometimes with a slight brownish or bluish-green, so when tremolite inclusions are present in large quantities, they give the emerald host a slight blue tinge (Gübelin et al., 1958).

Most of the main gem-quality emerald deposits in Zambia are situated in the Kafubu area, with smaller deposits in the Musakashi area of central Zambia. Emerald deposits in the Kafubu area were formed in Mesoproterozoic Muva supergroup rocks, which consist of metasedimentary rocks interspersed with ultrabasic rocks. The Muva Supergroup overlays a basement complex that’s about 1,800 to 2,050 million years old, which is ultramafic schist rich in talc-chlorite-amphibole (tremolite/actinolite)-magnetite schist intruded by pegmatite. Emeralds are usually found in reaction zones between schist and beryllium (Be)-bearing pegmatites (Zwaan et al., 2005; Sliwa and Nguluwe, 1984).

Tremolite is also a common guest in emeralds from other origins, it used to be the most typical inclusion of emeralds from the Sandawana mining area in Zimbabwe. However, unlike in Zambia, tremolite in Zimbabwean emeralds are much finer, the occurrence of dense masses is rare, and short needles are always present (Gübelin et al., 1958). 

Figure 1. A Zambian emerald contains a large number of prismatic tremolite crystals inside and distributed out of order. Photomicrograph by Huixin Zhao; field of view 4.18 mm. Darkfield illumination.

References

Gübelin E. J., Ph. D., C. G. (1958) Emeralds from Sandawana. The Journal of Gemmology, Vol. 6, No. 8, pp. 340-354.

Zwaan J. C., Seifert A. V., Vrána S., Laurs B. M., Anckar B., Simmons W. B., Falster A. U., Lustenhouwer W. J., Muhlmeister S., Koivula J. I., Guillerminet H. G. (2005) Emeralds from the Kafubu Area, Zambia. Gems & Gemology, Vol. 41, No. 2, pp. 116-148.

Sliwa A. S., Nguluwe C. A. (1984) Geological Setting of Zambian Emerald Deposits. Precambrian Research, Vol. 25, No. 1-3, pp. 213-228.