|
Colorado
Mineral Society
February 2025 General Meeting and Presentation
Friday, February 7th, 2025
Gemstone-Bearing
Pegmatites
by
Mike Menzies
Pegmatites
are produced from the very last remnants of some granitic
magmas, and are most commonly recognized by their exceptionally
coarse grain size.
Those
that host gem crystals are rare, representing a very
small proportion of the overall pegmatite population,
and are typically confined to specific districts. Key
characteristics include shallow emplacement depth, which
helps to create the required open space (miarolitic
cavities) for the crystals to grow. The most widely
accepted classification (introduced by Petr Cerný
(in 1991), divides pegmatites into two families, NYF
and LCT, which are named for their characteristic trace
elements (Niobium, Yttrium and Fluorine and Lithium,
Cesium and Tantalum). Most NYF bodies have crystallized
within their parent granites vs. LCT pegmatites, which
are emplaced within the surrounding country rocks. Despite
the difference in the characteristic trace elements,
there is significant overlap between the families in
their chemistry and the corresponding gem minerals.
Of
the two families, NYF pegmatites are typically much
less well represented in the literature than those of
LCT Family, with some of their features also less well-understood.
But, some characteristics (e.g., diversity of size,
physical form & internal structure) make them more
interesting than their LCT counterparts. Although NYF
bodies are mostly significantly smaller than LCT types,
a small proportion are exceptionally large and some
produce the world’s best and largest crystals of
gem minerals.
The
remainder of the talk focuses on the Little Gem Mine,
which is developed in a distinctly uncommon example
of an NYF pegmatite. The mine lies within the Boulder
Batholith, a few km east of Butte, Montana, just east
of the Continental Divide. Its characteristics include
an uncommon layered structure, and evidence of significant
late-stage hydrothermal alteration. Although it has
a very simple mineralogy (with less than 10 mineral
species), it is one of very few pegmatites worldwide
with abundant amethyst. It produces exceptional, world
class specimens of amethyst scepters, and jacaric overgrowths.
Mike
Menzies - Speaker biography
Mike was
born and grew up in New Zealand. He obtained an undergraduate
degree in New Zealand in 1966, emigrated to Canada and
obtained a Ph.D in Chemical Engineering at McMaster
University in Hamilton, Ontario in 1972. After brief
employment as an engineer in Montreal, he moved to Calgary
in 1976. He finished his career as a Chemical Engineer
and project manager in the Petroleum Industry in 2005.
With his move to Calgary, he became an avid mineral
collector, collecting in Idaho, Montana, Quebec, Ontario,
and Manitoba. He has published in the Mineralogical
Record on worldwide occurrences of topaz and several
locality articles for the Mineralogical Record and Rock
& Minerals. Since 1990, he has been a speak at various
mineral shows and mineralogical symposia in Canada and
the USA. His quest for knowledge about pegmatites and
their gem minerals has taken him on international trips
to Elba and Madagascar for mineralogical symposia and
field excursions. In 2022, the Mineralogical association
of Canada published his book, Pegmatites and their gem
minerals as Special Publication 15 of the Canadian Mineralogist.
Gem
Institute of American article on Gemstone-Bearing Pegmatites
(PDF file)
Many
of the important gem minerals seen on today's market
- aquamarine, tourmaline, and topaz, among others -
come from an unusual type of rock known as a pegmatite.
Gem-bearing pegmatites are crystalline igneous rocks
that are distinguished by their large-size crystals,
concentrations of certain chemical elements otherwise
rare in the earth's crust, and various unusual minerals.
Pegmatites are typically rather small bodies of rock
that are found in particular geologic environments;
the gem minerals occur in open cavities or "pockets"
within the pegmatite. This article surveys our current
understanding of pegmatites, beginning with a brief
description of their characteristics and following with
a discussion of the occurrence of gem minerals in them.
The article concludes with a summary of the specific
conditions necessary during pegmatite formation for
the crystallization of abundant gem minerals.
 |
A selection
of gemstones of
pegmatite origin ranging from 5.87 to
11.25 ct. Minerals shown (from left to
right, and from front to baclz) are us
follows: tourmuline (elbaite), garnet
(spessartine), chrysoberyl; topaz,
feldspar (or~hocluse)b, eryl
(morgunite); beryl (aquumarine),
tourmaline (elbaite), tourmaline
(elbaite); beryl (heliodor), spodumene
(kunzite). Photo by Tino Hammid.
(From GEMS & GEMOLOGY Summer
1984)
|
|
