Origin And Occurence Of Gems And Gem Minerals - Outline printable pdf download

Origin And Occurence Of Gems And Gem Minerals - Outline

M. Helper

Spring, 94

ORIGIN AND OCCURENCE OF GEMS AND GEM MINERALS

OUTLINE

I) Why are gem and gem minerals rare?

A) Composed in part of rare chemical elements

-e. g. Tourmaline (B), Beryl (Be), Topaz (F)

B) Formation requires unusual geologic conditions or mechanisms of exposure.

- e. g. Extreme pressure and temperature for diamond, v. high T for

corundum, slow growth in the presence of fluids (pegmatite minerals) for

large clear crystals, rapid transport of diamond and corundum via unusual,

deep-seated, volcanic eruptions.

II. Origin vs. Occurence

A) Origin - Relating to formation, geologic environment of formation.

B) Occurence - Where the gem or gem mineral is found today. May not be where

it formed.

III. Gem deposits

A) Definition - Economic accumulations of gems or gem minerals.

B) Types:

1) Placer - sedimentary accumulation of gems, gem gravels, formed by

weathering of original gem-bearing rocks. Natural concentration process

(winnowing) occurs due to high S.G. of most gem materials. Same true for

base and precious metals (gold, platinum, etc.).

2) Load - deposit of gem or gem minerals that occurs where the minerals

formed. Usually hard-rock mines. E.g. mining of pegmatites, diamonds.

Generally requires greater capital investment, more expertise, and larger-scale

operation to be profitable.

IV. Modes of Origin

A) Low T, involving surface and ground water near ambient temperature.

1) Dissolution, leaching and reprecipitation processes

a) Carbonic acid (HCO 3 ) from rainwater dissolution of limestone

leaches and carries metal cations (often from adjacent volcanic

rocks) to precipitate:

Malachite (CuCO 3 )

Rhodochrosite (MnCO 3 )

Origin And Occurence Of Gems And Gem Minerals - Outline