There are two kinds of thermal properties that can be valuable for mineral identification:
- The "feel" of the specimen (the touch test).
- Response to heating
Touch is helpful to categorize the thermal conductivity of
a specimen. You can tell a lot with an experienced touch, or (especially with a
small specimen with a glossy surface) you can also breathe on a cool specimen in your warm hand. If it is
insulating, your breath might condense on the surface. If it is conductive, the
warmth of your fingers may rapidly evaporate the moisture.
- These minerals are "warm" to the touch, and conduct heat poorly compared to most minerals.
Well known examples are gypsum (which is noticeably warmer to the touch than a
similar specimen of quartz), also
apatite. There can be some oddities. For example,
muscovite have average conductivity along the sheets of the crystals, but are insulating normal to those sheets.
The difference is much more dramatic in
approaching diamond along the plane of the crystal, highly insulating
normal to it. In reality, many minerals have different thermal
conduction on differing crystal axes, but generally not so great as the
micas let alone graphite.
- Average / normal
- Unfortunately, most minerals have a thermal conductivity that can't easily be distinguished by hand.
These include common minerals such as quartz and calcite.
- These minerals are "cold" to the touch, as they rapidly conduct away the warmth of your fingers. This includes most metals, but note that
diamond has the highest thermal conductivity known (five times higher than
There are a few other non-metallic minerals with anomalously high thermal conductivity,
including corundum (sapphire & ruby),
Response to Heating: Some minerals have characteristic responses to heating, including:
- Cinnabar easily reduces to metallic mercury, and
galena reduces to lead (as the sulfur burns out of the compounds). Other
minerals decompose by losing water of hydration (gypsum turns to anhydrite,
borax turns into
tincalconite which in turn dehydrates to powdery anhydrous borax, not a
recognized mineral). Likewise,
dehydrates into diopside.
- Minerals with a low melting point include
lead (all elements).
- Curling or Peeling
- Some of the phyllosilicates (a
clays and micas) are known for leafing when heated. The mineral
is named for its ability to exfoliate into a flaky mass when heated. Likewise,
the name apophyllite means "to leaf apart".
While not a phyllosilicate, the mineral
exfoliates when heated.
- Expanding / Swelling
- Vermiculite is known for extreme expansion when heated
(the vermiculite used in potting soil mixtures has already been
- Other chemical changes
- Amethyst can be turned into
citrine by heating; this involves a change in oxidation state of the iron impurities which give both their colors.
- Other characteristics:
- Minerals containing sulfur often smell of sulfur dioxide when heated; minerals containing arsenic often smell of garlic when heated.