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During game generation
System bodies Mineral composition (accessibility and availability) is determined upon each system generation. More gravity tends to lead to more minerals and lower accessibility. Comets always have moderate amounts of multiple minerals with high accessibility. Homeworlds have a different starting situation with regard to minerals, and start with the team survey flagged as complete.
When a body mineral generated check take place, a roll is made separately for each mineral. If this check is successful, it generates a quantity and accessibility. Duranium has twice the chance of any other mineral to be generated.
During geological survey
The first step for mineral exploitation is obviously to send geo-survey vessels. The second step is to use Geology Teams (aka: geoteams). A team can find hundreds of thousands of tons of minerals on a planet and improve the accessibility, which is a great boon to any empire.
(This part is slightly obsolete) The mechanics are fairly straightforward]. When a team is on a system body during the 5-day increment, it makes a mineral deposit check. If the check is successful a new mineral generation check takes place for that body. This check is identical to the one performed when the system was originally generated, except the chance for each individual mineral to be generated is 33% of normal. If the check for any of the individual minerals is successful, the size of the new deposit is compared to the one originally generated. If the size of the new deposit is greater then the amount of the original one, the amount of the mineral is changed to the new amount. If the accessibility is also higher then that is increased to the new level. If the size of the new deposit is less than the existing one but the accessibility is higher then the accessibility is increased to the new value. Whatever happens as a result of the successful check, including no new minerals, the survey is completed.
The chance of the survey team making multiple discoveries over a long period is now replaced with a chance of multiple discoveries during the single check.