Mining expansion missions are only offered by factions in EXPANSION mode.

This happens when that faction reaches 75% influence or above in one of the systems they're present in. Once they enter the EXPANSION state, it affects that faction in every system where they have a presence.

During the five-day expansion window, you can find 'Mining expansion missions' at Agricultural stations, preferably the larger kinds so you can dock a Type 9 or Cutter. (Other ships will work too.)

There is one vital thing to note: to get the missions with 50m rewards, you must be ALLIED with the faction in Expansion.

I recommend taking Courier delivery missions with the faction to get your reputation up.

EDIT: Inara now has a search feature to locate mining expansion missions[inara.cz], which makes things a whole lot easier.

The old way: The galaxy map can be filtered on factions in Expansion.

You can then look up that faction on Inara[inara.cz] to see if they hold any large stations, and if so, whether that station is Agricultural.

First thing you need to do is ally yourself with the faction in Expansion, else you won't see the lucrative missions. Instead, they'll be missions for much smaller quantities with much smaller rewards.

To become allied, choose missions for the faction in Expansion with decent Rep rewards.

The missions you want are the following:

Mining Expansion Missions for X tons of Betrandite
Mining Expansion Missions for X tons of Indite
Mining Expansion Missions for X tons of Gallite

All three of these can be purchased from stations in game.

You can buy a couple of others (Cobalt, Uraninite, Coltan) but the quantities involved are huge.

Also, avoid Bromellite which can only be obtained from mining. I strongly suggest you stick to the three listed at the top of this section.

As for the ship, you want the most cargo space possible. Some of these missions require 1200 tons of goods to be fetched, which is less than 2 trips in a T9 or a Cutter, but is an eye-watering amount of work in a small ship.

Bear in mind you can share the mission with up to 3 others, in a team, which means you each only have to fetch 1/4. When completed, each of you gets the full mission reward, which is usually 35-50 million.

(Don't forget the cost of goods, which run to about 20% of the mission total.)

Bertrandite, Gallite and Indite are sold at Extraction stations, whether small outposts or large orbiters.

Looking up goods on Inara is straightforward. This is the link to Bertrandite[inara.cz], and all you have to do is change the star system from 'HIP 20998' on the right to wherever you are now, then click Search.

The default will show you where to sell goods, not buy them, so click the Exports tab, then click Filter Carriers, Filter Small Pads, Filter Surface and Filter Odyssey.

What remains is the list of stations near you selling the item in question.

If you're using a large ship, you will want to buy from a large station to minimise the number of trips.

At this point, if you have a willing friend with a fleet carrier the whole job becomes about ten times easier.

Your friend (or you, if you own one) parks the carrier right next to the station selling the three materials, and you load it up with the required amount.

Then the carrier is jumped to the mission station, where you hand in the goods.

You can share the mission with team mates at any point before you actually claim the rewards. This is a great way to pay back the carrier owner with hundreds of millions of credits.

If you can't get your hands on a carrier you'll need to hop systems to fetch the goods. This is where the risk comes in, because every time you jump to another system there's a risk you'll trigger the mission enemies: a string of nine deadly anacondas who will interdict and attack you.

Fortunately they don't all attack together, but even one will destroy your cargo ship, so you'll want to avoid interdiction at all costs.

If you share a cargo mission, your team mates can deliver goods to that mission as well.

Once the required amount of cargo has been delivered the mission owner needs to be at the station to turn it in. Then you can share another mission.

Sometimes I do 10-20 of these missions on my own, completing them all but not handing them in. When my regular team mates log in, we wing up and I share out the missions, turning them in one by one. This usually spreads 400-500m in rewards to each of my team mates.

Since they've often been active doing the same missions, I usually get the same back again.

One or two of these 5-day expansions might even be enough to earn your own carrier.

Fraknoi Ring in Croatigae is controlled by Xiancheim Empire League.

About every 2-3 weeks I trigger an expansion for this faction, creating the five-day mission bonanza.

The great thing about Croatigae is that there are TWO large stations in the same system (Ambartsumiam and Carrington) which sell the goods I need!

I don't even have to jump out.

If you keep an eye on Xiancheim Empire League on Inara (you can bookmark a faction), when you see expansion pending or active you know that Croatigae is the place to be for these missions.